[محمد ابراهيم]

Retrograde Motion

One phenomenon that aincient astronomers had difficulty explaining was the retrograde motion of the planets. Over the course of a single night, a planet will move from East to West across the sky, like any other celestial object near the ecliptic.

(Most objects in our sky appear to rise somewhere on the Eastern horizon and set somewhere on the Western horizon. The only exceptions are stars near the North celestial pole, that stay above the horizon all the time and appear to make counterclockwise circles around the celestial pole. As one travels further North, the region of the sky that remains above the horizon at all times becomes larger, until the entire sky appears -to an observer at the North Pole- to be simply circling the North star. As one ravels South, the region that remains above the horizon becomes smaller, diminishing to zero size for an observer on the Equator. If one continues South of the Equator, one would observe a progressively larger region surrounding the South celestial pole that remains above the horizon at all times. Stars in that region would appear to circle the South celestial pole in clockwise circles.)

If observed from one night to the next, however, a planet appears to move from West to East against the background stars most of the time. Occasionally, however, the planet's motion will appear to reverse direction, and the planet will, for a short time, move from East to West against the background constellations. This reversal is known as retrograde motion, and is illustrated in the following animation.