Transits of the Martian Moons
Published on Nov 9, 2006 at 11:41 pm.
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Filed under planets.
Yesterday was all about the transit of Mercury. Well, today, I thought that I’d say a bit about some other transits. On Earth, our Moon is quite large, so when it passing in front of the Sun, sometimes it completely covers the Sun. We call this an eclipse. Since the Moon can cover the Sun, when it doesn’t, it is still an eclipse, only we call that a partial eclipse.
However, on Mars the situation is different. Mars’ two Moons Phobos and Deimos are so tiny that they never completely cover the Sun. So, technically, then can’t eclipse the Sun as seen from Mars. However, if you are in just the right place, you can see the moons pass directly in front of the Sun, much as Mercury did as seen from Earth yesterday. We call these transits. The Mars Exploration Rovers Spirit and Opportunity are both fairly close the the Martian equator, so they get to see this happen once in a while.
The rovers’ pancams regularly observe the Sun. This is done to measure the effects of the Martian atmosphere on the sunlight, thus shedding additional light onto the workings of that atmosphere. 
Well, the Moons have been caught passing right in front of the Sun on several occasions. Above, you see a transit of Deimos, and a partial transit of Phobos. A better transit was observed in March of 2004, as seen here. This is actually pretty telling, since Phobos is Mars’ largest moon, and the closest to the planet. Even so, it can’t cover all of the Sun’s disk (which is smaller as seen from Mars than from Earth!). The Martian moons are really quite tiny.
2004 was the year for this, with six transits by Phobos, as seen from Opportunity and one transit by Deimos seen by both Opportunity and Spirit. Both rovers also saw Deimos transits in 2005, and Opportunity got to see another one in January of 2006.
From Mars, you can also see Mercury transit the Sun, but it appears even smaller than it does from Earth. Venus can also transit as seen from Mars, but so can Earth. In fact, Earth transits would be particularly interesting, because both Earth and the Moon would be transiting, and the Moon as seen from Mars would appear about as big against the Sun’s disk as Mercury does from Earth, so it would make for a really cool double transit. The last time that happened was in 1984. But, the next time will be November 10, 2084. That might be interesting, because it is possible that there may be humans visiting Mars at that time. If so, they’d get so see something that no other human being has ever seen.
-Astroprof
A Ler…-- Rastos de Luz on November 10, 2006 at 8:09 am: 1
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