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Wednesday, November 3, 1999 Published at 15:50 GMT


The shadow of a Martian moon

Racing over the planet - Phobos's shadow

By BBC News Online Science Editor Dr David Whitehouse

The shadow of the Martian moon, Phobos, has been captured by the Mars Global Surveyor (MGS) spacecraft.

MGS was sent to orbit Mars and monitor changes in weather and surface conditions. Its wide angle cameras are also proving to be a good way to spot the frequent solar eclipses caused by the passage of Phobos between Mars and the Sun.

The shadow of Phobos was cast upon a region called Xanthe Terra on 26 August 1999 at about 1400 local time on Mars (Martian days are only about 30 mins longer than ours). The image covers an area about 250 km (155 miles).

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

Hall had been hunting for Martian satellites for some time, and was about to abandon the search when he was encouraged by his wife to continue. In honour of her role, the largest crater on Phobos was named Stickney, her maiden name.

Phobos is a tiny, potato-shaped world that is only about 13 by 11 by 9 km (8 by 7 by 6 miles) in size. It orbits Mars every 8 hours.

If you could stand on Mars and watch Phobos passing overhead, you would notice that it appeared to be only about half the size of the Earth's Moon.

In addition, the Sun would seem to be just over half of its size as seen from Earth. Martian eclipses are therefore dark but not as spectacular as total solar eclipses on Earth.

They are however thousands of times more common, occurring a few times a day somewhere on Mars whenever Phobos passes over the planet's sunlit side.

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