The Transit of the Earth... from Mars!

Several weeks ago we were witness to one of the rarest, yet predictable, astronomical events known: the transit of Venus. The next such transit will be in the year 2117. You can always catch the next transit of Mercury, though. These are far more common with 13-14 events per century: the next one is May 9, 2016. However, you do need a telescope to see it as Mercury is much smaller.

Perhaps you are now hooked on transits and want to catch an even more exciting event. You may actually have already seen the November 2006 Mercury transit. How about a (near) simultaneous transit of both Venus and Mercury? Good luck: the next one will be around September in the year 13,425 AD.

Perhaps it's best to focus on something that may happen sooner? Enter the transit of the Earth!
If you think Mercury and Venus transiting is exciting, then check out this simulation of transit of the Earth (and Moon!)... from Mars!

I created that video by using the Stellarium software, setting my location to Mars, and setting the correct date: November 10, 2084. I eliminated the atmosphere and ground in order to show the full transit. You can see it lasts for ~8 hours and the Moon gets to cross the disk as well (extending the event for another ~2 hours). Transits of Earth are just as rare as transits of Venus, however, the spacing of the events is different: 100.5, 79, 25.5, and 79 years and then repeat (a 284-year cycle). In comparison, Venus transits occur on a 243-year cycle: 121.5, 8, 105.5, and 8 years. In practice, you could catch 2 Venus transits in your lifetime thanks to the short 8-year period between the century-long waits. Transits of the Earth are more widely spaced. These Earth-transits happen when Mars is in opposition as seen from the Earth (ie, directly behind it) and also at its ascending or descending nodes. These nodes are when the plane of the orbit of Mars crosses that of the Earth. The same phenomenon holds for transits of Venus and is the reason why we don't see a transit every time the planets are 'lined up'. The last transit of the Earth as seen from Mars was on May 11, 1984 and the next one will be on November 10, 2084. Not surprisingly there has not been a single observations of this rare event (by humans or robots). We have, though, seen transits of the moons of Mars (see image below). We still have 72 years for the next Earth transit. Better get to work on establishing a Mars colony!

A transit of Deimos, one of the two moons of Mars. Taken by the Opportunity rover. Credit: NASA

What sort of science could a Martian do with the transit of the Earth? The basic science performed for the transit of Venus holds for that of Earth. A Martian would observe the aureole around Earth but not the Moon showing that Earth possess an atmosphere but the Moon does not. Timing the transit across Mars would also Martians to estimate the size of the solar system and thus the distance to the Earth. The Moon is an interesting new observable. Probably Martian astronomers would have already known the Moon was there and would have timed the period of its orbit. By combining the orbit, the measured separation during transit, and the distance to the Earth they would have an accurate measure of the separation between the Earth and the Moon in kilometers (or the Martian equivalent). That separation along with the period could then be used to infer the mass of the Earth. The angular size of the Earth could be converted to a real physical size (given the newly estimated distance) and Martian astronomers could comment on the average density of the Earth and its composition. The dimming of light from the Sun due to the transit is analogous to the way we search for extrasolar planets. To date, however, we have not found extrasolar moons. Martian astronomers could study the light from the Sun during the transit to see how a moon affects it. They would know if the instruments they've built would then be sensitive enough to find such moons around planets in distant stars. Other planets in our solar system also undergo transits, but these get rarer and rarer as you move out given that the planets move slower and slower. Hence, while you could see Uranus transit the Sun from Neptune, you may have to wait a few tens of thousands of years between events.

The Sun rises over the red planet; from Stellarium.