The Transit of Venus occurring Tuesday is the last one we’ll see for 105 years, but it’s not the first one we’ve seen! I saw the 2004 transit literally with my own eyes (wearing safe eclipse glasses – and which I plan to use to watch this one as well), and there were several witnessed before that. The last one before 2004 was in 1882, recent enough that photography was being used in astronomy. And it so happens that astronomers at Mt. Hamilton in California were able to take a series of 147 (!) images of the transit, 140 of which were used to make this amazing video:
That. Is. So. Cool! Not too often you see a movie made in 1882!
And the story behind these photos is interesting, too: they were made on glass plates, which, until the recent invention of electronic detectors were the go-to astronomical detectors. I’ve taken images on glass plates myself, and it’s a major pain in the neck. It’s incredible they got that many shots of the transit! The plates themselves languished, forgotten, until 2003, when they were re-discovered by astronomer Bill Sheehan, and eventually scanned and made into the above movie. Read that link for the details.
Here’s one of the plates for your perusal:
More of them can be found at the Naval Observatory site.
I expect we’ll be seeing far, far more than just a single video from the upcoming transit. If you get unusual or really interesting shots — especially video — please let me know! I’m hoping to put together a gallery of the best ones I’ve seen. Send me a tweet or email me at the bad astronomer at gmail dot com.
And remember, we’re doing a live Transit of Venus video star party tomorrow! I’ll have details coming soon.
Tip o’ the pinhole projector to David Griff.
– Everything you need to know about next week’s Transit of Venus
– Eclipse followup part 2: tons o’ links on how to safely watch
– Your last chance to see Venus for the next few weeks
– Venus rounds the corner
On Tuesday/Wednesday June 5/6, Earth will have the best seat — the only seat — for a great show: the Transit of Venus across the face of the Sun. This is a relatively rare event, and the next one won’t happen until December 10, 2117, so I’m guessing this will be the last time you’ll be able to see it.
I have a lot of info below, so I’ve broken it up into sections. Also, a special note: Fraser Cain, Pamela Gay, and I are hosting a live online video chat star party for the transit! That live video feed will be embedded here on the blog at the time of the transit, so if you read this blog — and you do — you’ll see it. I’ll have more info closer to the date.
Oh — you can click on the pictures here to embiggen them and get more info, too.
What Is the Transit?
A transit is when one object in the sky passes in front of another. In this case, we’ll see Venus move across the Sun’s face. Think of it as a mini-eclipse.
Venus orbits the Sun closer in than we do, taking about 225 days to circle it once. We don’t see a transit every time, though, because its orbit is tilted slightly (by about 3°) to the Earth’s, so most of the time it passes near the Sun, but misses it*. Only when the planets align just right do we see an actual transit.
The geometry works out in a funny way. Transits come in pairs 8 years between events, but each pair is separated from the next two by more than a century. The last pair was 1874/1882. The next transit after that was in 2004. Now we have the second of our current pair, and then the next two won’t be until 2117/2125!
I saw the last transit in 2004, and it was pretty awesome. Venus was just a small dot, barely a disk, seen in silhouette against the Sun, but it was clearly not a sunspot even without magnification. I’ll note here you shouldn’t observe the Sun without eye protection! I have more about that in the Observing the Transit section below.
Historically, the transit was used to measure the size of the solar system. In the 1600s, the distances to the planets were only known in units of the Earth’s orbit. So Venus was 0.7 times as far from the Sun as Earth, and Jupiter was 5 times as far. But the actual size of Earth’s orbit wasn’t known! Before we had space probes and radar (which we can use to bounce signals off planets and measure their distance directly) it was hoped the transit of Venus would allow it to be measured. It worked, but the details are pretty cool and well worth a read. The story is actually told pretty well on the Wikipedia page.
Even in modern times, transits are useful. We look for planets orbiting other stars using a similar method, watching for a dip in the light as the planet blocks the star. To help refine this method, astronomers will use Hubble (!) to look at the Moon (!) — since the Moon is lit by the Sun, the tiny drop in sunlight during the transit should dim the Moon a bit. They’re actually hoping to see if they can detect Venus’s atmosphere too, since that will affect how the light gets to the Moon from the Sun. Amazing.
Interestingly, as seen from Saturn in December 2012, Venus will transit the Sun as well, and astronomers are hoping to use Cassini to see if they can detect it. Also, in 2014, Earth will transit the Sun as seen from Jupiter! Hubble may be used to observe Jupiter at that time to see if it can be detected as well. [Thanks to BABloggee Garrett Curley for this info! You can read more about these space observations at Physics World – you have to register, but it’s free.]
* [Actually, because the Earth is moving around the Sun as well, after one complete orbit Venus still has to "catch up" to the Earth to line up with the Sun again. That’s called the synodic period, and it takes about 584 days. Still, the tilt of Venus’s orbit means it doesn’t always cross the Sun directly every synodic orbit.]
So I wanna see it! What do I do?
Because Venus is transiting the Sun, it has to be daytime during the transit for you to see it (duh). The transit occurs from about 22:00 June 5 to 05:00 June 6 (UTC; subtract 4 hours for Eastern US time). The exact time depends on where you are on Earth; NASA has a list of times for US cities and for the rest of the world.
For a more general overview, here’s a map of where the transit is visible: