Shallow Thoughts : : astro
Akkana's Musings on Open Source Computing, Science, and Nature.
Wed, 08 May 2013
A couple of months ago I wrote about
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
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.
[ 10:46 May 08, 2013
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Tue, 12 Mar 2013
I wrote last week about an upcoming
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
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
|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
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.
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
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.
[ 18:55 Mar 12, 2013
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Sat, 09 Mar 2013
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
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
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
|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,
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.
[ 10:30 Mar 09, 2013
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Wed, 31 Oct 2012
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)
>>> la.next_setting(sun, start=mid)
>>> la.next_setting(sun, start=mid) - la.next_rising(sun, start=mid)
>>> sf.next_rising(sun, start=mid)
>>> sf.next_setting(sun, start=mid)
>>> sf.next_setting(sun, start=mid) - sf.next_rising(sun, start=mid)
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))
>>> ((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
>>> ((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
And we have our answer -- there's about 11 minutes
difference in day length between SF and LA.
[ 10:46 Oct 31, 2012
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Fri, 21 Sep 2012
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
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.
[ 20:35 Sep 21, 2012
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Sun, 12 Aug 2012
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.
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
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:
Venus occultation, 2012-8-13.
[ 12:27 Aug 12, 2012
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Wed, 06 Jun 2012
After a heart-stopping day of rain on Monday, Tuesday, the day of the
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
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!
Venus Transit, June 5 2012.
[ 11:48 Jun 06, 2012
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Fri, 01 Jun 2012
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
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
San Francisco Amateur Astronomers,
or any of the others on the AANC's list of
Astronomy Clubs in Northern California
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
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
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
|First contact: ||3:06:20
|Internal ingress: ||3:23:56
|Maximum transit: ||6:25:30
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?
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!
[ 12:04 Jun 01, 2012
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