Shallow Thoughts : tags : astronomy

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Wed, 23 Jul 2014

Predicting planetary visibility with PyEphem

Part 1: Basic Planetary Visibility

All through the years I was writing the planet observing column for the San Jose Astronomical Association, I was annoyed at the lack of places to go to find out about upcoming events like conjunctions, when two or more planets are close together in the sky. It's easy to find out about conjunctions in the next month, but not so easy to find sites that will tell you several months in advance, like you need if you're writing for a print publication (even a club newsletter).

For some reason I never thought about trying to calculate it myself. I just assumed it would be hard, and wanted a source that could spoon-feed me the predictions.

The best source I know of is the RASC Observer's Handbook, which I faithfully bought every year and checked each month so I could enter that month's events by hand. Except for January and February, when I didn't have the next year's handbook yet by the time my column went to press and I was on my own. I have to confess, I was happy to get away from that aspect of the column when I moved.

In my new town, I've been helping the local nature center with their website. They had some great pages already, like a What's Blooming Now? page that keeps track of which flowers are blooming now and only shows the current ones. I've been helping them extend it by adding features like showing only flowers of a particular color, separating the data into CSV databases so it's easier to add new flowers or butterflies, and so forth. Eventually we hope to build similar databases of birds, reptiles and amphibians.

And recently someone suggested that their astronomy page could use some help. Indeed it could -- it hadn't been updated in about five years. So we got to work looking for a source of upcoming astronomy events we could use as a data source for the page, and we found sources for a few things, like moon phases and eclipses, but not much.

Someone asked about planetary conjunctions, and remembering how I'd always struggled to find that data, especially in months when I didn't have the RASC handbook yet, I got to wondering about calculating it myself. Obviously it's possible to calculate when a planet will be visible, or whether two planets are close to each other in the sky. And I've done some programming with PyEphem before, and found it fairly easy to use. How hard could it be?

Note: this article covers only the basic problem of predicting when a planet will be visible in the evening. A followup article will discuss the harder problem of conjunctions.

Calculating planet visibility with PyEphem

The first step was figuring out when planets were up. That was straightforward. Make a list of the easily visible planets (remember, this is for a nature center, so people using the page aren't expected to have telescopes):

import ephem

planets = [
    ephem.Moon(),
    ephem.Mercury(),
    ephem.Venus(),
    ephem.Mars(),
    ephem.Jupiter(),
    ephem.Saturn()
    ]

Then we need an observer with the right latitude, longitude and elevation. Elevation is apparently in meters, though they never bother to mention that in the PyEphem documentation:

observer = ephem.Observer()
observer.name = "Los Alamos"
observer.lon = '-106.2978'
observer.lat = '35.8911'
observer.elevation = 2286  # meters, though the docs don't actually say

Then we loop over the date range for which we want predictions. For a given date d, we're going to need to know the time of sunset, because we want to know which planets will still be up after nightfall.

observer.date = d
sunset = observer.previous_setting(sun)

Then we need to loop over planets and figure out which ones are visible. It seems like a reasonable first approach to declare that any planet that's visible after sunset and before midnight is worth mentioning.

Now, PyEphem can tell you directly the rising and setting times of a planet on a given day. But I found it simplified the code if I just checked the planet's altitude at sunset and again at midnight. If either one of them is "high enough", then the planet is visible that night. (Fortunately, here in the mid latitudes we don't have to worry that a planet will rise after sunset and then set again before midnight. If we were closer to the arctic or antarctic circles, that would be a concern in some seasons.)

min_alt = 10. * math.pi / 180.
for planet in planets:
    observer.date = sunset
    planet.compute(observer)
    if planet.alt > min_alt:
        print planet.name, "is already up at sunset"

Easy enough for sunset. But how do we set the date to midnight on that same night? That turns out to be a bit tricky with PyEphem's date class. Here's what I came up with:

    midnight = list(observer.date.tuple())
    midnight[3:6] = [7, 0, 0]
    observer.date = ephem.date(tuple(midnight))
    planet.compute(observer)
    if planet.alt > min_alt:
        print planet.name, "will rise before midnight"

What's that 7 there? That's Greenwich Mean Time when it's midnight in our time zone. It's hardwired because this is for a web site meant for locals. Obviously, for a more general program, you should get the time zone from the computer and add accordingly, and you should also be smarter about daylight savings time and such. The PyEphem documentation, fortunately, gives you tips on how to deal with time zones. (In practice, though, the rise and set times of planets on a given day doesn't change much with time zone.)

And now you have your predictions of which planets will be visible on a given date. The rest is just a matter of writing it out into your chosen database format.

In the next article, I'll cover planetary and lunar conjunctions -- which were superficially very simple, but turned out to have some tricks that made the programming harder than I expected.

Tags: , , ,
[ 21:32 Jul 23, 2014    More science/astro | permalink to this entry | comments ]

Sat, 07 Sep 2013

Daytime Venus-Moon-Saturn conjunction

Tomorrow, Sunday September 8th, is an interesting astronomical event: a nice conjunction of a slim crescent moon and gibbous Venus, with Saturn hanging above and to the left of the pair.

That alone isn't anything unusual, though they'll be a lovely naked-eye sight just after nightfall. But here's the kicker: they'll be quite a bit closest during the daytime, best around 2-3 in the afternoon, Which makes for a fun exercise: can you find the crescent moon during daylight, then use it to guide you to Venus (right above it, about a degree away) and Saturn (about 10 degrees away, down and left)?

They'll be just a little east of due south, and about 40 degrees up. You'll definitely need binoculars to find Saturn, and they might help in finding the other two as well, depending on how bright and how hazy your afternoon sky is. Once you find them, a low powered telescope view should show Venus' phase and Saturn's rings. Venus is gibbous, alas; it would have been fun to see two crescents lined up one above the other.

If you have trouble finding them, wait until 3:30 pm, when they'll be transiting. At that point, you should be able to point due south, sweep your binoculars (or just your eyes) up just short of halfway to the zenith, and the moon should be there.

If you don't get a chance to watch the daylight conjunction, or don't have binoculars or a telescope handy, at least take a naked eye look at the trio at nightfall.

Mars and an early view of Comet ISON

As long as I'm reposting tips from my SJAA Ephemeris Shallow Sky column, there's another interesting thing in the sky this month: Comet C/2012 S1 ISON. Yes, that's the "super comet" that's supposed to become brighter than the moon. No, it won't be bright yet. It's still super wimpy, and worse, it's still in the morning sky, so it's not an easy or convenient target.

On the other hand, through September and October, Mars and Comet ISON will be within a few degrees of each other. So if you're willing to stay up (or get up) for early morning dark-sky observing, and you have a big telescope, this could be a nice view.

The comet won't be very impressive yet -- it's only expected to be 10th magnitude in September -- but such close proximity to Mars makes it easy to find and keep track of. In September, the pair don't rise until about 3:30am, and that won't change much for the next few months. The comet will probably stay below naked eye visibility at least for the next two months, brightening from 11th magnitude in early September to maybe 7th magnitude by Halloween.

As September opens, ISON makes a triangle with Mars and M44, the Beehive cluster. The comet stands about 2 degrees north of the Beehive and about 5 degrees east of Mars. But it closes with Mars as the month progresses: by the end of September you can find the comet about two degrees north of Mars, and by the middle of October they'll be down to only a degree apart (with ISON brightening to about ninth magnitude).

About that Beehive cluster: right now (September 7 through 9), Mars is passing right through the Beehive, like an angry red wasp among the smaller bees. Should be a nice view even if the comet isn't. It's a good binocular or even naked eye view (though great with a telescope, too). So if you find yourself up before dawn, definitely take a look.

Tags:
[ 19:34 Sep 07, 2013    More science/astro | permalink to this entry | comments ]

Wed, 03 Jul 2013

Mad Moon Models

[This a slight revision of my monthly "Shallow Sky" column in the SJAA Ephemeris newsletter. Looks like the Ephemeris no longer has an online HTML version, just the PDF of the whole newsletter, so I may start reposting my Ephemeris columns here more often.]

[Plate IX: The Lunar Apennines, Archemedes &c.] Last month I stumbled upon a loony moon book I hadn't seen before, one that deserves consideration by all lunar observers.

The book is The Moon: Considered as a Planet, a World, and a Satellite by James Nasmyth, C.E. and James Carpenter, F.R.A.S. It's subtitled "with twenty-six illustrative plates of lunar objects, phenomena, and scenery; numerous woodcuts &c." It was written in 1885.

Astronomers may recognize the name Nasmyth: his name is attached to a modified Cassegrain focus design used in a lot of big observatory telescopes. Astronomy was just a hobby for him, though; he was primarily a mechanical engineer. His coauthor, James Carpenter, was an astronomer at the Royal Greenwich Observatory.

The most interesting thing about their book is the plates illustrating lunar features. In 1885, photography wasn't far enough along to get good close-up photos of the moon through a telescope. But Nasmyth and Carpenter wanted to show something beyond sketches. So they built highly detailed models of some of the most interesting areas of the moon, complete with all their mountains, craters and rilles, then photographed them under the right lighting conditions for interesting shadows similar to what you'd see when that area was on the terminator.

[David North explaining the moon] I loved the idea, since I'd worked on a similar but much less ambitious project myself. Over a decade ago, before we were married, Dave North got the idea to make a 3-D model of the full moon that he could use for the SJAA astronomy class. I got drafted to help. We started by cutting a 3-foot disk of wood, on which we drew a carefully measured grid corresponding to the sections in Rukl's Atlas of the Moon. Then, section by section, we drew in the major features we wanted to incorporate. Once the drawing was done, we mixed up some spackle -- some light, and some with a little black paint in it for the mare areas -- and started building up relief on top of the features we'd sketched. The project was a lot of fun, and we use the moon model when giving talks (otherwise it hangs on the living room wall).

Nasmyth and Carpenter's models cover only small sections of the moon -- Copernicus, Plato, the Apennines -- but in amazing detail. Looking at their photos really is like looking at the moon at high magnification on a night of great seeing.

So I had to get the book. Amazon has two versions, a paperback and a hardcover. I opted for the paperback, which turns out to be scanned from a library book (there's even a scan of the pocket where the book's index card goes). Some of the scanning is good, but some of the plates come out all black. Not very satisfying.

But once I realized that an 1885 book was old enough to be public domain, I checked the web. I found two versions: one at Archive.org and one on Google Books. They're scans from two different libraries; the Archive.org scan is better, but the epub version I downloaded for my ebook reader has some garbled text and a few key plates, like Clavius, missing. The Google version is a much worse scan and I couldn't figure out if they had an epub version. I suspect the hardcover on Amazon is likely a scan from yet a fourth library.

At the risk of sounding like some crusty old Linux-head, wouldn't it be nice if these groups could cooperate on making one GOOD version rather than a bunch of bad ones?

I also discovered that the San Jose library has a copy. A REAL copy, not a scan. It gave me a nice excuse to take the glass elevator up to the 8th floor and take in the view of San Jose. And once I got it, I scanned all the moon sculpture plates myself. Sadly, like the Archive.org ebook, the San Jose copy is missing Copernicus. I wonder if vandals are cutting that page out of library copies? That makes me wince even to think of it, but I know such things happen.

Whichever version you prefer, I'd recommend that lunies get hold of a copy. It's a great introduction to planetary science, with very readable discussions of how you measure things like the distance and size of the moon. It's an even better introduction to lunar observing: if you merely go through all of their descriptions of interesting lunar areas and try to observe the features they mention, you'll have a great start on a lunar observing program that'll keep you busy for months. For experienced observers, it might give you a new appreciation of some lunar regions you thought you already knew well. Not at super-fine levels of detail -- no Alpine Valley rille -- but a lot of good discussion of each area.

[Plate XVIII: Aristarchus & Herodotus ] Other parts of the book are interesting only from a historical perspective. The physical nature of lunar features wasn't a settled issue in 1885, but Nasmyth and Carpenter feel confident that all of the major features can be explained as volcanism. Lunar craters are the calderas of enormous volcanoes; mountain ranges are volcanic too, built up from long cracks in the moon's crust, like the Cascades range in the Pacific Northwest.

There's a whole chapter on "Cracks and Radiating Streaks", including a wonderful plate of a glass ball with cracks, caused by deformation, radiating from a single point. They actually did the experiment: they filled a glass globe with water and sealed it, then "plunged it into a warm bath". The cracks that resulted really do look a bit like Tycho's rays (if you don't look TOO closely: lunar rays actually line up with the edges of the crater, not the center).

It's fun to read all the arguments that are plausible, well reasoned -- and dead wrong. The idea that craters are caused by meteorite impacts apparently hadn't even been suggested at the time.

Anyway, I enjoyed the book and would definitely recommend it. The plates and observing advice can hold their own against any modern observing book, and the rest ... is a fun historical note.

Here are some places to get it:
Amazon:

Online:

Or, try your local public library -- they might have a real copy!

Tags: , , , ,
[ 16:12 Jul 03, 2013    More science/astro | permalink to this entry | comments ]

Wed, 08 May 2013

Gamymede whac-a-moon tonight

A couple of months ago I wrote about watching an eclipse of Europa by Jupiter's shadow. It's a game I call "Whac-a-Moon", where a moon comes out from behind Jupiter, but stays there for only a short time then disappears into eclipse. If you aren't ready for it, it's gone.

This can only happen when Jupiter's shadow is offset from Jupiter that there's a gap between the planet and the shadow as seen from Earth. Jupiter is getting low in the west, and soon we'll lose it behind the sun, but tonight, Wednesday May 8, there's a decent Ganymede Whac-a-Moon opportunity for those of us on the US west coast.

Ganymede disappears behind Jupiter at 6:45 pm PDT, still during daylight. Some time around 9:43 Ganymede reappears from behind Jupiter, but it only stays visible for a couple of minutes before entering Jupiter's eclipse. Don't trust these times I'm giving you: set up at least five minutes early, preferably more than that. And set up somewhere with a good western horizon, because Jupiter will be very low, less than 8 degrees above the horizon.

You can simulate the event on my Javascript Jupiter. When the G goes blue, that means Ganymede is in eclipse. But the simulation won't show you the interesting part: how gradual the eclipse is, as the moon slides through the edge of Jupiter's shadow. During the Europa eclipse a few months ago, I wanted to record the time of disappearance so I could adjust my code accordingly, but I found I couldn't pin it down at all -- Europa started dimming almost as soon as it emerged from behind Jupiter, and kept dimming until I couldn't convince myself I saw it any more.

So far, I've only watched Europa as it slid into eclipse by Jupiter's shadow; I haven't whacked Ganymede. But Ganymede is so much larger that I suspect the slow dimming effect will be even more obvious. Unfortunately, I'm not optimistic about being able to see it myself; we've had cloudy skies here for the last few nights, and that combined with the low western horizon may do me in. I may have to wait until autumn, when Jupiter will next be visible in our evening skies. But I hope someone reading this gets a chance to see this month's eclipse.

Tags: ,
[ 11:46 May 08, 2013    More science/astro | permalink to this entry | comments ]

Tue, 12 Mar 2013

The Europa Eclipse -- results

I wrote last week about an upcoming eclipse of Europa by Jupiter's shadow. One of the interesting things I'd found was how much the predicted times of Europa's appearance from behind Jupiter, and subsequent disappearance into Jupiter's shadow, varied depending on which program you were using. I had just recently managed to get my own Javascript Jupiter page showing eclipse events, and its times didn't agree with any of the other programs either. So I was burning with curiosity to know who was right.

The predicted times were:
Europa appears Europa disappears
XEphem 7:43 7:59
S&T Jupiter's Moons 7:40 7:44
my Javascript Jupiter 7:45 7:52
Stellarium 6:49 7:05

I was out of town on March 10. I brought along a travel scope, an Orion 80mm f/6 Orion Express. Not the perfect planetary scope, but certainly enough to see Europa. (The Galilean moons are even visible in binoculars, as long as you mount the binoculars on a tripod or otherwise hold them steady.)

I synchronized my watch and had the telescope set up by 7:35. Sure enough, there was no Europa there. But at 7:38 on the dot, I saw the first hint of Europa peeking out. No question about it. I watched, and timed, and by 7:41 the whole disk of Europa was visible and I could start to think I could see blackness between it and Jupiter. I'd been to a school star party a few days earlier and hadn't cleaned my eyepieces afterward -- oops! -- so the view was a little foggy and it was hard to tell for sure exactly when Europa's disk cleared Jupiter.

In fact, no matter which eyepiece I used, the fogginess seemed to get worse and worse. I had a hard time seeing Europa at all. Finally I realized that I was looking through a tree branch, and moved the scope. But by the time I got it moved again, Europa had gotten even harder to see. That was when I realized that it had been going into eclipse practically the whole time I was watching it. It was already significantly dimmed by 7:43, very dim indeed by 7:48 and gone -- in the 80mm -- by 7:49:20, though I suspect it still would have been visible in a larger scope with clean eyepieces.

So that's why the times in different programs varied so much! Galilean moons aren't point sources: you can't predict a single time for a moon disappearance, appearance or eclipse. Do you want to predict the beginning of the event, the end of the event or the time at the moon's center point? And that goes double for eclipses, where the moon is gradually sliding into the shadow of Jupiter's atmosphere. I found that it took over seven minutes the moon to go from full brightness to fully eclipsed. So what part of that do you predict?

All in all, a very interesting observing session. I'm looking forward to observing more of these eclipses, doing more timings, and tuning my program to give better predictions. (I notice my program was significantly late on both the appearance and the eclipse. I'll work on that. Better to err on the early side, and not miss anything!)

While I was adding eclipses to my Jupiter program, I also added longer-range predictions, so it would be easier to find out when these events will happen. Once that was implemented, I looked for upcoming Whac-a-Moon events. I found one on Mar 26, when Ganymede appears at 7:29pm PDT (add 7 hours for GMT). Europa and its shadow are transiting Jupiter's disk, too, so there's plenty to look at. Ganymede then enters eclipse at 9:40pm PDT. A long time between the events, I know, but it's easy enough to leave a scope set up in the backyard and go out to check it now and then.

These times are from my Javascript Jupiter program and may be a few minutes late. Always be ready at least five minutes early in case the predictions are off, no matter which program you use. Don't say I didn't warn you.

I found no events in April visible at night in California (for other time zones, you can generate predictions on my Javascript Jupiter page). But May 8 has a decent one: Ganymede appears at 9:44pm PDT, then disappears into eclipse at 9:46. Based on what I saw tonight with Europa, that means the moon should start to fade almost immediately after it has emerged from behind Jupiter, maybe even before it has fully emerged. Ganymede's larger size may also mean the fade-out will take longer. Stay tuned. Jupiter will be very low by then, only 7 degrees above the horizon.

Not many events to observe -- this is a bit rarer than I'd thought. Of course, there are lots of moons disappearing into eclipse and appearing from out of it every night, so watching that long gradual appearance or disappearance isn't difficult; the only rare part is when they appear briefly between Jupiter and Jupiter's shadow. That is relatively rare, and I'm glad I had a chance to catch it.

Tags: ,
[ 19:55 Mar 12, 2013    More science/astro | permalink to this entry | comments ]

Sat, 09 Mar 2013

Whac-a-Moon: Watch Europa appear and disappear this Sunday

This is an edited and updated version of my "Shallow Sky" column this month in the SJAA Ephemeris newsletter.

A few months ago, I got email from a Jupiter observer calling my attention to an interesting phenomenon of Jupiter's moons that I hadn't seen before. The person who mailed me described himself as a novice, and wasn't quite sure what he had seen, but he knew it was odd. After some further discussion we pinned it down.

He was observing Jupiter at 11/11/12 at 00.25 UT (which would have been mid-afternoon here in San Jose). Three of the moons were visible, with only Ganymede missing. Then Ganymede appeared: near Jupiter's limb, but not right on it. As he watched over the next few minutes, Ganymede seemed to be moving backward -- in toward Jupiter rather than away from it. Eventually it disappeared behind the planet.

It turned out that what he was seeing was the end of an eclipse. Jupiter was still a few months away from opposition, so the shadow thrown by the planet streamed off to one side as seen from our inner-planet vantage point on Earth. At 0:26 UT on that evening, long before he started observing, Ganymede, still far away from Jupiter's limb, had entered Jupiter's shadow and disappeared into eclipse. It took over two hours for Ganymede to cross Jupiter's shadow; but at 2:36, when it left the shadow, it hadn't yet disappeared behind the planet. So it became visible again. It wasn't until 2:50 that Ganymede finally disappeared behind Jupiter.

So it was an interesting effect -- bright Ganymede appearing out of nowhere, moving in toward Jupiter then disappearing again fourteen minutes later. It was something I'd never seen, or thought to look for. It's sort of like playing Whac-a-mole -- the moon appears only briefly, so you've got to hit it with your telescope at just the right time if you want to catch it before it disappears again.

A lot of programs don't show this eclipse effect -- including, I'm sad to say, my own Javascript Jupiter's moons web page. (I have since remedied that.) The open source program Stellarium shows the effect; on the web, Sky and Telescope's Jupiter's Moons page shows it, and even prints out a table of times of various moon events, including eclipses.

[Europa eclipse on Mar 10 2013]

These eclipse events aren't all that uncommon -- but only when the sun angle is just right. Searching in late February and early March this year, I found several events for Ganymede and Europa (though, sadly, many of them were during our daytime). By mid-March, the angles have changed so that Europa doesn't leave Jupiter's shadow until after it's disappeared behind the planet's limb; but Ganymede is farther out, so we can see Ganymede appearances all the way through March and for months after.

The most interesting view, it seems to me, is right on the boundary when the moon only appears for a short time before disappearing again. Like the Europa eclipse that's happening this Sunday night, March 10.

Reporting on that one got a little tricky -- because that's the day we switch to Daylight Savings time. I have to confess that I got a little twisted up trying to compare results between programs that use UTC and programs that use local time -- especially when the time zone converter I was using to check my math told me "That time doesn't exist!" Darnit, if we'd all just use UTC all the time, astronomy calculations would be a lot easier! (Not to mention dropping the silly Daylight Savings Time fiasco, but that's another rant.)

Before I go into the details, I want to point out that Jupiter's moons are visible even in binoculars. So even if you don't have a telescope, grab binoculars and set them up in as steady a way as you can -- if you don't have a tripod adaptor, try bracing them on the top of a gate or box.

On Sunday night, March 10, at some time around 7:40 pm PDT, Europa peeks out from behind Jupiter's northeast limb. (All times are given in PDT; add 7 hours for GMT.) The sky will still be bright here in California -- the sun sets at 7:12 that night -- but Jupiter will be 66 degrees up and well away from the sun, so it shouldn't give you too much trouble. Once Europa pops out, keep a close eye on it -- because if Sky & Tel's calculations are right, it will disappear again just four minutes later, at 7:44, into eclipse in Jupiter's shadow. It will remain invisible for almost three hours, finally reappearing out of nowhere, well off Jupiter's limb, at around 10:24 pm.

Here's a link to my Javascript Jupiter just before Europa reappears.

I want to stress that those times are approximate. In fact, I tried simulating the event in several different programs, and got wildly varying times:
Io disappears Europa appears Europa disappears Europa reappears Io appears
XEphem 7:15 7:43 7:59 10:06 10:43
S&T Jupiter's Moons 7:16 7:40 7:44 10:24 10:48
Javascript Jupiter 7:17 7:45 7:52 10:15 10:41
Stellarium 6:21 6:49 7:05 9:32 10:01

You'll note Stellarium seems to have a time zone problem ... maybe because I ran the prediction while we were still in standard time, not daylight savings time.

I'm looking forward to timing the events to see which program is most accurate. I'm betting on XEphem. Once I know the real times, maybe I can adjust my Javascript Jupiter's code to be more accurate. If anyone else times the event, please send me your times, in case something goes wrong here!

Anyway, the spread of times makes it clear that when observing this sort of phenomenon, you should always set up the telescope ten or fifteen minutes early, just in case. And ten extra minutes spent observing Jupiter -- even without moons -- is certainly never time wasted! Just keep an eye out for Europa to appear -- and be ready to whack that moon before it disappears again.

Tags: , ,
[ 11:30 Mar 09, 2013    More science/astro | permalink to this entry | comments ]

Wed, 31 Oct 2012

Comparing sunset times with PyEphem

We were marveling at how early it's getting dark now -- seems like a big difference even compared to a few weeks ago. Things change fast this time of year.

Since we're bouncing back and forth a lot between southern and northern California, Dave wondered how Los Angeles days differed from San Jose days. Of course, San Jose being nearly 4 degrees farther north, it should have shorter days -- but by the weirdness of orbital mechanics that doesn't necessarily mean that the sun sets earlier in San Jose. His gut feel was that LA was actually getting an earlier sunset.

"I can calculate that," I said, and fired up a Python interpreter to check with PyEphem. Since PyEphem doesn't know San Jose (hmph! San Jose is bigger than San Francisco) I used San Francisco.

Since PyEphem's Observer class only has next_rising() and next_setting(), I had to set a start date of midnight so I could subtract the two dates properly to get the length of the day.

>>> sun = ephem.Sun()
>>> la = ephem.city('Los Angeles')
>>> sf = ephem.city('San Francisco')
>>> 
>>> mid = ephem.Date('2012/10/31 8:00')
>>> 
>>> la.next_rising(sun, start=mid)
2012/10/31 14:11:57
>>> la.next_setting(sun, start=mid)
2012/11/1 01:00:45
>>> la.next_setting(sun, start=mid) - la.next_rising(sun, start=mid)
0.45055988136300584
>>> 
>>> sf.next_rising(sun, start=mid)
2012/10/31 14:34:19
>>> sf.next_setting(sun, start=mid)
2012/11/1 01:11:44
>>> sf.next_setting(sun, start=mid) - sf.next_rising(sun, start=mid)
0.4426457611261867

So Dave's intuition was right: northern California really does have a later sunset than southern California at this time of year, even though the total day length is shorter -- the difference in sunrise time makes up for the later sunset.

How much shorter?

>>> (la.next_setting(sun, start=mid) - la.next_rising(sun, start=mid)) - (sf.next_setting(sun, start=mid) - sf.next_rising(sun, start=mid))
0.007914120236819144
>>> ((la.next_setting(sun, start=mid) - la.next_rising(sun, start=mid)) - (sf.next_setting(sun, start=mid) - sf.next_rising(sun, start=mid))) * 24
0.18993888568365946
>>> ((la.next_setting(sun, start=mid) - la.next_rising(sun, start=mid)) - (sf.next_setting(sun, start=mid) - sf.next_rising(sun, start=mid))) * 24 * 60
11.396333141019568

And we have our answer -- there's about 11 minutes difference in day length between SF and LA.

Tags: , ,
[ 11:46 Oct 31, 2012    More science/astro | permalink to this entry | comments ]

Fri, 21 Sep 2012

Farewell, Space Shuttle Endeavour

[Space shuttle Endeavour flyby] This morning, the last space shuttle, Endeavour, made a piggyback fly-by of California cities prior to landing at LAX, where it will be trucked to its final resting place in Exposition Park. And what science and astronomy fan could resist a once in a lifetime chance to see the last shuttle in flight, piggyback on its 747 transporter?

Events kept me busy all morning, so I was late getting away. Fortunately I'd expected that and planned for it. While watching the flyby from Griffith Observatory sounded great, I suspected there would be huge crowds, no parking and there's no way I could get there in time. The Times suggested Universal City -- which I took to mean that there would be huge crowds and traffic there too. So I picked a place off the map, Blair Dr., that looked like it was easy to get to, reasonably high and located in between Griffith and Universal.

It turned out to be a good choice. There were plenty of people there, but I found a parking spot a few blocks away from where everybody was hanging out and walked back to the viewpoint where I'd seen the crowds.

[Universal Studios back lot] I looked down and the first thing I saw was a smashed jumbo jet among the wreckage of some houses. Um ... not the way I wanted to see the shuttle! But then I realized I was looking at the Universal Studios back lot. Right. Though binoculars I could even see the tram where the folks on the studio tour went right by the "plane crash". And I could look across to Universal City, where the crowds made me happy I'd decided against going there -- I bet they had some traffic jams too.

The crowd was friendly and everybody was sharing the latest rumors of the shuttle's location -- "It just flew over Santa Barbara!" "It's over West Hollywood -- get ready!" "Nope, now it's going west again, might be a while." That helped with the wait in the hot sun.

[Space shuttle Endeavour flyby] Finally, "It's coming!" And we could see it, passing south of the crowds at Universal City and coming this way ... and disappearing behind some trees. We all shifted around so we'd see it when it cleared the trees.

Only it didn't! We only got brief glimpses of it, between branches, as the shuttle flew off toward Griffith Observatory. Oh no! Were we in exactly the wrong location?

Then the word spread, from people farther down the road -- "It's turning -- get ready for another pass!" This time it came by south of us, giving us all a beautiful clear view as the 747 flew by with the shuttle and its two fighter-plane escorts.

We hung around for a few more minutes, hoping for another pass, but eventually we dispersed. The shuttle and its escorts flew on to LAX, where it will be unloaded and trucked to Exposition Park. I feel lucky to have gotten such a beautiful view of the last shuttle flight.

Photos: Space shuttle Endeavour flyover.

Tags: ,
[ 21:35 Sep 21, 2012    More science/astro | permalink to this entry | comments ]

Sun, 12 Aug 2012

A daytime Venus occultation

Tomorrow -- Monday, August 13th -- starting a little after 1 pm PDT (20 UT), the moon passes in front of Venus. That's during the day for those of us in the US, but don't worry -- both Venus and the moon are easily visible during the daytime.

The RASC handbook lists the time as exactly 1pm, but XEphem and some web sources show Venus disappearing at more like 1:30. The time isn't critical, because the most interesting part of this occultation is the lead-up, where you can see both Venus and the moon at once. The nearness of the moon will make it easy to locate Venus during the day, something that's usually a bit challenging even with this bright magnitude -4 planet.

Binoculars should show both objects just fine, though a telescope is even better. In a telescope, you'll be able to compare the phases of the two objects: the slim crescent of the moon contrasted with the half Venus.

If you've never seen a Venus occultation before, you'll be amazed at the difference between the brightness of Venus and the dimness of the moon's limb. We think of the moon as bright, but it's actually dark grey, about the same albedo (reflectivity) as asphalt; whereas Venus is covered with brightly reflective clouds.

It's a great excuse to set up a telescope or binoculars for a late lunchtime observing session and share some photons with your co-workers or anyone else who happens by. I've heard an amazing number of adults express amazement at the idea of seeing the moon during the daytime (even though they've undoubtedly seen it themselves at some point, and just don't remember it). So seeing both objects, and their phases, should be a great conversation starter outside the cafeteria or local coffeehouse.

I'd suggest setting up no later than 12:30, and earlier works fine. Even before 11, a low power eyepiece should show both the moon and Venus in the same field. Watch out for the sun! Try to find a place where you're shaded from the sun but can still see the moon. That way, not only do you stay cooler, but you're protected against accidentally swinging binoculars toward the sun and blinding yourself.

Of course, what goes behind must come out again: Venus should re-emerge from behind the dark side of the moon around 2:30 to 3 pm.

[Daytime Venus occultation] And now it's over. A fun event! It disappeared at about 1:35pm. You can see my low-tech photos here: Daytime Venus occultation, 2012-8-13.

Tags:
[ 13:27 Aug 12, 2012    More science/astro | permalink to this entry | comments ]

Wed, 06 Jun 2012

There's a big black spot on the sun today ...

[Transit of Venus, June 5 2012] After a heart-stopping day of rain on Monday, Tuesday, the day of the Venus transit astronomers have been anticipating for decades, dawned mostly clear.

For the 3 pm ingress, Dave and I set up in the backyard -- a 4.5-inch Newtonian, a Takahashi FS102, and an 80mm f/6 refractor with an eyepiece projection camera mount. I'd disliked the distraction during the annular eclipse of switching between eyepiece and camera mount, and was looking forward to having a dedicated camera scope this time.

Venus is big! There wasn't any trouble seeing it once it started its transit. I was surprised at how slowly it moved -- so much slower than a Mercury transit, though it shouldn't have been a surprise, since I knew the event would take the rest of the evening, and wouldn't be finished until well past our local sunset.

The big challenge of the day was to see the aureole -- the arc of Venus' atmosphere standing out from the sun. With the severely windy weather and turbulent air (lots of cumulus clouds) I wasn't hopeful. But as Venus reached the point where only about 1/3 of its disk remained outside the sun, the aureole became visible as a faint arc. We couldn't see it in the 4.5-inch, and it definitely isn't visible in the poorly-focused photos from the 80mm, but in the FS102 it was definitely there.

About those poorly focused pictures: I hadn't used the 80mm, an Orion Express, for photography before. It turned out its 2-inch Crayford focuser, so nice for visual use, couldn't hold the weight of a camera. With the sun high overhead, as soon as I focused, the focuser tube would start to slide downward and I couldn't lock it. I got a few shots through the 80mm, but had better luck holding a point-and-shoot camera to the eyepiece of the FS102.

Time for experiments

[Binocular projection of Venus transit] Once the excitement of ingress was over, there was time to try some experiments. I'd written about binocular projection as a way anyone, without special equipment, could watch the transit; so I wanted to make sure that worked. I held my cheap binoc (purchased for $35 many years ago at Big 5) steady on top of a tripod -- I never have gotten around to making a tripod mount for it; though if I'd wanted a more permanent setup, duct tape would have worked.

I couldn't see much projecting against the ground, and it was too windy to put a piece of paper or cardboard down, but an old whiteboard made a perfect solar projection screen. There was n trouble at all seeing Venus and some of the larger sunspots projected onto the whiteboard.

As the transit went on, we settled down to a routine of popping outside the office every now and then to check on the transit. Very civilized. But the transit lasted until past sunset, and our western horizon is blocked by buildings. I wanted some sunset shots. So we took a break for dinner, then drove up into the hills to look for a place with a good ocean view.

The sunset expedition

Our first idea, a pullout off Highway 9, had looked promising in Google Earth but turned out to have trees and a hill (that somehow hadn't shown up in Google Earth) blocking the sunset. So back up highway 9 and over to Russian Ridge, where I remembered a trail entrance on the western side of the ridge that might serve. Sure enough, it gave us a great sunset view. There was only parking space for one car, but fortunately that's all we needed. And we weren't the only transit watchers there -- someone else had hiked in from the main parking lot carrying a solar filter, so we joined him on the hillside as we waited for sunset.

[Mak 90 with solar filter] I'd brought the 80mm refractor for visual observing and the 90 Mak for camerawork. I didn't have a filter for the Mak, but Dave had some Baader solar film, so earlier in the afternoon I'd whipped up a filter. A Baskin-Robbins ice cream container lid turned out to be the perfect size. Well, almost perfect -- it was just a trifle too large, but some pads cut from an old mouse pad and taped inside the lid made it fit perfectly. Dave used the Baader film, some foam and masking tape to make a couple of filters for his binocular.

The sun sank through a series of marine cloud layers. Through the scopes it looked more like Jupiter than the sun, with Jupiter's banding -- and Venus' silhouette even looked like the shadow of one of Jupiter's moons.

[off-axis aperture stops from ice cream containers] Finally the sun got so low, and so obscured by clouds, that it seemed safe to take the solar filter off the 90mm camera rig. (Of course, we kept the solar filters on the other scope and binocular for visual observing.) But even at the camera's fastest shutter speed, 1/4000, the sun came out vastly overexposed with 90mm of aperture feeding it at f/5.6.

I had suspected that might be a problem, so I'd prepared a couple of off-axis stops for the Mak, to cover most of the aperture leaving only a small hole open. Again, BR ice cream containers turned out to be perfect. I painted the insides flat black to eliminate reflections, then cut holes in the ends -- one about the size of a quarter, the other quite a bit larger. It turned out I didn't use the larger stop at all, and it would have been handy to have one smaller than the quarter-sized one -- even with that stop, the sun was overexposed at first even at 1/4000 and I had to go back to the solar filter for a while.

[Venus transit at sunset] [Venus transit at sunset] I was happy with the results, though -- I got a nice series of sunset photos complete with Venus silhouette.

More clouds rolled in as we packed up, providing a gorgeous blue-and-pink sunset sky backdrop for our short walk back to the car. What a lovely day for such a rare celestial event!

Photos here: Venus Transit, June 5 2012.

Tags: , ,
[ 12:48 Jun 06, 2012    More science/astro | permalink to this entry | comments ]

Fri, 01 Jun 2012

Observe the 2012 Venus transit!

[Venus transit 2004 from Chicago. Copyright © 2004 by Bill Arnett] June 5 brings the last Venus transit until 2117, when Venus will pass across the face of the sun -- the second of the only two Venus transits any of us will see in our lives. (The first, pictured in this lovely image from Bill Arnett, was in 2004, visible on the east coast of the US but not visible here in California.)

Venus is just a small spot against the vastness of the sun -- the skies won't dim like in an eclipse, and you need equipment to see it. So why should a non-astronomer care?

Mostly because it's so rare. Venus transits happen in pairs with more than a century between successive pairs: the last transit before 2004 was in 1882, and the next one after this week's won't happen until 2117. The entire 20th century passed without a single Venus transit.

They're historically interesting, too. It was in 1663 that Scottish mathematician James Gregory proposed that you could calculate the distance to the Sun by measuring a Venus transit, by observing at many different places on earth and measuring parallax. Edmund Halley (of Halley's Comet fame) tried this method during the Mercury transit of 1676, but since Mercury is so much closer to the sun and farther from us, the results weren't good enough. Unfortunately, Halley died in 1742, too early for the Venus transit of 1761.

But lots of other astronomers were ready, mounting expeditions that year to Siberia, Norway, Newfoundland and Madagascar (some of these expeditions were major adventures, and several books have been written about them). They followed up in 1769 with expeditions to Hudson Bay, Baja California, Norway, St Petersburg, Philadelphia... and the first voyage of Captain Cook to Tahiti, where they observed the transit at a location that's still called "Point Venus" today.

[Black drop effect] Alas, their measurements weren't as accurate as they had hoped. The exact times of a Venus transit turn out to be difficult to measure due to the dreaded black drop effect, where the black circle of Venus can seem to elongate into a teardrop shape as it "tears away" from the edge of the sun. The effect seems to be caused by blurring from our own atmosphere (poor seeing) combined with telescope diffraction. So the steadier your seeing is, and the bigger and better your optics, the less likely you are to see the black drop.

How to see the transit

First, if you don't have a telescope of your own, don't despair -- head to your local astronomy club. Here in the bay area there are dozens of clubs, and just about all of them have public star parties planned for the Venus transit. There are events planned at local science museums, planetaria and schools as well. A few bay area links: San Jose Astronomical Association, Peninsula Astronomical Society, San Francisco Sidewalk Astronomers, San Francisco Amateur Astronomers, or any of the others on the AANC's list of Amateur Astronomy Clubs in Northern California or the SF Chronicle's list of astronomy clubs. And the Hubble Space Telescope will be watching the transit by looking at light reflected off the moon.

But suppose you're viewing it on your own?

Of course, this being a solar event, you can't look at it directly -- you need a filter or other apparatus. No need for a fancy H-alpha filter -- a white light solar filter is fine, the kind that covers the aperture of the telescope. (Don't use the kind that screw into the eyepiece! They can overheat and crack while you're looking through them.)

You don't need a big telescope. I used an Orion solar filter on my little 80mm f/7 refractor for the last Mercury transit and it worked great. And Venus is much larger than Mercury, at about 50 arcseconds versus Mercury's 12 (the sun is half a degree, or 30 arcminutes). So if you've seen a Mercury transit, you can imagine how much easier and more spectacular a Venus transit can be.

If you use binoculars, either make sure that you have solar filters for both sides, or keep one side covered at all times. If your telescope has a finderscope, keep it covered.

If you can't find a solar filter in time for the transit, you can set up your telescope to project the sun's image onto a white board or sheet of paper. (This is how Jeremiah Horrocks made the first known Venus transit observation.)

Use a cheap, low powered eyepiece for this: the eyepiece will get hot, and you don't want to risk damaging a fancy eyepiece. Be careful with solar projection -- make sure nobody nearby can walk between the telescope and the surface you're using as a projection screen, or place their hands or eyes in the light path. A web search for solar projection will uncover other tips.

You can project the sun's image with binoculars, too, so don't feel left out if you don't have a telescope. You'll definitely want a tripod mount. I tried binocular projection during last month's annular eclipse, and found it very fiddly to hold the binoculars just right. Don't count on being able to hold them steady while also looking for Venus on the projected image. If you don't have a tripod adapter (try Orion), cobble something together with duct tape and a block of wood, or whatever you have handy.

And do try to get a good white surface to project onto. Concrete worked well enough for the solar eclipse, but you'll want better resolution for Venus.

Timeline

When does this all happen?

Seen from the bay area, Venus begins its ingress onto the disc of the sun on 3:06 PDT on the afternoon of June 5. The transit continues until after the sun sets at 8:26. So we won't get to see egress. Venus's exit from the face of the sun, but it's the mirror image of what we'll see at ingress.

Ingress has two parts: first contact, when the edge of Venus's disk first touches the outside of the sun's disk, and "internal ingress" or second contact, when Venus's disk is fully inside that of the sun. Second contact is the most interesting period of the transit, since it's when the "black drop effect" occurs.

[Venus transit aureole by Lorenzo Comolli [Gruppo Astronomico Tradatese] and the VT-2004 programme] And if you have a good telescope and filter and you're blessed with especially good seeing around the time of second contact, try looking for the aureole, an arc of light just outside of the solar disk made by the refraction of sunlight through Venus's atmosphere. Amazingly, the aureole has the same surface brightness as the sun's surface, and is said to be possible to see even through a solar filter. That's something you'll never see in a Mercury transit! (Follow the link on the image to see Lorenzo Comolli's amazing aureole photo in more detail, along with other great aureole images courtesy of the VT-2004 programme.)

Here's the time table for the bay area (from the table on NASA's eclipse website):
First contact: 3:06:20
Internal ingress: 3:23:56
Maximum transit: 6:25:30
Sunset: 8:26

At first contact, the sun will still be high for bay area observers, 60° up. By maximum transit the pair will have sunk to 21°, still plenty high enough to see the spectacle. Photographers will want to wait around for sunset for a chance at some spectacular photos, like the Bill Arnett photo at the top of this article, taken from Chicago.

Want more details or times from other locations? transitofvenus.org has plenty of links, as does Everything you need to know about next week’s Transit of Venus.

Stay safe, and enjoy this once-in-a-lifetime event!

Tags: ,
[ 13:04 Jun 01, 2012    More science/astro | permalink to this entry | comments ]

Tue, 22 May 2012

Saw the "Ring of Fire" 2012 annular eclipse

[Annular eclipse 2012] I've just seen the annular eclipse, and what a lovely sight it was!

This was only my second significant solar eclipse, the first being a partial when I was a teenager. So I was pretty excited about an annular so nearby -- the centerline was only about a 4-hour drive from home.

We'd made arrangements to join the Shasta astronomy club's eclipse party at Whiskeytown Lake, up in the Trinity Alps. Sounded like a lovely spot, and we'd be able to trade views with the members of the local astronomy club as well as showing off the eclipse to the public. As astronomers bringing telescopes, we'd get reserved parking and didn't even have to pay the park fee. Sounded good!

Not knowing whether we might hit traffic, we left home first thing in the morning, hours earlier than we figured was really necessary. A good thing, as it turned out. Not because we hit any traffic -- but because when we got to the site, it was a zoo. There were cars idling everywhere, milling up and down every road looking for parking spots. We waited in the queue at the formal site, and finally got to the front of the line, where we told the ranger we were bringing telescopes for the event. He said well, um, we could drive in and unload, but there was no parking so we'd just have to drive out after unloading, hope to find a parking spot on the road somewhere, and walk back.

What a fiasco!

After taking a long look at the constant stream of cars inching along in both directions and the chaotic crowd at the site, we decided the better part of valor was to leave this vale of tears and high-tail it back to our motel in Red Bluff, only little farther south of the centerline and still well within the path of annularity. Fortunately we'd left plenty of extra time, so we made it back with time to spare.

The Annular Eclipse itself

[early stage of annular eclipse 2012, showing sunspots] One striking thing about watching the eclipse through a telescope was how fast the moon moves. The sun was well decorated with several excellent large sunspot groups, so we were able to watch the moon swallow them bit by bit.

Some of the darker sunspot umbras even showed something like a black drop effect as they disappeared behind the moon. We couldn't see the same effect on the smaller sunspot groups, or on the penumbras. [black drop at end of annularity] There was also a pronounced black drop effect at the onset and end of annularity.

The seeing was surprisingly good, as solar observing goes. Not only could we see good detail on the sunspot groups and solar faculae, but we could easily see irregularities in the shape of the moon's surface -- in particular one small sharp mountain peak on the leading edge, and what looked like a raised crater wall farther south on that leading edge. We never did get a satisfactory identification on either feature.

[pinhole eclipse viewing] After writing and speaking about eclipse viewing, I felt honor bound to try viewing with pinholes of several sizes. I found that during early stages of the eclipse, the pinholes had to be both small (under about 5 mm) and fairly round to show much. Later in the eclipse, nearly anything worked to show the crescent or the annular ring, including interlaced fingers or the shadow of a pine tree on the wall. I wish I'd remembered to take an actual hole punch, which would have been just about perfect.

[binocular projection for eclipse] I also tried projection through binoculars, and convinced myself that it would probably work as a means of viewing next month's Venus transit -- but only with the binoculars on a tripod. Hand-holding them is fiddly and difficult. (Of course, never look through binoculars at the sun without a solar filter.) Look for an upcoming article with more details on binocular projection.

The cast of characters

For us, the motel parking lot worked out great. We were staying at the Crystal Motel in Red Bluff, an unassuming little motel that proved to be clean and quiet, with friendly, helpful staff and the fastest motel wi-fi connection I've ever seen. Maybe not the most scenic of locations, but that was balanced by the convenience of having the car and room so close by.

And we were able to show the eclipse to locals and motel guests who wouldn't have been able to see it otherwise. Many of these people, living right in the eclipse path, didn't even know there was an eclipse happening, so poor had the media coverage been. (That was true in the bay area too -- most people I talked to last week didn't know there was an eclipse coming up, let alone how or where to view it.)

We showed the eclipse to quite a cast of characters --

In between visitors, we had plenty of time to fiddle with equipment, take photos, and take breaks sitting in the shade to cool down. (Annularity was pleasantly cool, but the rest of the eclipse stayed hot on an over 90 degree central valley day.)

There's a lot to be said for sidewalk astronomy! Overall, I'm glad we ended up where we did rather than in that Whiskeytown chaos.

Here's my collection of Images from the "Ring of Fire" Annular Eclipse, May 2012, from Red Bluff, CA.

Tags: , , ,
[ 11:42 May 22, 2012    More science/astro | permalink to this entry | comments ]

Wed, 16 May 2012

Ring of Fire: 2012 annular eclipse

[Solar annular eclipse of January 15, 2010 in Jinan, Republic of China, by A013231 on Wikimedia Commons.] This Sunday, May 20th, the western half of the US will be treated to an annular solar eclipse.

Annular means that the moon is a bit farther away than usual, so it won't completely cover the sun even if you travel to the eclipse centerline. Why? Well, the moon's orbit around the earth isn't perfectly circular, so sometimes it's farther away, sometimes nearer. Remember all the hype two weeks ago about the "supermoon", where it was unusually close at full moon? The other side of that is that during this eclipse, at new moon, the moon is unusually far away, and therefore a little smaller, not quite big enough to cover the sun.

Since the sun will never be totally covered, make sure you have a safe solar filter for this one -- don't look with your naked eyes! You want a solar filter anyway, if you have any kind of telescope or even binoculars, because of next month's once-in-a-lifetime Venus transit (I'll write about that separately). But if you don't have a solar filter and absolutely can't get one in time, read on -- I'll have some suggestions later even for people without any sort of optical aid.

But first, the path of the eclipse. Here in the bay area, we're just a bit south of the southern limit of the annular path, which passes just south of the town of Redway, through Covelo, just south of Willows, then just misses Yuba City and Auburn. If you want to be closer to the centerline, go camping at Lassen National Park or Lake Shasta, or head to Reno or Tahoe If you're inclined to travel, NASA has a great interactive 2012 eclipse map you can use to check out possible locations.

Even back in the bay area, we still get a darn good dinner show. The partial eclipse starts at 5:17 pm PDT, with maximum eclipse at 6:33. The sun will be 18 degrees above the horizon at that point, and 89% eclipsed. Compare that with 97% for a site right on the centerline -- remember, since this is an annular eclipse, no place sees 100% coverage. The partial eclipse ends at 7:40 -- still well before sunset, which isn't until 8:11.

Photographers, if you want a shot of an annular eclipse as the sun sets, you'll need to head east, to Albuquerque, NM or Lubbock, TX. A little before sunset, the centerline also crosses near a lot of great vacation spots like Bryce, Zion and Canyon de Chelly.

[eclipse viewed through leaves] I mentioned that even without a solar filter, there are ways of watching the eclipse. The simplest is with a pinhole. You don't need to use an actual pin -- the size and shape of the hole isn't critical, as you can see in this image of the sun through the leaves of a tree during a 2005 eclipse in Malta. If you don't have a leafy tree handy, you can even lace your fingers together and look at the shadow of your hands. This eclipse will be very low in the sky, continuing through sunset, so you may need to project its shadow onto a wall rather than the ground.

If you have some time to prepare, take a piece of cardboard and punch a few holes through it. Try different sizes -- an actual pinhole, a BBQ skewer, a 3-hole punch, maybe even bigger holes up to the size of a penny. You might also try using aluminum foil -- you can get very clean circular holes that way, which might give a crisper image. Here's a good page on eclipse pinhole projection. What works best? I don't remember! It's been a very long time since the last eclipse here! Do the experiment! I know I will be.

[Solar projection with a Dobsonian] If you do have a telescope or binoculars but couldn't get a solar filter in time, don't despair. Instead of looking through the eyepiece, you can project the sun's image onto a white screen or even the ground or a wall. Use a cheap, low-power eyepiece -- any eyepiece you use for solar projection will get very hot, and you don't want to risk ruining a fancy one.

Point the telescope at the sun -- it's easy to tell when it's lined up by watching the shadow of the telescope -- and rotate the eyepiece so that it's aimed at your screen, which can be as simple as a sheet of paper. Be careful where that eyepiece is aimed -- make sure no one can walk through the path or put their hand in the way, and if you have a finderscope, make sure it's covered. This solar projection method works with binoculars too, but you'll want to mount them on a tripod so you don't have to hold them the whole time.

Of course, another great way to watch the eclipse is with your local astronomy club. I expect every club in the bay area -- and there are a lot of them -- will have telescopes out to share the eclipse with the public. So check with your local club -- San Jose Astronomical Association, Peninsula Astronomical Society, San Francisco Sidewalk Astronomers, San Francisco Amateur Astronomers, or any of the others on the AANC's list of Amateur Astronomy Clubs in Northern California or the SF Chronicle's list of astronomy clubs.

This eclipse should be pretty cool -- and a great chance to test out your solar equipment before next month's Venus transit.

When I went to put the event on my wall calendar last month, I discovered the calendar already had an entry for May 20: it's the start of Bear Awareness Week. So if you head up to Lassen or Shasta to watch the eclipse, be sure to be aware of the bears! (Also, maybe I should get a calendar that's a little more in tune with the sky.)

Tags: , ,
[ 21:12 May 16, 2012    More science/astro | permalink to this entry | comments ]

Fri, 27 Apr 2012

Venus is at its brightest -- why? And how to calculate it

Venus has been a beautiful sight in the evening sky for months, but at the end of April it's reaching a brightness peak, magnitude -4.7.

By then, if you look at it in a telescope or even good binoculars, you'll see it has waned to a crescent. That's a bit non-obvious: when the moon is a crescent, it's a lot fainter than a full moon. So why is Venus brightest in its crescent phase?

It has to do with their orbits. The moon is always about the same distance away, about 385,000 km or 239,000 miles (I've owned cars with more miles than that!), though it varies a little, from 362,600 km at perigee to 405,400 km at apogee.

When we look at the full moon, not only are we seeing the whole Earth-facing surface illuminated, but the central part of that light is reflecting straight up off the moon's surface. When we look at a crescent moon, we're seeing light that's near the moon's sunrise or sunset point -- dimmer and more spread out than the concentrated light of noon -- and in addition we're seeing less of it.

Venus, in contrast, varies its distance from us immensely. We can't see Venus when it's "full", because it's on the other side of the sun from us and lost in the sun's glow. It'll next be there a year from now, in April of 2013. But if we could see it when it's full, Venus would be a distant 1.7 AU from us. An AU is an Astronomical Unit, the average distance of the earth from the sun or about 89 million miles, so Venus when it's full is about 170 million miles away. Its disk is a tiny 9.9 arcseconds (an arcsecond is 1/3600 of a degree) -- about the size of Mars this month.

In contrast, when we look at the crescent Venus around the end of this month, although we're only seeing about 28% of its surface illuminated, and that only with glancing twilight rays, it's much closer to us -- less than half an AU, or about 45 million miles -- and its disk extends a huge 37 arcseconds, bigger than Jupiter this month.

Of course, eventually, as Venus pulls between us and the sun, its crescent gets so slim that even its huge size can't compensate. So its peak brightness happens when those two curves cross, when the disk is somewhere around 27% illuminated, as happens at the end of this month and the beginning of May.

Exactly when? Good question. The RASC Handbook says Venus' "greatest illuminated extent" is on April 30, but PyEphem and XEphem say Venus is actually brighter from May 3-8 ... and when it emerges from the sun's glare and moves into the morning sky in June, it'll be slightly brighter still, peaking at magnitude -4.8 in the first week of July.)

Tracking Venus with PyEphem

When I started my Shallow Sky column this month, I saw the notice of Venus's maximum brightness and greatest illuminated extent in the RASC Handbook. But I wanted more details -- how much did its distance and size really change, when would the brightness peak again as it emerged from the sun's glare, when would it next be "full"?

PyEphem made it easy to calculate all this. Just create an ephem.Venus() object, calculate its values for any date of interest, then print out parameters like phase, mag, earth_distance and size. In just a few minutes of programming, I had a nice table of Venus data.

import ephem

venus = ephem.Venus()

print '%10s   %6s %6s %6s %6s' % ('date', '%', 'mag', 'dist', 'size')
def print_venus(when) :
    venus.compute(when)
    fmt = '%02d-%02d-%02d   %6.2f %6.2f %6.2f %6.2f'
    trip = when.triple()
    print fmt % (trip[0], trip[1], trip[2],
                 venus.phase, venus.mag, venus.earth_distance, venus.size)

# Loop from the beginning of 2012 through the middle of 2013:
d = ephem.date('2012')
end_date = ephem.date('2013/6/1')
while d < end_date :
    print_venus(d)
    # Add a week:
    d = ephem.date(d + ephem.hour * 24)

I've found PyEphem very handy for calculations like this -- and it's great to be able to double-check listings in other publications.

Tags: , ,
[ 14:44 Apr 27, 2012    More science/astro | permalink to this entry | comments ]

Thu, 29 Dec 2011

Plotting the Analemma

My SJAA planet-observing column for January is about the Analemma and the Equation of Time.

The analemma is that funny figure-eight you see on world globes in the middle of the Pacific Ocean. Its shape is the shape traced out by the sun in the sky, if you mark its position at precisely the same time of day over the course of an entire year.

The analemma has two components: the vertical component represents the sun's declination, how far north or south it is in our sky. The horizontal component represents the equation of time.

The equation of time describes how the sun moves relatively faster or slower at different times of year. It, too, has two components: it's the sum of two sine waves, one representing how the earth speeds up and slows down as it moves in its elliptical orbit, the other a function the tilt (or "obliquity") of the earth's axis compared to its orbital plane, the ecliptic.

[components of the Equation of time] The Wikipedia page for Equation of time includes a link to a lovely piece of R code by Thomas Steiner showing how the two components relate. It's labeled in German, but since the source is included, I was able to add English labels and use it for my article.

But if you look at photos of real analemmas in the sky, they're always tilted. Shouldn't they be vertical? Why are they tilted, and how does the tilt vary with location? To find out, I wanted a program to calculate the analemma.

Calculating analemmas in PyEphem

The very useful astronomy Python package PyEphem makes it easy to calculate the position of any astronomical object for a specific location. Install it with: easy_install pyephem for Python 2, or easy_install ephem for Python 3.

import ephem
observer = ephem.city('San Francisco')
sun = ephem.Sun()
sun.compute(observer)
print sun.alt, sun.az

The alt and az are the altitude and azimuth of the sun right now. They're printed as strings: 25:23:16.6 203:49:35.6 but they're actually type 'ephem.Angle', so float(sun.alt) will give you a number in radians that you can use for calculations.

Of course, you can specify any location, not just major cities. PyEphem doesn't know San Jose, so here's the approximate location of Houge Park where the San Jose Astronomical Association meets:

observer = ephem.Observer()
observer.name = "San Jose"
observer.lon = '-121:56.8'
observer.lat = '37:15.55'

You can also specify elevation, barometric pressure and other parameters.

So here's a simple analemma, calculating the sun's position at noon on the 15th of each month of 2011:

    for m in range(1, 13) :
        observer.date('2011/%d/15 12:00' % (m))
        sun.compute(observer)

I used a simple PyGTK window to plot sun.az and sun.alt, so once it was initialized, I drew the points like this:

    # Y scale is 45 degrees (PI/2), horizon to halfway to zenith:
    y = int(self.height - float(self.sun.alt) * self.height / math.pi)
    # So make X scale 45 degrees too, centered around due south.
    # Want az = PI to come out at x = width/2.
    x = int(float(self.sun.az) * self.width / math.pi / 2)
    # print self.sun.az, float(self.sun.az), float(self.sun.alt), x, y
    self.drawing_area.window.draw_arc(self.xgc, True, x, y, 4, 4, 0, 23040)

So now you just need to calculate the sun's position at the same time of day but different dates spread throughout the year.

[analemma in San Jose at noon clock time] And my 12-noon analemma came out almost vertical! Maybe the tilt I saw in analemma photos was just a function of taking the photo early in the morning or late in the afternoon? To find out, I calculated the analemma for 7:30am and 4:30pm, and sure enough, those were tilted.

But wait -- notice my noon analemma was almost vertical -- but it wasn't exactly vertical. Why was it skewed at all?

Time is always a problem

As always with astronomy programs, time zones turned out to be the hardest part of the project. I tried to add other locations to my program and immediately ran into a problem.

The ephem.Date class always uses UTC, and has no concept of converting to the observer's timezone. You can convert to the timezone of the person running the program with localtime, but that's not useful when you're trying to plot an analemma at local noon.

At first, I was only calculating analemmas for my own location. So I set time to '20:00', that being the UTC for my local noon. And I got the image at right. It's an analemma, all right, and it's almost vertical. Almost ... but not quite. What was up?

Well, I was calculating for 12 noon clock time -- but clock time isn't the same as mean solar time unless you're right in the middle of your time zone.

You can calculate what your real localtime is (regardless of what politicians say your time zone should be) by using your longitude rather than your official time zone:

    date = '2011/%d/12 12:00' % (m)
    adjtime = ephem.date(ephem.date(date) \
                    - float(self.observer.lon) * 12 / math.pi * ephem.hour)
    observer.date = adjtime

Maybe that needs a little explaining. I take the initial time string, like '2011/12/15 12:00', and convert it to an ephem.date. The number of hours I want to adjust is my longitude (in radians) times 12 divided by pi -- that's because if you go pi (180) degrees to the other side of the earth, you'll be 12 hours off. Finally, I have to multiply that by ephem.hour because ... um, because that's the way to add hours in PyEphem and they don't really document the internals of ephem.Date.

[analemma in San Jose at noon clock time] Set the observer date to this adjusted time before calculating your analemma, and you get the much more vertical figure you see here. This also explains why the morning and evening analemmas weren't symmetrical in the previous run.

This code is location independent, so now I can run my analemma program on a city name, or specify longitude and latitude.

PyEphem turned out to be a great tool for exploring analemmas. But to really understand analemma shapes, I had more exploring to do. I'll write about that, and post my complete analemma program, in the next article.

Tags: , , , , ,
[ 20:54 Dec 29, 2011    More science/astro | permalink to this entry | comments ]

Thu, 22 Dec 2011

Calculating the Solstice and shortest day

Today is the winter solstice -- the official beginning of winter.

The solstice is determined by the Earth's tilt on its axis, not anything to do with the shape of its orbit: the solstice is the point when the poles come closest to pointing toward or away from the sun. To us, standing on Earth, that means the winter solstice is the day when the sun's highest point in the sky is lowest.

You can calculate the exact time of the equinox using the handy Python package PyEphem. Install it with: easy_install pyephem for Python 2, or easy_install ephem for Python 3. Then ask it for the date of the next or previous equinox. You have to give it a starting date, so I'll pick a date in late summer that's nowhere near the solstice:

>>> ephem.next_solstice('2011/8/1')
2011/12/22 05:29:52
That agrees with my RASC Observer's Handbook: Dec 22, 5:30 UTC. (Whew!)

PyEphem gives all times in UTC, so, since I'm in California, I subtract 8 hours to find out that the solstice was actually last night at 9:30. If I'm lazy, I can get PyEphem to do the subtraction for me:

ephem.date(ephem.next_solstice('2011/8/1') - 8./24)
2011/12/21 21:29:52
I used 8./24 because PyEphem's dates are in decimal days, so in order to subtract 8 hours I have to convert that into a fraction of a 24-hour day. The decimal point after the 8 is to get Python to do the division in floating point, otherwise it'll do an integer division and subtract int(8/24) = 0.

The shortest day

The winter solstice also pretty much marks the shortest day of the year. But was the shortest day yesterday, or today? To check that, set up an "observer" at a specific place on Earth, since sunrise and sunset times vary depending on where you are. PyEphem doesn't know about San Jose, so I'll use San Francisco:

>>> import ephem
>>> observer = ephem.city("San Francisco")
>>> sun = ephem.Sun()
>>> for i in range(20,25) :
...   d = '2011/12/%i 20:00' % i
...   print d, (observer.next_setting(sun, d) - observer.previous_rising(sun, d)) * 24
2011/12/20 20:00 9.56007901422
2011/12/21 20:00 9.55920379754
2011/12/22 20:00 9.55932991847
2011/12/23 20:00 9.56045709446
2011/12/24 20:00 9.56258416496
I'm multiplying by 24 to get hours rather than decimal days.

So the shortest day, at least here in the bay area, was actually yesterday, 2011/12/21. Not too surprising, since the solstice wasn't that long after sunset yesterday.

If you look at the actual sunrise and sunset times, you'll find that the latest sunrise and earliest sunset don't correspond to the solstice or the shortest day. But that's all tied up with the equation of time and the analemma ... and I'll cover that in a separate article.

Tags: , , , ,
[ 11:28 Dec 22, 2011    More science/astro | permalink to this entry | comments ]

Tue, 07 Jun 2011

Make your own Saturn sketching template with GIMP

My SJAA Ephemeris planetary astronomy column for next month will discuss Saturn, among other topics, since Saturn is the main planet visible in the evening sky right now.

Saturn has some storms visible right now in the north polar and equatorial bands, and a great way to focus your attention to see more detail through a telescope, especially on subtle details like Saturnian storms, is to take pencil and paper and sketch what you see. I've recommended sketching in my column many times before, but don't talk about it on the blog very often.

When sketching Saturn, it helps to start with a template, so you can concentrate on the interesting details of the rings and bands rather than fussing over trying to get the exact width of the rings right. Saturn's tilt changes with time -- right now it's tilted at 8° to observers here on Earth -- so sometimes the rings are open wide, sometimes they're narrow, and sometimes (as last year) they're edge-on and invisible to us. That's a hassle to try to get right in a sketch, when you'd rather be focusing on the gaps in the rings and the pastel colors of the cloud bands on the planet.

ALPO, the Association of Lunar and Planetary Observers, makes templates for sketching Saturn; but I had trouble finding any online that showed a tilt appropriate for this month's Saturn. You can get observing materials by joining ALPO, but sheesh! you shouldn't have to join an organization just to get a simple sketching template. And I wanted one for my column. Besides, the ALPO templates fill in too much detail -- they don't really give you a chance to do your own ring sketch.

So here's an easy way to make a Saturn sketching template with GIMP.

[Saturn sketch template]

Start with an image

You can calculate the aspect ratio you need for the planet from the ring tilt, but why go to all that trouble? I started with an image of Saturn I got by running XEphem. Call up View->Saturn, then make the window as big as you can. Of course, you may substitute any planetarium program of your choosing, as long as it shows Saturn with the right ring tilt.

I used GIMP's screenshot facility to open this as an image: File->Create->Screenshot..., then Select a region to grab.

You can also use a recent photo of Saturn. The point here is to get something that's the right shape: it doesn't matter if it's beautiful or large.

Fix the rotation and size

You want the rings horizontal, if they're not already. Use GIMP's Free Rotate tool to do that. You can eyeball it to make it approximately right, or if you want to be more accurate, use the Measure tool (the icon looks like a drawing compass) to measure from one edge of the rings to the other and note the angle in the status bar at the bottom of the window. Then when you use Free Rotate, type in the number you measured.

You'll be printing this out on sketching paper, so if the original image is small, use Image->Scale to expand it. Remember, you won't be looking at this original image -- it's just for tracing -- so don't worry if the image comes out fuzzy after you scale it up. I made mine about 1000 pixels wide.

Make a white background layer

Layer->New Layer... to make a new layer; check "white" in the dialog. Then click the eyeball icon next to it in the Layers dialog to make it invisible. You'll want it later.

Outline the planet on its own layer

Layer->New Layer... to make a new layer; this time make it transparent, not white. I named mine "planet", since this is where I'll draw the ellipse for the planet. (Yes, Saturn is an ellipse, not a sphere. So is the Earth, for that matter, but Saturn is a lot less spherical than Earth is.)

Choose the ellipse selection tool and drag out a selection that matches the outer edges of the planet. Use the resize handles to adjust the selection until it fits as closely as you can manage.

In the Toolbox or the Brushes dialog, choose the smallest hard brush, "Circle (01)".

Then Edit->Stroke Selection.... Click "Stroke with a paint tool", and click Stroke.

Tip: You may notice my template ended up with very jaggy lines. That's a common artifact of GIMP's Stroke Selection. I'm not worried about it for a sketching template, but if the jaggies bother you, you can get a much smoother line by converting the selection to a path and stroking the path instead of the selection.

Preview your work so far

Go back to the Layers dialog and make that white layer visible again, so you can see the outline you just made. You may want to do Select->None and click on some tool other than ellipse select, so the selection outline disappears and you can see the line better.

If you're not happy with your planet outline, Edit->Undo and repeat with a different selection, a thicker line or whatever.

Outline the rings on their own layer

Repeat what you just did for the planet, this time for the rings. I recommend using a new layer for just the rings (you'll see why in the next step).

I outlined just the outside of the rings, so the sketch can show the ring thickness. ALPO's templates don't do this, but how much ring you can see can vary based on seeing conditions. If you want the inner edge of the ring on your template, add it now.

Erase the hidden parts of the ring and planet outlines

You can't see the rings where they go behind the planet, or the part of the planet hidden by the rings. And you don't want your template lines spoiling your sketch in those regions. So use GIMP's eraser tool and a large brush to erase the appropriate parts.

This is a little easier if you used separate layers for the rings and planet: you won't have to be as careful with the eraser. But it's not a big deal: this is a template, not a finished artwork, and you're going to be drawing over it anyway. So don't sweat it too much.

Optionally, make the lines fainter

I made the template lines fainter using the Opacity slider in the Layers dialog on the planet and ring layers. Of course, you can just draw in grey in the first place, but I like being able to decide afterward what color I want, or change it later.

Label the template

Trust me, you'll be really annoyed if you decide in 2026 that you want to make another Saturn sketch, find your old template but can't remember what ring tilt it's for. So use the Text tool to label either the current date or the approximate ring tilt. Or put that information in an image comment under Image->Image Properties..., or in the filename.

Save your template as XCF.gz, save a copy in some other format like jpg, png or gif, and you're ready to print templates on paper. Then go out and sketch Saturn!

Tags: , ,
[ 15:13 Jun 07, 2011    More science/astro | permalink to this entry | comments ]

Sun, 13 Mar 2011

Measuring Mars Methane -- Messy!

I write a monthly column for the San Jose Astronomical Association. Usually I don't reprint the columns here, but last month's column, Worlds of Controversy, discussed several recently controversial topics in planetary science.

One of the topics was the issue of methane on Mars -- or lack thereof. We've all read the articles about how the measurements of Mars methane points to possible signs of life, woohoo! But none of the articles cover the problems with those measurements, as described in a recent paper by Kevin Zahnle, Richard S. Freedmana and David C. Catling: Is there methane on Mars?

Lack of life on Mars isn't sexy, I guess; The Economist was the only mainstream publication covering Kevin's paper, in an excellent article, Methane on Mars: Now you don't...

Here's the short summary from my column last month:

I'm sure you've seen articles on Martian methane. Methane doesn't last long in the atmosphere -- only a few hundred years -- so if it's there, it's being replenished somehow. On Earth, one of the most common ways to produce methane is through biological processes. Life on Mars! Whoopee! So everyone wants to see methane on Mars, and it makes for great headlines.

The problem, according to Kevin, is that the Mars measurements show changes on a scale much shorter than hundreds of years: they fluctuate on a seasonal basis. That's tough to explain. Known atmospheric oxidation processes wouldn't get rid of methane fast enough, so you'd need to invent some even more exotic process -- perhaps methane-eating bacteria in the Martian soil? -- to account for the drops.

Worse, the measurements showing methane aren't very reliable. The evidence is spectroscopic: methane absorbs light at several fixed wavelengths, so you can measure methane by looking for its absorption lines.

But any Earth-based measurement of Martian methane has to cope with the fact that Earth's atmosphere has far more methane than Mars. How do you separate possible Mars methane absorption lines from Terran ones? There's one clever way: you can measure Mars at quadrature, when it's coming toward us or going away from us, and any methane spectral lines would be red- or blue-shifted compared to the Terran ones. But then the lines overlap with other absorption lines from Earth's atmosphere. It's very difficult to get a reliable measurement. Of course, a measurement from space would avoid those problems, so the spectrograph on the ESA Mars orbiter has been pressed into service. But there are questions about its accuracy.

The published evidence so far for Martian methane just isn't convincing, especially with those unlikely seasonal fluctuations. That doesn't mean there's no methane there; it means we need better data. The next Mars Rover, dubbed "Curiosity", will include a laser spectrometer which can give us much more accurate methane measurements. Curiosity is set to launch this fall and arrive at Mars in August of next year.

It gets worse: the kapton tape issue

But it gets worse. That Curiosity rover whose sensitive equipment is going to answer the question for us? Well, check out an article in Wired last week: Space Duct Tape Could Confuse Mars Rover.

It seems the materials used to build Curiosity, notably the kapton tape used in large quantities to hold the rover together ... emit methane! Andrew C. Schuerger, Christian Clausen and Daniel Britt, in Methane Evolution from UV-irradiated Spacecraft Materials under Simulated Martian Conditions: Implications for the Mars Science Laboratory (MSL) Mission (abstract), take a selection of materials used in the rover, plus bacteria that might be expected to contaminate it, and subject them to simulated Mars conditions. They conclude

... the large amount of kapton tape used on the MSL rover (lower bound estimated at 3 m2) is likely to create a significant source of terrestrial methane contamination during the early part of the mission.

A skeptical eye

So let's sum up:

* We desperately want to see methane on Mars, because it might point to biological processes and that would be cool.
* But we don't currently have any reliable way to measure Martian methane.
* So we build a special mission one of whose primary purposes is to get accurate measurements of Martian methane.
* But we build the probe with materials that will make the measurements unreliable.

It's apparently too late to fix the problem; so instead, just shrug and say, well, it might not be so bad if we measure at night, or if we wait a while (how long?) until most of the methane outgasses. The methane emission from the kapton tape is fairly small -- though it's hard to know exactly how small, since it's impossible to test it in a real Martian environment.

So in a couple of years, when you start seeing news releases trumpeting Curiosity's methane measurements and talking about life on Mars, read them with a skeptical eye.

Maybe Curiosity will see methane levels on Mars so large that they swamp any contamination issues. Maybe not. But we won't be able to tell from the reports we read in the popular press.

Tags: , ,
[ 12:38 Mar 13, 2011    More science/astro | permalink to this entry | comments ]

Sat, 06 Feb 2010

Making "Citizen Science" compelling

I had the opportunity to participate in a focus group on NASA's new "citizen science" project, called Moon Zoo, with a bunch of other fellow lunatics, amateur astronomers and lunar enthusiasts.

Moon Zoo sounds really interesting. Ordinary people will analyze high-resolution photos of the lunar surface: find out how many boulders and craters are there. I hope it will also include more details like crater type and size, rilles and so forth, though that wasn't mentioned. These are all tasks that are easy for a human and hard for a computer: perfect for crowdsourcing. Think Galaxy Zoo for the moon. The resulting data will be used for planning future lunar missions as well as for general lunar science.

It sounds like a great project and I'm excited about it. But I'm not going to write about Moon Zoo today -- it doesn't exist yet (current estimate is mid-March), though there is a preliminary PDF. Instead, I want to talk about some of the great ideas that came out of the focus group.

The primary question: How do we get people -- both amateur astronomers and the general public, people of all ages -- interested in contributing to a citizen science project like Moon Zoo?

Here are some of the key ideas:

Make the data public

This was the most important point, echoed by a lot of participants. Some people felt that many of the existing "citizen science" projects project the attitude "We want something from you, but we're not going to give you anything in return." If you use crowdsourcing to create a dataset, make it available to the crowd.

Opening the data has a lot of advantages:

Projects like Wikipedia and Open Street Map, as well as Linux and the rest of the open source movement, show how much an open data model can inspire contributions.

Give credit to individuals and teams

People cited the example of SETI@Home, where teams of contributors can compete to see who's contributed the most. Show rankings for both individuals and groups, so they can track their progress and maybe get a bit competitive with other groups. Highlight groups and individuals who contribute a lot -- maybe even make it a formal competition and offer inexpensive prizes like T-shirts or mugs.

A teenaged panel member had the great suggestion of making buttons that said "I'm a Moon Zookeeper." Little rewards like that don't cost much but can really motivate people.

Offer an offline version

They wanted to hear ideas for publicizing Moon Zoo to groups like our local astronomy clubs.

I mentioned that I've often wanted to spread the word about Galaxy Zoo, but it's entirely a web-based application and when I give talks to clubs or school groups, web access is never an option. (Ironically, the person leading the focus group had planned to demonstrate Galaxy Zoo to us but couldn't get connected to the wi-fi at the Lawrence Hall of Science.)

Projects are so much easier to evangelize if you can download an offline demo.

And not just a demo, either. There should be a way to download a real version, including a small data set. Imagine if you could grab a Moon Zoo pack and do a little classifying whenever you got a few spare minutes -- on the airplane or train, or in a hotel room while traveling.

Important note: this does not mean you should write a separate Windows app for people to download. Keep it HTML, Javascript and cross platform so everyone can run it. Then let people download a local copy of the same web app they run on your site.

Make sure it works on phones and game consoles

Lots of people use smartphones more than they use a desktop computer these days. Make sure the app runs on all the popular smartphones. And lots of kids have access to handheld web-enabled game consoles: you can reach a whole new set of kids by supporting these platforms.

Offer levels of accomplishment, like a game

Lots of people are competitive by nature, and like to feel they're getting better at what they're doing. Play to that: let users advance as they get more experienced, and give them the option of doing harder projects. "I'm up to level 7 in Moon Zoo!"

Use social networking

Facebook. Twitter. Nuff said.

Don't keep results a secret

Quite a few scientific publications have arisen out of Galaxy Zoo -- yet although most of us were familiar with Galaxy Zoo, few of us knew that. Why so secretive? They should be trumpeting achievements like that.

How many times have you volunteered for a survey or study, then wondered for years afterward how the results came out? Researchers never contact the volunteers when the paper is finally published. It's frustrating and demotivating; it makes you not want to volunteer again. Lots of us sign up because we're curious about the science -- but that means we're also curious about the results.

With citizen science projects, this is particularly easy. Set up a mailing list or forum (or both) to discuss results and announce when papers are published. Set up a Twitter account and a Facebook group to announce new papers to anyone who wants to follow. This is the age of Web 2.0, folks -- there's no excuse for not communicating.

I don't know if NASA will listen to our ideas. But I hope they do. Moon Zoo promises to be a terrific project ... and the more of these principles they follow, the more dedicated volunteers they'll get and that will make the project even better.

Tags: , , ,
[ 20:25 Feb 06, 2010    More science/astro | permalink to this entry | comments ]

Fri, 18 Sep 2009

A new theory of orbital dynamics

[PGE billboard: Solar Power: Making planets orbit and bagels toast] This PG&E billboard just went up down the street from where I live.
"Solar Power: Making planets orbit and bagels toast."

And here all this time I'd been under the impression that orbits had mostly to do with gravity. Somehow I'd missed the influence of light pressure when writing my orbital software.

Or is the sun's gravitational influence considered a part of "solar power"? Can we look forward to the upcoming generation of gravitovoltaic solar cells?

Tags: ,
[ 20:13 Sep 18, 2009    More humor | permalink to this entry | comments ]

Sun, 31 May 2009

JS Jup: now, with variable animation speed

I wrote last week about the sorts of programmer compulsions that lead to silly apps like my animated Javascript Jupiter. I got it working well enough and stopped, knowing there were more features that would be easy to add but trying to ignore them.

My mom, immediately upon seeing it, unerringly zeroed in on the biggest missing feature I'd been trying to ignore. "Can you make it go faster or slower?"

I put it off for a while, but of course I had to do it -- so now there are Faster and Slower buttons. It still goes by hour jumps, so the fastest you can go is an hour per millisecond. Fun to watch. Or you can slow it down to 1 hour per 3600000 milliseconds if you want to see it animate in real time. :-)

Tags: , ,
[ 11:42 May 31, 2009    More programming | permalink to this entry | comments ]

Sat, 23 May 2009

Javascript Jupiter

It's a sickness, I tell you.

It's not like I needed another Jupiter's moons application. I've already written more or less the same app for four platforms.

I don't use the Java web version, Juplet, very much any more, because I often have Java disabled or missing. And I don't use my Zaurus any more so Juplet for Zaurus isn't very relevant. But I can always call up my Xlib or PalmOS Jupiter's moons app if I need to check on those Galilean moons. They work fine. Another version would be really pointless. A waste of time.

So it should have been no big deal when, during the course of explaining to someone the difference between Java and Javascript, it suddenly occurred to me that it would be awfully easy to re-implement that Java Juplet web page using Javascript, HTML and CSS. I mean, a rational person would just say "oh, yeah, I suppose that's true" and go on with life.

But what I'm trying to say is that programming isn't a career path, or a hobby, or a field of academic study. It's a disease. It's a compulsion, where, sometimes, just realizing that something could be done renders you unable to think about anything else until you just ... try ... just a few minutes ... see how well it works ... oh, wow, that really looks a lot better than the Java version, wouldn't it look even nicer if you just added in this one other little tweak ... but wait, now it's so close to working, I bet it wouldn't be all that hard to take the Java class and turn it into ...

... and before you know it, it's tomorrow and you have something that's almost a working app, and it's just really a shame to get that far and not finish it at least to the point where you can share it.

But then, Javascript and web pages are so easy to work on that it really isn't that much extra work to add in some features that the old version didn't have, like an animate button ...

... and your Saturday morning is gone forever, and there's not much you can do about that, but at least you have a nice animated Jupiter's moons (and shadows) page when the sickness passes and you can finally think about other things.

Tags: , ,
[ 21:10 May 23, 2009    More programming | permalink to this entry | comments ]

Wed, 01 Apr 2009

Pluto Visits the States

This is a reprinting of an article I wrote for my monthly planet column in the SJAA Ephemeris:

Is Pluto a planet, or not? Maybe you caught the news last month that Illinois, birthplace of Clyde Tombaugh, has declared Pluto a planet. It joins New Mexico, Tombaugh's longtime home, which made a similar declaration two years ago.

When I first heard about the New Mexico resolution, I was told that they had declared that Pluto would be a planet within the state's boundaries. [Size of Pluto and Charon vs. the US] That made me a bit curious: would Pluto even fit inside New Mexico? I looked it up: Pluto has a diameter of 2300km, while New Mexico is about 550km in longitude and a bit more in latitude. Not even close (see Figure 1). Too bad -- I liked the image of Pluto and Charon coming to visit and hang out with friends. Though at Pluto's orbital velocity (it takes it just under 248 years to complete its 18 billion kilometer orbit, meaning an average speed of 23 million km/year or 63,000 km/day) and its current distance of about 32 AU (4.8 billion km), it whould take it about 207 years to get here.

But it turns out that's not what the resolution said anyway. Both states' resolutions said roughly the same thing:

BE IT RESOLVED BY THE LEGISLATURE OF THE STATE OF NEW MEXICO that, as Pluto passes overhead through New Mexico's excellent night skies, it be declared a planet and that March 13, 2007 be declared "Pluto Planet Day" at the legislature.

RESOLVED, BY THE SENATE OF THE NINETY-SIXTH GENERAL ASSEMBLY OF THE STATE OF ILLINOIS, that as Pluto passes overhead through Illinois' night skies, that it be reestablished with full planetary status, and that March 13, 2009 be declared "Pluto Day" in the State of Illinois in honor of the date its discovery was announced in 1930.

So the law applies to anyone (though it's probably not enforceable outside state boundaries) -- but only when Pluto is overhead in New Mexico or Illinois.

But wait -- does Pluto ever actually pass overhead in those states?

New Mexico stretches from 31.2 to about 37 degrees latitude, while Illinois spans 36.9 to 42.4. Right now Pluto is in Sagittarius, with a declination of -17° 41'; there's no way anyone in the US is going to see it directly overhead this year. Worse, it's on its way even farther south. It won't cross into the northern hemisphere until the beginning of 2111. But how far north will it go?

My first thought was to add Pluto's inclination -- 17.15 degrees, very high compared to other planets -- to the 23 degrees of the ecliptic to get 40.4°. Way far north -- no problem in either state! But unfortunately it's not as simple as that.

It turns out that when Pluto gets to its maximum north inclination, it's in Bootes (bet you didn't know Bootes was a constellation of the zodiac, did you? It's that 17° inclination that puts Pluto just past the Virgo border). That'll happen in February of 2228.

But in the Virgo/Bootes region, the ecliptic is 8° south of the equator, not 23° north. So we don't get to add 23 and 17; in fact, Pluto's declination will only be about 7.3° north. That's no help!

To find the time when Pluto gets as far north as it's going to get, you have to combine the declination of the ecliptic and the angle of Pluto above the ecliptic. The online JPL HORIZONS simulator is very helpful for running data like that over long periods -- much easier than plugging dates into a planetarium program. HORIZONS told me that Pluto's maximum northern declination, 23.5°, will happen in spring of 2193.

Unfortunately, 23.5° isn't far enough north to be overhead even from Las Cruces, NM. So Pluto, sadly, will never be overhead from either New Mexico or Illinois, and thus by the text of the two measures, it will never be a planet.

With that in mind, I'm asking you to support my campaign to persuade the governments of Ecuador and Hawaii to pass resolutions similar to the New Mexico and Illinois ones. Please give generously -- and hurry, because we need your support before April 1!

Tags: , , , ,
[ 20:09 Apr 01, 2009    More science/astro | permalink to this entry | comments ]

Tue, 24 Mar 2009

For Ada Lovelace Day: Vera Rubin

For Ada Lovelace Day I'm honoring Vera Rubin.

In 1948, when she applied to Princeton as an aspiring astronomy grad student, they wouldn't let her in because women weren't allowed. (They finally started admitting women in 1975.) Fortunately, Cornell was more accommodating.

For her thesis, she worked on a project that seemed useful and uncontroversial. She took other people's data on the redshifts of galaxies, and catalogued them to see how fast they were all moving away from us.

Except something unexpected happened. She found that galaxies in one direction weren't moving away as fast as galaxies in the other directions. The universe was supposed to be expanding evenly in all directions -- but that's not what her data showed.

In 1950 she presented her results to a conference of the American Astronomical Society. The results were not promising. Famous astronomers she'd read about but never met stood up in the audience to ridicule her paper and say it couldn't be true. No one would publish her master's thesis. It wasn't a good start to her career. She decided to try to find something less controversial to study.

Her husband finished at Cornell and moved to Washington, D.C.. Rubin and her new baby moved with him, and she enrolled as a PhD student at Georgetown. They had two children by now; her parents watched the kids while she took night classes.

She hooked up with George Gamow at Georgetown. He called her to ask her about her research -- but said they'd have to talk in the lobby, not in his office, because women weren't allowed in the office area of the building.

After Rubin finished her PhD with Gamow in 1954, Her experience trying to present her 1950 paper made her leery of confrontation. She's said, "I wanted a problem that no one would bother me about." Working with Kent Ford at the Carnegie Institute in Washington, she helped design a super-sensitive digital spectrograph, and they set out to make a huge catalog of data on boring "normal" galaxies no one else was looking at. They started with the Andromeda galaxy, M31, the closest large galaxy to us (and the easiest one to see with the naked eye, if you go somewhere away from city lights).

And right away they found something weird. Normally, you'd expect the outer parts of the galaxy to be rotating a lot slower than the inner parts. Think of our solar system: Mercury goes around the sun really fast (a Mercury year is only 88 days), Earth goes not quite as fast, and when you get all the way out to Pluto, it takes 247 years to go around the sun once. It's not just that it has farther to go to make a circuit around the sun; it's that the sun's influence is so weak way out there that Pluto goes a lot slower in its orbit than we do.

Galaxies should be the same way: stars in the center should just whiz around in no time, while stars at the outer edge take forever.

But Rubin and Ford found that Andromeda wasn't like that. When they started looking at the stars farther out, they were all going about the same speed. If anything, the stars at the edge were going a little faster than the stars in the center.

That made no sense. It didn't follow any normal model of gravity or galaxy formation. They published their results in 1970, but no one took them seriously. They decided that maybe something was wrong, or their equipment was faulty. They decided to try studying a simpler problem: just measure the redshift of some faint galaxies and make a catalog of those.

That went well for a while -- except that pretty soon, they ran into the same thing Rubin had discovered as a graduate student back at Cornell. Galaxies in the direction of Pegasus were moving away from us at a different speed from galaxies in other parts of the sky. She and Ford tried again to present that, but the reaction wasn't any more positive this time.

Discouraged, they went back to trying to measure galaxy rotation, hoping Andromeda had just been a fluke. But every galaxy they studied looked the same as Andromeda, with the stars far out near the edge of the galaxy rotating as fast, or faster, than the stars near the hub.

There were only two possible explanations. Either the law of gravity doesn't work the way we think it does ... or there's a lot more matter inside a galaxy than what we see with a telescope.

When they tried to present this result, no one believed it, so they kept measuring more galaxies, always with the same result.

By 1985, they had enough evidence that people finally started paying attention. As their results got talked about more and taken more seriously, they came up with a name for the extra mass that makes the galaxy rotation flat: "dark matter". Yes, the dark matter you hear about that apparently makes up more than 90% of all matter in the universe. Not a bad discovery for someone who was just trying to lay low and catalogue a lot of data that might be useful to other people! (Rubin's first graduate project, on the rotation of the universe, has also since been vindicated.)

Vera Rubin is still working at the Department of Terrestrial Magnetism. Her intellect, hard work and perseverance are an inspiration, and I salute her on Ada Lovelace Day. (You can read other people's Ada Lovelace Day posts in the Ada Lovelace Day Collection.)

Tags: , ,
[ 20:12 Mar 24, 2009    More science/astro | permalink to this entry | comments ]

Tue, 20 Jan 2009

LCA 2009 Tuesday

I missed a lot of the miniconf talks on Tuesday because I wanted to make some last-minute changes to my talk. But I do want to comment on one: Simon Greener's talk on "A Review of Australian Geodata Providers." Of course, I'm not in Australia, but it was quite interesting to hear how similar Australia's problematic geodata siguation is to the situation in the US. His presentation was entertaining, animated and I learned some interesting facts about GPS and geodata in general.

And Dave and I got another good astronomy opportunity with the dark skies at Peppermint Bay at the Speakers' Dinner. Despite occasional intrusive clouds we managed to get a great view of the Large Magellanic Cloud and a decent view of the small one, as well as eta Carinae and the star clouds between Crux and Carina. Pity I'd forgotten to bring my thumpin' travel optics that I'd been using the previous evening: a 6x20 monocular.

Tags: , , ,
[ 17:15 Jan 20, 2009    More conferences/lca2009 | permalink to this entry | comments ]

Mon, 19 Jan 2009

LCA 2009 Monday

On day one of LCA 2009, I divided my time between the LinuxChix and Kernel miniconfs.

In the morning, Paul McKenney, in "Why is parallel Programming Hard?", discussed some of the background of parallel programming research, then gave an entertaining demonstration of instruction overhead using a roll of toilet paper. Each square represented one clock cycle -- he estimated there were a few hundred clock cycles in the full roll -- and he had audience members unroll the roll carefully, passing it from one person to the next. It took a long time.

Over at the LinuxChix miniconf, Jacinta Richardson gave a wonderfully entertaining (and useful) talk "On Speaking". She explained how to hack audience members' brains, particularly the corpus callosum and the hippcampus, by using emotion, visual images and suspenseful stories to give your audience whole-brain entertainment.

After Jacinta's talk we spent some time going around the room introducing ourselves, and speakers got a chance to plug their upcoming talks.

I skipped the panel on Geek Parenting (not being a parent) to go back to the kernel miniconf's "Problem Solving Hour". Questions involved network performance, solid state disk performance, how to debug crashes, tracing (the moderator commented that if you're thinking of getting involved in the kernel effort but aren't quite sure what to do, there's a huge need for better tracing and performance analysis tools), solid-state disks (someone plugged the talk on that subject on Friday) and similar interesting topics.

I asked about an overheating problem I've been having with my laptop. I mentioned that even in single-user mode, the CPU temperature keeps going up, so I was pretty sure it was a kernel and not userspace issue. Matthew Garrett said that a lot of drivers are optimized for a normal use case -- meaning X -- and may work very poorly in text mode. You can have something that's overheating in single-user mode, then you start X and a bunch of power management systems kick in and the temperature actually goes down. So how do you figure out what's causing a temperature problem? Open up the laptop when it's hot, poke around then figure out what's hot. Then debug that component.

Lunch was a lovely BBQ provided by Google.

After lunch, Matthew Garrett, in "How I learned to stop worrying and love ACPI", was entertaining, as all his talks are. I'm not sure I actually learned much in the way of practical advice for helping ACPI work better on my machines, but at least I learned lots of new ways in which ACPI sucks more than I ever realized.

Then it was back to LinuxChix for a workshop on getting schoolgirls more interested in IT. We saw short presentations from the four workshop leaders, then split into groups -- our group went outside and sat in the hazy sunshine and talked about how to get girls, teachers, parents and school IT staff on board.

After tea, all the LinuxChix groups reported back on the discussions and there was a full-room discussion on how to get involved with educational programs like that. Then we ended with lightning talks; I got roped into giving one, so I didn't take notes on the rest, but they were all fun and interesting.

Then in the evening, after dinner, we found a spot somewhat sheltered from the lights of the hotel for some quick astronomy before bed. The sky was hazy and picking up lots of sky glow from a light beam shining from the hotel, but fortunately the sky around the Southern Cross was clear. We found both the Large and Small Magellanic clouds, as well as Eta Carina and some other clusters around the Southern Cross. A lovely view, unmatched by anything I saw from around Sydney or Melbourne. Tasmania definitely wins for stargazing!

Tags: , ,
[ 05:17 Jan 19, 2009    More conferences/lca2009 | permalink to this entry | comments ]

Sat, 29 Nov 2008

Upheaval Dome: New research confirms impact theory

Kurt Fisher wrote to draw my attention to the latest Lunar Photo Of the Day (LPOD), a lovely shot he made of one of my favorite places anywhere, Upheaval Dome in Utah's Canyonlands National Park.

Upheaval Dome has long been strongly suspected to be a massive, eroded impact crater, but the LPOD highlights a study that finally puts this (non-)controversy to rest, Elmar Buchner and Thomas Kenkmann's Upheaval Dome, Utah, USA: Impact origin confirmed, documenting shocked quartz grains in the Kayenta sandstone of Upheaval's outer ring.

[Upheaval Dome] It's about time -- it's been pretty clear for many years that this structure was an impact formation, not a collapsed salt dome (the relative lack of salt in the core might have been a clue) but the park service doesn't seem to have gotten the message, giving equal weight to the salt-dome theory in all its Canyonlands literature and signs. Perhaps the Buchner and Kenkmann paper will finally convince them.

Reading about this gave me the push I needed to update my own Upheaval Dome page, adding links to the latest research and to the excellent Upheaval Dome Bibliography Kurt has put together. My page also badly needed a bigger view of the crater itself, so I stitched together a quick panorama of the view from the rim that I'd shot on a trip several years ago but never assembled.

Tags: , , ,
[ 13:15 Nov 29, 2008    More science/geology | permalink to this entry | comments ]

Sun, 16 Nov 2008

Cleaning up the edges of Moonroot's transparent images

[moonroot] I wrote moonroot more to figure out how to do it than to run it myself. But on the new monitor I have so much screen real estate that I've started using it -- but the quality of the images was such an embarrassment that I couldn't stand it. So I took a few minutes and cleaned up the images and made a moonroot 0.6 release.

Turned out there was a trick I'd missed when I originally made the images, years ago. XPM apparently only allows 1-bit transparency. When I was editing the RGB image and removing the outside edge of the circle, some of the pixels ended up semi-transparent, and when I saved the file as .xpm, they ended up looking very different (much darker) from what I had edited.

Here are two ways to solve that in GIMP:

  1. Use the "Hard edge" option on the eraser tool (and a hard-edged brush, of course, not a fuzzy one).
  2. Convert the image to indexed, in which case GIMP will only allow one bit's worth of transparency. (That doesn't help for full-color images, but for a greyscale image like the moon, there's no loss of color since even RGB images can only have 8 bits per channel.)

Either way, the way to edit a transparent image where you're trying to make the edges look clean is to add a solid-color background layer (I usually use white, but of course it depends on how you're going to use the image) underneath the layer you're trying to edit. (In the layers dialog, click the New button, chose White for the new layer, click the down-arrow button to move it below the original layer, then click on the original layer so your editing will all happen there.)

Once you're editing a circle with sharp edges, you'll probably need to adjust the colors for some of the edge pixels too. Unfortunately the Smudge tool doesn't seem to work on indexed images, so you'll probably spend a lot of time alternating between the Color Picker and the Pencil tool, picking pixel colors then dabbing them onto other pixels. Key bindings are the best way to do that: o activates the Color Picker, N the Pencil, P the Paintbrush. Even if you don't normally use those shortcuts it's worth learning them for the duration of this sort of operation.

Or use the Clone tool, where the only keyboard shortcut you have to remember is Ctrl to pick a new source pixel. (I didn't think of that until I was already finished, but it works fine.)

Tags: , ,
[ 15:48 Nov 16, 2008    More gimp | permalink to this entry | comments ]

Mon, 20 Oct 2008

Requesting no window decorations (and moonroot 0.4)

Someone on #openbox this morning wanted help in bringing up a window without decorations -- no titlebar or window borders.

Afterward, Mikael commented that the app should really be coded not to have borders in the first place.

Me: You can do that?

Turns out it's not a standard ICCCM request, but one that mwm introduced, MWM_HINTS_DECORATIONS. Mikael pointed me to the urxvt source as an example of an app that uses it.

My own need was more modest: my little moonroot Xlib program that draws the moon at approximately its current phase. Since the code is a lot simpler than urxvt, perhaps the new version, moonroot 0.4, will be useful as an example for someone (it's also an example of how to use the X Shape extension for making non-rectangular windows).

Tags: , , ,
[ 12:06 Oct 20, 2008    More programming | permalink to this entry | comments ]

Mon, 22 Sep 2008

Linux Planet: Linux Astronomy part III: Stellarium and Celestia

Part III in the Linux Astronomy series on Linux Planet covers two 3-D apps, Stellarium and Celestia.

Writing this one was somewhat tricky because the current Ubuntu, "Hardy", has a bug in its Radeon handling and both these apps lock my machine up pretty quickly, so I went through a lot of reboot cycles getting the screenshots. (I found lots of bug reports and comments on the web, so I know it's not just me.) Fortunately I was able to test both apps and grab a few screenshots on Fedora 8 and Ubuntu "Feisty" without encountering crashes. (Ubuntu sure has been having a lot of trouble with their X support lately! I'm going to start keeping current Fedora and Suse installs around for times like this.)

Tags: , , , ,
[ 22:10 Sep 22, 2008    More writing | permalink to this entry | comments ]

Fri, 12 Sep 2008

Linux Planet: Linux Astronomy part II: XEphem

I have a new article on XEphem on Linux Planet, following up to the KStars article two weeks ago: Viewing the Night Sky with Linux, Part II: Visit the Planets With XEphem.

Tags: , ,
[ 11:50 Sep 12, 2008    More writing | permalink to this entry | comments ]

Thu, 28 Aug 2008

Writing for Linux Planet: Stargazing with KStars

I have an article on Linux Planet! The first of many, I hope. At least the first of a short series on Linux astronomy programs, starting with the one that's easiest to use: KStars. It's oriented toward binocular observing, with suggestions for good targets for beginners.

Viewing the Night Sky with Linux, Part I: KStars

Tags: , ,
[ 22:46 Aug 28, 2008    More writing | permalink to this entry | comments ]

Tue, 18 Mar 2008

Setting app name and class in Xlib

I was looking at Dave's little phase-of-the-moon Mac application, and got the urge to play with moonroot, the little xlib ditty I wrote several years ago to put a moon (showing the right phase) on the desktop.

I fired it up, and got the nice moon-shaped window ... but with a titlebar. I didn't want that! Figuring out how to get rid of the titlebar in openbox was easy, just

<application name="moonroot">
    <decor>no</decor>
    <desktop>all</desktop>
</application>
... but it didn't work! A poke with xwininfo showed the likely cause: instead of "moonroot", the window was listed as "Unnamed window". Whoops!

A little poking around revealed three different ways to set "name" for a window: XStoreName, XSetClassHint (which sets both class name and app name), and XSetWMName. Available online documentation on these functions was not very helpful in explaining the differences; fortunately someone hanging out on the openbox channel knew the difference (thanks, Crazy_Hopper). Thus:

I didn't see much in the way of example code for what an app ought to do with these, so I'll post mine here:

    char* appname;
    XClassHint* classHint;
[ ... ]
    if (argv && argc > 1)
        appname = basename(argv[0]);
    else
        appname = "moonroot";

    /* set the titlebar name */
    XStoreName(dpy, win, appname);

    /* set the name and class hints for the window manager to use */
    classHint = XAllocClassHint();
    if (classHint) {
        classHint->res_name = appname;
        classHint->res_class = "MoonRoot";
    }
    XSetClassHint(dpy, win, classHint);
    XFree(classHint);

And if anyone is interested in my silly moon program, it's at moonroot-0.3.tar.gz. moonroot gives you a large moon, moonroot -s gives a smaller one. I'm not terribly happy with its accuracy and wasted too much time today fiddling with it and verifying that it's doing the right time conversions. All I can figure is that the approximation in Meeus' Astronomical Algorithms is way too approximate (it's sometimes off by more than a day) and I should just rewrite all my moon programs to calculate moon phase the hard (and slow) way.

Tags: , ,
[ 21:15 Mar 18, 2008    More programming | permalink to this entry | comments ]

Sun, 28 Oct 2007

Bright naked-eye comet: 17/P Holmes

I finally got a chance to take a look at Comet 17/P Holmes. I'd been hearing about this bright comet for a couple of days, since it unexpectedly broke up and flared from about 17th magnitude (fainter than most amateur telescopes can pick up even in dark skies) to 2nd magnitude (easily visible to the naked eye from light-polluted cities). It's in Perseus, so only visible from the northern hemisphere, pretty much any time after dark (but it's higher a little later in the evening).

And it's just as bright as advertised. I grabbed my binoculars, used a finder chart posted by one of our local SJAA members, and there it was, bright and obviously fuzzy. Without the binoculars it was still easy to see, and still noticably fuzzy.

So I dragged out the trusty 6" dobsonian, and although it has no visible tail, it has lots of structure. It looked like this:
[Comet 17/P Holmes] It has a stellar nucleus, a bright inner area (the coma?) and a much larger, fainter outer halo. There's also a faint star just outside the coma, so it'll be fun (if we continue to get holes in the clouds) to see how fast it moves relative to that star. (Not much motion in the past hour.)

It's nice to have a bright comet in the sky again! Anyone interested in astronomy should check this one out in the next few days -- since it may be in the process of breaking up, there's no telling how long it'll last or what will happen next. Grab some binoculars, or a 'scope if you have one, and take a look.

Tags: ,
[ 22:51 Oct 28, 2007    More science/astro | permalink to this entry | comments ]

Wed, 08 Nov 2006

Mercury Transits and Titan Occultations

Mercury transited the sun today. The weather forecast predicted rain, and indeed, I awoke this morning to a thick overcast which soon turned to drizzle. But miraculously, ten minutes before the start of the transit the sky cleared, and we were able to see the whole thing, all five hours of it (well, we weren't watching for the whole five hours -- the most interesting parts are the beginning and end).

I had plenty of practice with solar observing yesterday, showing the sun to a group of middle school girls as part of an astronomy workshop. This is organized by the AAUW, the same group that runs the annual Tech Trek summer science girls' camps. (The Stanford Tech Trek has a star party, which is how I got involved with this group.) It's the second year I've done the astronomy workshop for them; this year went pretty smoothly and everybody seemed to have a good time observing the sun, simulating moon phases, learning about the Doppler effect and plotting relative distances of the planets on a road map.

But what I really wanted to write about was the amazing video shown by last weekend's SJAA speaker, Dr. Ivan Linscott of Stanford. As one of the team members on the New Horizons mission to Pluto, he was telling us about Pluto's tenuous atmosphere. There isn't a lot of information on Pluto's atmosphere yet, but one of the goals of New Horizons is to take readings as Pluto occults the sun to see how sunlight is refracted through Pluto's atmosphere.

But that's no problem: it turns out we've already done more challenging occultation studies than that. Back in December 2001, Titan occulted a binary star, and researchers using Palomar's Adaptive Optics setup got a spectacular video of the stars being refracted through Titan's atmosphere as the occultation progresses.

This is old news, of course, but most of us hadn't seen it before and everyone was blown away. Remember, this is a video from Earth, of the atmosphere of a moon of Saturn, something most Earth-based telescopes would have trouble even resolving as a disk. Watch the Titan occultation video here.

Tags: ,
[ 23:38 Nov 08, 2006    More science/astro | permalink to this entry | comments ]

Fri, 25 Aug 2006

Pluto is too a planet

The BBC had a good article today about the International Astronomical Union vote that demoted Pluto from planet status.

It was fairly obvious that the previous proposal, last week, that defined "planet" as anything big enough that its gravity made it round, was obviously a red herring that nobody was going to take very serious. Fercryinoutloud, it made the asteroid Ceres a planet, as well as Earth's moon (in a few billion years when it gets a bit farther away from us and ceases to be considered a moon).

But apparently there were several other dirty tricks played by the anti-Pluto faction, and IAU members who weren't able to be in the room at the time of the vote are not happy and are spoiling for a rematch. The new definition doesn't make much more sense than the previous one, anyway: it's based on gravitationally sweeping out objects from an orbit, but that also rules out Earth, Mars, Jupiter and Neptune, all of which have non-satellite objects along their orbits.

And of course the public is pretty upset about it for sentimental, non-scientific reasons. Try searching for Pluto or "Save Pluto" on Cafe Press to see the amazing selection of pro-Pluto merchandise you can buy barely a day after the IAU decision. (Personally, I want a Honk if Pluto is still a planet bumper sticker.)

It'll be interesting to see if the decision sticks.

So do I have a viable definition of "planet" which includes Pluto but not Ceres or the various other Kuiper belt objects which are continually being discovered?

Why, no, I don't. But the discussion is purely semantic anyway. Whether we call Pluto a planet doesn't make any difference to planetary science. But it does make a difference to an enormous collection of textbooks, museum exhibits, and other science-for-the-public displays.

Pluto is big enough to have been discovered in 1930, back in the days before computerized robotic telescopes and satellite imaging; it's been considered a planet for 76 years. There's no scientific benefit to changing that, and a lot of social and political reason not to -- especially now with New Horizons headed there to give us our first up-close look at what Pluto actually looks like.

There are two possible bright notes to the Pluto decision. First, Mark Taylor pointed out that it has become much easier to observe all the planets in one night, even with a very small telescope or binoculars.

And second, maybe Christine Lavin will make a new updated version of her song Planet X and go on tour with it.

Tags: ,
[ 22:56 Aug 25, 2006    More science/astro | permalink to this entry | comments ]

Sun, 10 Jul 2005

Dark Side of the Moon; M51 Supernova

Yesterday was the annual Fremont Peak Star-b-q. This year the weather managed to be fairly perfect for observing afterward: the fog came in for a while, making for fairly dark skies, and it wasn't too cold though it was a bit breezy. It was even reasonably steady.

I had my homebuilt 8" dob, while Dave brought his homebuilt 12.5". Incredibly, we were all alone in the southwest lot: the most Star-b-q was fairly lightly attended, and most of the handful who stayed to observe afterward set up at Coulter row.

The interesting sight of the evening was the supernova in M51 (the Whirlpool galaxy). It was fairly easy in the 12.5" once we knew where to look (Mike Koop came over to visit after looking at it in the 30"), and once we found it there all three of us could see it in the 8" as well.

We had excellent views of Jupiter in the 8", with detail in the red spot, the thin equatorial band easily visible, and long splits in both the northern and southern equatorial bands. I didn't make any sketches since a family wandered by about then so I let them look instead.

We also had lovely low-power views of Venus and crescent Mercury, and we spent some time traversing detail on the dark side of the slim crescent moon due to the excellent earthshine. All the major maria were visible, and of course Aristarchus, but we could also see Plato, Sinus Iridum, Kepler, Copernicus and its ray system, Tycho (only in the 12" -- the 8" was having glare problems that close to the lit part of the moon) and one long ray from Tycho that extended across Mare Nubium and out to near Copernicus. Pretty good for observing the "dark" side!

Neither of us was able to find Comet Tempel-1 (the Deep Impact comet), even with the 12.5". But after moonset I picked up the Veil and North American in the 8" unfiltered (having left my filters at home), and we got some outstanding views of the nebulae in Sagittarius, particularly the Trifid, which was showing more dust-lane detail without a filter than I've ever seen even filtered.

It was a good night for carnivores, too. We saw one little grey fox cub trotting up the road to the observatory during dinner, and there was another by the side of the road on the way home. Then, farther down the road, I had to stop for three baby raccoons playing in the street. (Very cute!) They eventually got the idea that maybe they should get off the road and watch from the shoulder. The parents were nowhere to be seen: probably much more car-wise than their children (I don't often see raccoon roadkill). I hope the kids got a scolding afterward about finding safer places to play.

Tags: ,
[ 23:31 Jul 10, 2005    More science/astro | permalink to this entry | comments ]

Fri, 17 Jun 2005

The Game of Telephone

Remember the game of "Telephone" when you were a kid? Everybody gets in a big circle. One kid whispers a message in the ear of the kid next to them. That kid repeats the message to the next kid, and so on around the circle. By the time the message gets back to the originator, it has usually changed beyond recognition.

Sometimes the Internet is like that.

Background: a year and a half ago, in August 2003, there was an unusually favorable Mars opposition. Mars has a year roughly double ours, so Mars "oppositions" happen about every two years (plus a few months). An opposition is when we and Mars are both on the same side of the sun (so the sun is opposite Mars in our sky, and Mars is at its highest at midnight). We're much closer to Mars at opposition than at other times, and that makes a big difference on a planet as small as Mars, so for people who like to observe Mars with a telescope, oppositions are the best time to do it.

The August 2003 opposition was the closest opposition in thousands of years, because Mars was near its perihelion (the point where it's closest to earth) at the time of the opposition. Much was made of this in the press (the press loves events where they can say "best in 10,000 years") to the point where lots of people who aren't normally interested in astronomy decided they wanted to see Mars and came to star parties to look through telescopes.

That's always nice, and we tried to show them Mars, though Mars is very small, even during an opposition. The 2003 opposition wasn't actually all that favorable for those of us in northern hemisphere. because Mars was near the southernmost part of its orbit. That means it was very low in the sky, which is never good for seeing detail through a telescope. Down near the horizon you're looking through a lot more of Earth's atmosphere, and you're down near all the heat waves coming off houses and streets and even rocks. That disturbs the view quite a bit, like trying to see detail on a penny at the bottom of a swimming pool.

This year's opposition, around Halloween, will not be as close as the 2003 opposition, but it's still fairly close as oppositions go. Plus, this year, Mars will be much farther north. So we're expecting a good opposition -- weather permitting, both on Earth, which is sometimes cloudy in November, and on Mars, where you never know when a freak dust storm might appear.

Which brings me back to the game of Telephone.

A few weeks ago I got the first of them. An email from someone quoting a message someone had forwarded, asking whether it was true. The message began:

The Red Planet is about to be spectacular! This month and next, Earth is catching up with Mars in an encounter that will culminate in the closest approach between the two planets in recorded history.
and it ended:
Share this with your children and grandchildren. NO ONE ALIVE TODAY WILL EVER SEE THIS AGAIN
(sic on the caps and the lack of a period at the end).

I sent a reply saying the email was two years out of date, and giving information on this year's Mars opposition and the fact that it may actually be better for observing Mars than 2003 was. But the next day I got a similar inquiry from someone else. So I updated my Mars FAQ to mention the misleading internet message, and the inquiries slowed down.

But today, I got a new variant.

Subject: IS MARS GOING TO BE AS BIG AS THE MOON IN AUGUST?

As big as the moon! That would be a very close opposition!

(Dave, always succinct, said I should reply and say simply, "Bigger." Mars is, of course, always bigger than the moon, even if its apparent size as viewed from earth is small.)

It looks like the story is growing in the telling, in a way it somehow didn't two years ago.

I can't wait to see what the story will have become by August. Mars is going to hit us?

Tags: ,
[ 11:48 Jun 17, 2005    More science/astro | permalink to this entry | comments ]

Mon, 17 Jan 2005

Open Source Scientific Image Processing

Anthony Liekens has a wonderful page on open-source Cassini-Huygens image analysis.

A group of people from a space IRC channel took the raw images from the descent of the Huygens probe onto Titan's surface, and applied image processing: they stitched panoramas, created animations, created stereograms, added sharpening and color. The results are very impressive!

I hope NASA takes notice of this. There's a lot of interest, energy and talent in the community, which could be very helpful in analysis of astronomical data. Astronomy has a long history of amateur involvement in scientific research, perhaps more so than any other science; extending that to space-based research seems only a small step.

Tags: ,
[ 19:30 Jan 17, 2005    More science/astro | permalink to this entry | comments ]

Sun, 18 Jul 2004

FPOA Star-b-q

Hiking up to the top of Fremont Peak before the FPOA Star-b-q started, we saw the Ghost and the Darkness, squirrel style. A couple of ground squirrels hidden in the tall grass startled as we walked by, and whisked off through the grass, occasionally twitching a tail-tip up above the tops of the grasses but otherwise mostly invisible.

Down in the parking lots, there were some interesting ant or wasp-like insects: furry scarlet head, black thorax, furry scarlet abdomen. The wings were black, too, and they could fly at least a little. No idea what they were.

Learned a new word reading scoops on the way down: Anecdotage, that advanced age where all one does is relate stories about "the good, old days."

Turned out Jeff Moore was the speaker at FPOA. He always gives good talks, but this one was especially good: interpretation of the Mars Rover geologic results so far. Some of his slides showed terrestrial scenes (mostly Death Valley) for comparison with the Martian geologic features, and he mentioned that the terrestrial slides were easy to tell because they were the ones with the pocketknife showing (for scale). So the following morning, I got inspired to whip up a few counterexamples.

Tags: ,
[ 10:00 Jul 18, 2004    More science/astro | permalink to this entry | comments ]

Syndicated on:
LinuxChix Live
Ubuntu Women
Women in Free Software
Graphics Planet
DevChix
Ubuntu California
Planet Openbox
Devchix
Planet LCA2009

Friends' Blogs:
Morris "Mojo" Jones
Jane Houston Jones
Dan Heller
Long Live the Village Green
Ups & Downs
DailyBBG

Other Blogs of Interest:
DevChix
Scott Adams
Dave Barry
BoingBoing

Powered by PyBlosxom.