During the Venus transit, I found myself wondering about how often Venus might be seen to transit the Sun if you were living on Mars, and found myself in a brief conversation with @mike_peel about it. He directed me to a convenient Wikipedia page which was seemingly created shortly after the last Venus transit back in 2004.
Mars is apparently due to see its next Venus transit in 2030. In fact, Venus transits are apparently a lot more frequent on Mars – a fact which Mike suggested was probably due to the difference in inclination between the orbits of the different planets. While all the planets orbit in roughly the same plane, they aren’t exact. Earth’s orbit is actually tilted by about 7.2°. Venus and Mars have orbits tilted by 3.2° and 5.7° respectively, so because Mars has an orbital inclination closer to that of Venus, Venus is thus more likely to come between Mars and the Sun. In fact, while only 7 Venus transits have been seen from Earth since the invention of the telescope, Mars is due to have 7 Venus transits before the next one occurs here on Earth – in the Earth years of 2030, 2032, 2059, 2064, 2091, 2096, and 2098. So if anyone on Earth missed the Venus transit this last time, they could sign up to be a martian colonist, and then they’d get to see several from Mars. Possibly.
Which in turn made me wonder about other rare astronomical events. From Mars, Earth is seen to transit the Sun twice every 79 years or so. But if you think these events are uncommon, far rarer is the chance of seeing two planets cross the Sun at the same time. If you were on Mars and you wanted to see both Earth and Venus transit the Sun at the same time, you’d have to wait until the year 571471. Pragmatically, it has to be wondered if the human race will even exist by then, at least in its current form. Should there still be humans recognisable as humans, precisely what kind of society might exist half a million years into the future is anyone’s guess.
Interestingly, there’s a greater chance of seeing a double transit from here on Earth. On September 17, in the year 13425, both Venus and Mercury will transit the Sun’s disk as seen from Earth. Then again on July 26, in the year 69163. It may require waiting for a few thousand years, but that’s bound to be quite a sight!
Assuming anyone’s still around to see it by then…
Apparently, given distance from the Sun and the Sun’s apparent size from much further away, the rarest planetary transit of all is a transit of Uranus seen from Neptune – then next of which is due for the year 38172.
Image credits: NASA & Stzeman @ Wikimedia Commons respectively – manipulated subtly by yours truly. And no, it wouldn’t actually look quite like in these images. Mercury transits are actually very difficult to spot!