In this series the shadow of Phobos is visible as it crosses the southern Elysium Planitia, the northern Lunae Planum and Kasei Valles areas of Mars. The images were taken September 1, 8 and 25, respectively

November 3, 1999
Web posted at: 11:29 a.m. EST (1629 GMT)

(CNN) -- Martian eclipses are not as spectacular as total solar eclipses on Earth can be. But in compensation, they are thousands of times more common, occurring a few times a day somewhere on Mars whenever Phobos passes over the planet's sunlit side.

The Global Surveyor's wide-angle cameras, designed to monitor changes in martian weather and surface conditions, are also proving to be a good way to spot the frequent eclipses as Phobos passes between the red planet and the sun.

Phobos is a tiny, potato-shaped moon, only about 13-by-11-by-9 kilometers (8-by-7-by-6 miles) in size.

If you could stand on Mars and watch Phobos passing overhead, you would notice that it appears to be only about half the size of what Earth's moon looks like when viewed from the ground.

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In addition, the sun would seem to have shrunk to about 2/3 (or nearly 1/2) of its size as seen from Earth.

The shadow of Phobos was seen during the Viking missions in the late 1970s, and in fact one day the shadow was observed to pass right over the Viking 1 lander.

Color composite view of the shadow of Phobos as it was cast upon western Xanthe Terra on August 26, about 2 p.m. local time on Mars The images were taken September 1, 8 and 25, respectively

The surface of Phobos itself was first imaged by Mariner 9 in 1971, and global coverage was obtained by the Viking orbiters in 1976-80.

Phobos and Mars' more distant moon, Deimos, were discovered in 1877 by Asaph Hall, an astronomer at the United States Naval Observatory in Washington, D.C.

The $150 million Mars Global Surveyor mission was launched in 1996 and arrived at Mars the following year. It has orbited the red planet for a full martian year, or 26 months. It started collecting extensive data earlier this year.