Venus, Venera, Ishtar:
(My apologies, this post never made it out of draft in time for the actual event described...)
On June 8, Venus crossed the face of the Sun ("transited," in astonomy-speak). A mini-eclipse. "Mini" because even though Venus is four times larger than the Moon, it is so far away that it only covered about 0.1% of the Sun's surface. Big deal, you say. Planets pass in front of their suns all the time, don't they? Well yes, they do. But catching them in the act is the trick. Having all the alignments right is a rare thing. Last time Venus was seen pulling this, it was 1882. Cameras were still relatively new, and they were deployed as an aid in making observations. Out of approximately 10,000 photographs taken of the 1882 transit, fewer than ten survive. Old celluloid is fragile.
If you think about it, you can quickly see why only planets that orbit closer to the Sun than Earth (the "inferior" planets, in astronomy-speak) ever make transits. They have to be in-between the Earth and the Sun at some point for a transit to be visible - they have to be "inferior" to the Earth. So, from Mars, you would occasionally get to see an Earth transit, in addition to Venus and Mercury transits. From Pluto, you might get to see all the other planets make transits - but that's not a sure thing, since if you also think about how all the planes of the orbits would have to line up, and since Pluto is such an odd-ball, transits visible from Pluto are probably quite rare (the Sun is also pretty darned small from there, too). There is a really nice website with an applet that gives you a good look at the geometry of the whole thing here.
For this transit of Venus, we had a lot more help to see it happen - the Internet was of course involved, allowing those who happened to be on the wrong side of the Earth to enjoy the sight during the six hours Venus took to cross the Solar disk. The best place to have been happened to be centered on a longitude passing through Europe - and the Penteli Observatory in Greece had a webcast via The Exploratorium starting at 01:00 Eastern, and the European Southern Observatory in Germany also had live images. Images from the Americas and Asia were accessible through this NASA-Goddard site. However, because the Americas are West of Europe, and therefore behind solar time-wise, most of the transit was over before sunrise (an afternote: I managed to see sunrise on the 8th from Nassau, as far East as I could get at that time, but my simple camera lucida could not resolve the tiny dot that had already started its exit contact with the solar limb - I had to be satisfied with the TV broadcasts that day...).
There is a fairly long history of observing transits. Observing a Mercury transit in 1677, Edmund Halley figured out that if we had a good estimate for the speed of light, and timed the four times of contact between the two disks' edges from widely separated spots on the Earth, we could get an estimate of the Earth-Sun distance. Halley laid out the mathematics for this calculation, but did not see a Venus transit, dying 19 years before the next one in 1761. Capt. James Cook took an expedition to Tahiti for the 1769 transit, and George Biddle Airy worked on the maths for the 1874 and 1882 transits. It took about five years to do those maths, resulting in distances of 149.59 million kilometers and 148.672 million km, compared to the value of 150.34 million km accepted today. Even John Philip Sousa got involved.
But what is the deal with watching this again? Haven't we already figured out how far from the Sun we are? Well, yes. But this time there were other things to see, since we have invented a few new instruments since 1882. Fr'instance, Tim Brown, a scientist at NCAR's High Altitude Observatory, looked at the spectrum of Venus' high atmosphere (65 - 85 km) to determine the concentration of particular molecules. Usually this part of the atmosphere can't be detected, since it is so diffuse, but the bright light of the Sun does the job of lighting it up quite nicely. What is amazing is that this technique has already been used to probe the atmospheres of planets circling other stars. This time, we have a sort of inverted technology: something used to observe a planet 150 light years from Earth is being adapted to observe a planet a few light-minutes away. Given the closeness of Venus, the observations will also be used to look at layering and wind velocities. In another twist that would amaze astronomers from 1882, Brown operated a telescope on the Canary Islands from a lab in Freiburg, Germany.
Here is one of the most interesting photos I found. Not because of the visual impact - that is actually small, and there are better-looking photos out there. What is interesting about this one is that it captured not only Venus crossing the solar disk, but also captured the International Space Station. The calculations involved in doing this would be staggering even for Airy, and yet we are quite blithely doing them on our Palms.
(c) Tomas Maruska and VT-2004 Programme, 2004
OK, so how long do you have to wait for the next Venus transit?
Until June 6, 2012. And where do you have to be to see it? East Asia or the Pacific - here's Fred Espenak's map of the transit visibility.
If you miss that one, you are out of luck - December 11, 2117 and December 8, 2125 are the next pair. Do you detect a pattern? You should - it's the harmony of the spheres ringing in your ears - 105.5 years separate the 8-year pairs of Venus transits. They are quite rare, and it is sobering to think that only six Venus transits have occurred since the invention of the telescope!
You would have better luck chasing a transit of Mercury - they occur about 13 - 14 times a century with the next one ocurring on November 8, 2006.
Material taken from articles in EOS 85/21, Science 304/5675, Physics Today 56/12, Fred Espenak's transit webpages, and NSF press releases.