Shallow Thoughts : : science

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

Sat, 20 Jun 2020

Solstice Sun Dagger

Today is the summer solstice. Happy solstice!

[Solstice sun dagger] When I was in grade school -- probably some time around 7th grade -- I happened upon an article in Scientific American about the Anasazi Sun Dagger on Fajada Butte in Chaco Canyon. On the solstices and equinoxes, a thin dagger of light is positioned just right so that it moves across a spiral that's carved into the rock.

I was captivated. What an amazing sight it must be, I thought. I wondered if ordinary people were allowed to go see it.

Well, by the time I was old enough to do my own traveling, the answer was pretty much no. Too many people were visiting Fajada Butte ...

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[ 17:35 Jun 20, 2020    More science/astro | permalink to this entry | comments ]

Mon, 06 Apr 2020

F is for Food Waste

I keep seeing people claim that 40% of consumer food in the US is thrown away uneaten, or hear statistics like 20 pounds of wasted food per person per month.

I simply don't believe it.

There's no question that some food is wasted. It's hard to avoid having that big watermelon go bad before you have a chance to finish it all, especially when you're a one- or two-person household and the market won't sell you a quarter pound of cherries or half a pound of ground beef. And then there's all the stuff you don't want to eat: the bones, the fat, the banana peels and apple cores, the artichoke leaves and corn cobs.

But even if you count all that ... 40 percent? 2/3 of a pound per day per person? And that's supposed to be an average -- so if Dave and I are throwing out a few ounces, somebody else would have to be throwing out multiple pounds a day. It just doesn't seem possible. Who would do that?

When you see people quoting a surprising number -- especially when you see the same big number quoted by lots of people -- you should always ask yourself the source of the number.

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[ 20:08 Apr 06, 2020    More science | permalink to this entry | comments ]

Sun, 01 Mar 2020

Plotting Epicycles

Galen Gisler, our master of Planetarium Tricks, presented something strange and cool in his planetarium show last Friday.

[inner planet orbits from north ecliptic pole, with Venus pentagram] He'd been looking for a way to visualize the "Venus Pentagram", a regularity where Venus' inferior conjunctions -- the point where Venus is approximately between Earth and the Sun -- follow a cycle of five. If you plot the conjunction positions, you'll see a pentagram, and the sixth conjunction will be almost (but not quite) in the same place where the first one was. Supposedly many ancient civilizations supposedly knew about this pattern, though as Galen noted (and I'd also noticed when researching my Stonehenge talk), the evidence is sometimes spotty.

Galen's latest trick: he moved the planetarium's observer location up above the Earth's north ecliptic pole. Then he told the planetarium to looked back at the Earth and lock the observer's position so it moves along with the Earth; then he let the planets move in fast-forward, leaving trails so their motions were plotted.

The result was fascinating to watch. You could see the Venus pentagram easily as it made its five loops toward Earth, and the loops of all the other planets as their distance from Earth changed over the course of both Earth's orbits and theirs.

You can see the patterns they make at right, with the Venus pentagram marked (click on the image for a larger version). Venus' orbit is white, Mercury is yellow, Mars is red. If you're wondering why Venus' orbit seems to go inside Mercury's, remember: this is a geocentric model, so it's plotting distance from Earth, and Venus gets both closer to and farther from Earth than Mercury does.

He said he'd shown this to the high school astronomy club and their reaction was, "My, this is complicated." Indeed. It gives insight into what a difficult problem geocentric astronomers had in trying to model planetary motion, with their epicycles and other corrections.

Of course that made me want one of my own. It's neat to watch it in the planetarium, but you can't do that every day.

So: Python, Gtk/Cairo, and PyEphem. It's pretty simple, really. The goal is to plot planet positions as viewed from high above the north ecliptic pole: so for each time step, for each planet, compute its right ascension and distance (declination doesn't matter) and convert that to rectangular coordinates. Then draw a colored line from the planet's last X, Y position to the new one. Save all the coordinates in case the window needs to redraw.

[planet orbits from north ecliptic pole] At first I tried using Skyfield, the Python library which is supposed to replace PyEphem (written by the same author). But Skyfield, while it's probably more accurate, is much harder to use than PyEphem. It uses SPICE kernels (my blog post on SPICE, some SPICE examples and notes), which means there's no clear documentation or list of which kernels cover what. I tried the kernels mentioned in the Skyfield documentation, and after running for a while the program died with an error saying its model for Jupiter in the de421.bsp kernel wasn't good beyond 2471184.5 (October 9 2053).

Rather than spend half a day searching for other SPICE kernels, I gave up on Skyfield and rewrote the program to use PyEphem, which worked beautifully and amazed me with how much faster it was: I had to rewrite my GTK code to use a timer just to slow it down to where I could see the orbits as they developed!

It's fun to watch; maybe not quite as spacey as Galen's full-dome view in the planetarium, but a lot more convenient. You need Python 3, PyEphem and the usual GTK3 introspection modules; on Debian-based systems I think the python3-gi-cairo package will pull in most of them as dependencies.

Plot your own epicycles: epicycles.py.

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[ 13:04 Mar 01, 2020    More science/astro | permalink to this entry | comments ]

Tue, 10 Dec 2019

Planetarium Show Friday: Hitchhiker's Guide to the Moon

[Schroter's Valley on the Moon] This Friday, Dave and I will be presenting a planetarium show called The Hitchhiker's Guide to the Moon: Visit the Moon Without Leaving Your Home Planet.

I'm jazzed about this show. I think it'll be the most fun planetarium show we've given so far. We'll be showing a variety of lunarfeatures: maria, craters, mountains, rilles, domes, catenae and more. For each one, we'll discuss what the feature actually is and how it was created, where to see good examples on the moon, and -- the important part -- where you can go on Earth, and specifically in the Western US, to see a similar feature up close.

Plus: a short flyover of some of the major features using the full-dome planetarium. Some features, like Tycho, the Straight Wall, Reiner Gamma, plus lots of rilles, look really great in the planetarium.

If you can't get to the moon yourself, this is the next best thing!

The Hitchhiker's Guide to the Moon: 7pm at the PEEC nature center. Admission is free. Come find out how to explore the moon without leaving your home planet!

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[ 18:06 Dec 10, 2019    More science/astro | permalink to this entry | comments ]

Mon, 11 Nov 2019

Mercury Transit: Comparing Between H-alpha and White Light

[Mercury transit 11/11/2019] The Mercury transit is over. But we learned some interesting things.

I'd seen Mercury transits before, but this is the first time we had an H-alpha scope (a little 50mm Coronado PST) in addition to a white light filter (I had my 102mm refractor set up with the Orion white-light filter).

As egress approached, Dave was viewing in the H-alpha while I was on the white light scope. When I saw the black-drop effect at third contact, Mercury was still nowhere near the edge in the H-alpha: the H-alpha shows more of the solar atmosphere so the sun's image is noticably bigger. This was the point when we realized that we should have expected this and been timing and recording. Alas, it was too late.

Mercury was roughly 60% out in the white light filter -- just past the point where the "bite" it made in the limb of the sun -- by the time Dave called out third contact. We guessed it was roughly a minute, but that could be way off.

For fourth contact, Dave counted roughly 45 seconds between when I couldn't see Mercury any more and when he lost track of it. This is pretty rough, because it was windy, seeing was terrible and there was at least a 15-second slop when I wasn't sure if I could any indentation in the limb; I'm sure it was at least as hard in the Coronado, which was running at much lower magnification.

So we had a chance to do interesting science and we flubbed it. And the next chance isn't til 2032; who knows if we'll still be actively observing then.

I wanted to at least correlate those two numbers: 45 seconds and 60% of a Mercury radius.

Mercury is about 10" (arcseconds) right now. That was easy to find. But how fast does it move? I couldn't find anything about that, searching for terms like mercury transit angular speed OR velocity. I tried to calculate it with PyEphem but got a number that was orders of magnitude off. Maybe I'll figure it out for a later article, but I wanted to get this posted quickly.

[Mercury transit 11/11/2019 in H-alpha] I didn't spend much time trying photography. I got a couple afocal snaps with my pocket digital camera through the white-light scope that worked out pretty well. I wasn't sure that would work for the Coronado: the image is fairly dim. The snaps I did get show Mercury, though none of the interesting detail like faculae and the one tiny prominence that was visible. But the interesting thing is the color. To the eye, the H-alpha scope image is a slightly orangy red, but in the digital camera it came out a startling purplish pink. This may be due to the digital camera's filters passing some IR, confusing the algorithms that decide how to shift the color. Of course, I could have adjusted the color in GIMP back to the real color, but I thought it was more interesting to leave it the hue it came out of the camera. (I did boost contrast and run an unsharp mask filter, to make it easier to see Mercury.)

Anyway, fun and unexpectedly edifying! I wish we had another transit happening sooner than 2032.

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[ 12:15 Nov 11, 2019    More science/astro | permalink to this entry | comments ]

Fri, 08 Nov 2019

Mercury Transit Next Monday

[Mercury Transit 2006, photo by Brocken Inaglory]
Mercury Transit 2006, photo by Brocken Inaglory
Next Monday, November 11, is a transit of Mercury across the sun.

Mercury transits aren't super rare -- not once- or twice-in-a-lifetime events like Venus transits -- but they're not that common, either. The last Mercury transit was in 2016; the next one won't happen til 2032.

This year's transit isn't ideal for US observers. The transit will already be well underway by the time the sun rises, at least in the western US. Here in New Mexico (Mountain time), the sun rises with Mercury transiting, and the transit lasts until 11:04 MST. Everybody else, check timeanddate's Mercury Transit page for your local times.

Mercury is small, unfortunately, so it's not an easy thing to see without magnification. Of course, you know that you should never look at the sun without an adequate filter. But even if you have safe "eclipse glasses", it may be tough to spot Mercury's small disk against the surface of the sun.

[binocular projection of a solar eclipse] One option is to take some binoculars and use them to project an image. Point the big end of the binoculars at the sun, and the small end at a white surface, preferably leaning so it's perpendicular to the sun. I don't know if binocular projection will give a big enough image to show Mercury, so a very smooth and white background, tilted so it's perpendicular to the sun, will help. (Don't be tempted to stick eclipse glasses in front of a binocular or telescope and look through the eyepiece! Stick to projection unless you have filters specifically intended for telescopes or binoculars.)

Of course, a telescope with a safe solar filter is the best way to see a transit. If you're in the Los Alamos area, I hear the Pajarito Astronomers are planning to set up telescopes at Overlook Park. They don't seem to have announced it in any of the papers yet, but I see it listed on the Pajarito Astronomers website. There's also an event planned at the high school where the students will be trying to time Mercury's passage, but I don't know if that's open to the public. Elsewhere in the world, check with your local astronomy club for Mercury transit parties: I'm sure most clubs have something planned.

I was discussing the transit with a couple of local astronomers earlier this week, and one of them related it to the search for exoplanets. One of the main methods of detecting exoplanets is to measure the dimming of a star's light as a planet crosses its face. For instance, in 55 Cancri e, you can see a dimming as the planet crosses the star's face, and a much more subtle dimming when the planet disappears behind the star. As Mercury crosses the Sun's face, it blocks some of the sun's light in the same way. By how much?

The radius of Mercury is 0.0035068 solar radii, and the dimming is proportional to area so it should be 0.00350682, or 0.0000123, a 0.00123% dimming. Not very much!

But it looks like in the 55 Cancri e case, they're detecting dips of around .001% -- it seems amazing that you could detect a planet as small as Mercury this way (and certainly the planet is much bigger in the case of 55 Cancri e) ... but maybe it's possible.

Anyway, it's fun to think about exoplanets as you watch tiny Mercury make its way across the face of the Sun. Wherever you are, I hope you get a chance to look!

Update: A report from the transit: Mercury Transit: Comparing Between H-alpha and White Light.

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[ 11:36 Nov 08, 2019    More science/astro | permalink to this entry | comments ]

Thu, 13 Jun 2019

Finding Astronomical Alignments in Ancient Monuments (or anywhere else)

Dave and I will be presenting a free program on Stonehenge at the Los Alamos Nature Center tomorrow, June 14.

The nature center has a list of programs people have asked for, and Stonehenge came up as a topic in our quarterly meeting half a year ago. Remembering my seventh grade fascination with Stonehenge and its astronomical alignments -- I discovered Stonehenge Decoded at the local library, and built a desktop model showing the stones and their alignments -- I volunteered. But after some further reading, I realized that not all of those alignments are all they're cracked up to be and that there might not be much of astronomical interest to talk about, and I un-volunteered.

But after thinking about it for a bit, I realized that "not all they're cracked up to be" makes an interesting topic in itself. So in the next round of planning, I re-volunteered; the result is tomorrow night's presentation.

The talk will include a lot of history of Stonehenge and its construction, and a review of some other important or amusing henges around the world. But this article is on the astronomy, or lack thereof.

The Background: Stonehenge Decoded

Stonehenge famously aligns with the summer solstice sunrise, and that's when tens of thousands of people flock to Salisbury, UK to see the event. (I'm told that the rest of the time, the monument is fenced off so you can't get very close to it, though I've never had the opportunity to visit.)

Curiously, archaeological evidence suggests that the summer solstice wasn't the big time for prehistorical gatherings at Stonehenge; the time when it was most heavily used was the winter solstice, when there's a less obvious alignment in the other direction. But never mind that.

[Gerald Hawkins' Stonehenge alignments from Stonehenge Decoded] In 1963, Gerald Hawkins wrote an article in Nature, which he followed up two years later with a book entitled Stonehenge Decoded. Hawkins had access to an IBM 7090, capable of a then-impressive 100 Kflops (thousand floating point operations per second; compare a Raspberry Pi 3 at about 190 Mflops, or about a hundred Gflops for something like an Intel i5). It cost $2.9 million (nearly $20 million in today's dollars).

Using the 7090, Hawkins mapped the positions of all of Stonehenge's major stones, then looked for interesting alignments with the sun and moon. He found quite a few of them. (Hawkins and Fred Hoyle also had a theory about the fifty-six Aubrey holes being a lunar eclipse predictor, which captured my seventh-grade imagination but which most researchers today think was more likely just a coincidence.)

But I got to thinking ... Hawkins mapped at least 38 stones if you don't count the Aubrey holes. If you take 38 randomly distributed points, what are the chances that you'll find interesting astronomical alignments?

A Modern Re-Creation of Hawkins' Work

Programmers today have it a lot easier than Hawkins did. We have languages like Python, with libraries like PyEphem to handle the astronomical calculations. And it doesn't hurt that our computers are about a million times faster.

Anyway, my script, skyalignments.py takes a GPX file containing a list of geographic coordinates and compares those points to sunrise and sunset at the equinoxes and solstices, as well as the full moonrise and moonset nearest the solstice or equinox. It can find alignments among all the points in the GPX file, or from a specified "observer" point to each point in the file. It allows a slop of a few degrees, 2 degrees by default; this is about four times the diameter of the sun or moon, but a half-step from your observing position can make a bigger difference than that. I don't know how much slop Hawkins used; I'd love to see his code.

[Astronomical alignments between pairs of New Mexico peaks] My first thought was, what if you stand on a mountain peak and look around you at other mountain peaks? (It's easy to get GPS coordinates for peaks; if you can't find them online you can click on them on a map.) So I plotted the major peaks in the Jemez and Sangre de Cristo mountains that I figured were all mutually visible. It came to 22 points; about half what Hawkins was working with.

My program found (114 alignments.

[Astronomical alignments between pairs of New Mexico peaks] Yikes! Way too many. What if I cut it down? So I tried eliminating all but the really obvious ones, the ones you really notice from across the valley. The most prominent 11 peaks: 5 in the Jemez, 6 in the Sangres.

That was a little more manageable. Now I was down to only 22 alignments.

Now, I'm pretty sure that the Ancient Ones -- or aliens -- didn't lay out the Jemez and Sangre de Cristo mountains to align with the rising and setting sun and moon. No, what this tells us is that pretty much any distribution of points will give you a bunch of astronomical alignments.

And that's just the sun and moon, all Hawkins was considering. If you look for writing on astronomical alignments in ancient monuments, you'll find all people claiming to have found alignments with all sorts of other rising and setting bodies, like Sirius and Orion's belt. Imagine how many alignments I could have found if I'd included the hundred brightest stars.

So I'm not convinced. Certainly Stonehenge's solstice alignment looks real; I'm not disputing that. And there are lots of other archaeoastronomy sites that are even more convincing, like the Chaco sun dagger. But I've also seen plenty of web pages, and plenty of talks, where someone maps out a collection of points at an ancient site and uses alignments among them as proof that it was an ancient observatory. I suspect most of those alignments are more evidence of random chance and wishful thinking than archeoastronomy.

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[ 14:54 Jun 13, 2019    More science/astro | permalink to this entry | comments ]

Sun, 28 Oct 2018

How to Extend a Moonrise

Last night, as we drove home from the Pumpkin Glow -- one of Los Alamos's best annual events, a night exhibition of dozens of carved pumpkins all together in one place -- I noticed a glow on the horizon right around Truchas Peak and wondered if the moon was going to rise that far north.

Sure enough, I saw the first sliver of the moon poking over the peak as we passed the airport. "We may get an extended moonrise tonight", I said, realizing that as the moon rose, we'd be descending the "Main Hill Road", as that section of NM 502 is locally known, so we'd get lower with respect to the mountains even as the moon got higher. Which would win?

As it turns out, neither. The change of angle during the descent down the Main Hill Road exactly matches the rate of moonrise, so the size of the moon's sliver stayed almost exactly the same during the whole descent, until we got down to the "Y" where a nearby mesa blocked our view entirely. By the time we could see the moon again, it was just freeing itself of the mountains.

Neat! Made me think of The Little Prince: his home asteroid B6-12 (no, that's not a real asteroid desgination) was small enough that by moving his chair, he could watch sunset over and over again. I'm a sucker for moonrises -- and now I know how I can make them last longer!

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[ 19:32 Oct 28, 2018    More science/astro | permalink to this entry | comments ]