FROM LEFT: Deimos, Phobos and the Earth's moon. (NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M University)But Mars' moons orbit at a much closer distance than our own Moon orbits ours. While the moon is about 238,000 miles away from Earth (give or take), Phobos is only about 6,000 miles away from the surface of Mars.
Among other things, that proximity causes it to rotate incredibly fast, circling around Mars in under eight hours. A person standing on Mars would see it cross the sky twice in one day. Because of its small size, it appears smaller than our own moon does to us.
Here's what Phobos looks like in the Martian afternoon sky to the Mars Curiosity rover:
Phobos' close, fast orbit makes it cross paths with the sun fairly often — near-daily. But because the moon is so small it never fully occludes the sun to create a total eclipse. Part of the sun's disc is always visible.
The Mars Curiosity rover captured real-time video of this happening on Aug. 20, 2013.
Here are a few still shots captured by the rover during that event.
Against the solar backdrop, you can clearly make out Phobos' irregular shape, which it partly owes to being so small that it doesn't exert enough gravity to pull itself into a proper sphere. A person weighing 150 pounds on Earth would weigh only two ounces on Phobos.
“Because this eclipse occurred near midday at Curiosity's location on Mars,” NASA explains, “Phobos was nearly overhead, closer to the rover than it would have been earlier in the morning or later in the afternoon. This timing made Phobos' silhouette larger against the sun — as close to a total eclipse of the sun as is possible from Mars.”
Here's a wide-angle shot of a 2010 Mars eclipse taken by the Opportunity rover.
What about the other Martian moon? Deimos orbits more than twice as far away from Mars and is smaller to boot, making it much less visible in the Martian sky. When Deimos crosses paths with the sun, it's more properly called a transit, rather than an eclipse.
In this photo of a Deimos transit taken by the Opportunity rover in 2004, the moon basically looks like a sunspot.
Other planets experience eclipses, too, although we haven't observed any of them from the ground up. Here, for instance, is a Hubble telescope image of Jupiter's moon Io casting a shadow on Jupiter's surface.
From Jupiter, the sun appears much smaller than it does in our own sky. A number of the planet's moons obscure it completely, creating not an eclipse but an occultation — an astronomical term for when one object is completely hidden by another one of much larger apparent size. Because Jupiter has at least 69 moons, it sometimes experiences multiple eclipses and occultations simultaneously.
But our own total eclipses on Earth are one-of-a-kind. Because of the similarity between the apparent sizes of the moon and sun when viewed from Earth, our total eclipses block out the entirety of the sun's disc while leaving the luminous corona — the sun's fiery atmosphere — plainly visible in the darkened sky.
That event happens nowhere else in the solar system — not even on Mars.