lunar image of the month

august: moon shadow

Solar eclipses seen from the Earth are probably the most spectacular in the solar system, due to the coincidence of the apparent sizes of the Moon and Sun. The larger of Mars' moons, Phobos, is only 22km across, compared with the Moon's diameter of 3476km. However, it orbits at a distance of only 9000km (less than 1/40 of the Moon's distance from Earth) giving it roughly half of the Moon's apparent size. Couple that with the Sun's greater distance from Mars, making it seem 2/3 as large as from the Earth, and Martian eclipses become impressively dark, as can be seen in this Mars Global Surveyor image showing Phobos' shadow as seen from orbit.

At its low altitude, Phobos orbits faster than the planet rotates, so it rises in the west and sets in the east just 51/2 hours later. Martian eclipses are very frequent, occurring several times each day somewhere on the surface of the Red Planet, but are also very brief.

Image: NASA, JPL, Malin Space Science Systems.