dipper@utastro.UUCP (Debbie Byrd) (09/21/84)
The Martian moon Phobos rises in the west -- twice a day. We'll talk more about this tiny moon of Mars -- after this. September 14 Phobos and Mars The planet Mars has two small moons -- Phobos and Deimos. The two Martian moons aren't round like Earth's moon -- they're so irregular and elongated in shape that they're sometimes called the potato moons. Phobos and Deimos look very much like small asteroids -- and may really be asteroids that were captured by the gravity of Mars. Like Earth's moon, the surfaces of the two Martian moons are heavily cratered. Also like Earth's moon, Phobos and Deimos always keep one face turned toward the larger planet. The two moons travel almost directly over the equator of Mars -- their orbits are tilted less than two degrees to the Martian equator. And they orbit so close to the planet that an observer near one of the Martian poles wouldn't be able to see the moons. Phobos is the larger moon -- and orbits closer to the planet than Deimos. When our grandchildren visit Mars on vacations they'll see a unique sight. Tiny as it is, Phobos will appear about half the diameter of our moon -- though not nearly as bright. But an extraordinary thing happens because Phobos moves around Mars FASTER than the planet turns on its axis. As a result Phobos appears to move across the dome of the sky from WEST to EAST -- in the opposite direction from all the other celestial objects -- even its sister moon! Twice each Martian day, the moon Phobos rises in the west, takes about four and a half hours to cross the dome of the Martian sky, and sets in the east -- the strange case of the backwards moon! Script by Diana Hadley and Deborah Byrd. Patrick Moore, "Astronomy Facts & Feats", p.87. "McGraw-Hill Encyclopedia of Astronomy," p. 179. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin