Shallow Thoughts : : science

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.

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.

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

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.

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.

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.

Farewell, Space Shuttle Endeavour

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

Observe the 2012 Venus transit!

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.

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.

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!