The imminent transit of Venus across the Sun is understandably generating a great deal of excitement around the world, and I wish I was able to get a good view of it myself. Watching a planetary silhouette creep across a stellar disc in real time would be a spectacularly visual experience for someone who spends a lot of their time working with the lightcurves of transiting exoplanets! Unfortunately, in England we’re only going to catch the last hour or so at dawn, and there’s a roughly 100% chance of cloud here in Oxford.
One thing the Venus transit certainly reminds us of is what a dynamical and interesting planetary system we inhabit. There are alignments between the Sun and various other bodies occurring all the time, most of which will go unobserved by any human eyes. For instance, if you were standing within Hellas Planitia on Mars during the local daytime on the 11th of May, 1984, you would have had the opportunity to witness a full transit of Earth across the Sun, and in this short and easy-to-read paper, Meeus & Goffin did the calculations to work out what you could have seen.
To start with, the geometry of the transit would have been similar to the Venus transit, in terms of the stellar-to-planetary disc size ratio. From Earth, the diameter of Venus will appear to be ~3% that of the Sun, which happens to be about the same as the Earth-to-Sun diameter ratio as seen from Mars. In addition, the 1984 Earth transit occurred at a solar mid-latitude, which will also be the case for the 2012 Venus transit. On the other hand, while the upcoming Venus transit will only last about 7 hours, the Earth transit from Mars would have been more like 13 hours from start to end. And something else you get with an Earth transit that you don’t get with a Venus transit is the possibility of seeing the Moon transit the Sun as well, because the Moon is close enough to the Earth and it would only appear about 4 times smaller.
As well as the one that occurred in 1984, Meeus & Goffin calculated details for all 47 Earth transits that have been or will be observable from Mars between 4BC and AD3015. Transits usually occur either 25.5, 79.0, or 100.5 years apart, with the next one due to happen in 2084. I have a feeling we might need to wait until at least the transit after that, in 2163, before an intrepid human finally has the opportunity to stand on Mars and watch one.
Feature Image: The Sun setting over the surface of Mars, photographed in 2005 by the Mars Exploration Rover Spirit. NASA.