The Rhythmic Sky Page 15

4. Motion of the planets

1This animation image shows Jupiter and Saturn moving with respect to the stars over a period of almost a year at two-week intervals.

In the old days planets were referred to as “wandering stars” for that reason. Astrology has been interested in the position of planets for the same reason.


Image www.williams.edu





The reason for this apparent motion is that the planets are very much closer to earth than any of the stars and they move, like Earth, around the Sun. When the Earth "overtakes" one of the outer planets, that planet seems to move backwards in the sky. This is called retrograde motion.

The main differences between planets and stars

Stars are very numerous. Whereas planets are few in number  (5 are visible to the unaided eye).

Stars are essentially “fixed” relative to each other. Planets “wander” relative to the fixed stars. So they are not in the same location each night nor in the same position year to year.

Stars have a wide range of declination and right ascension and planets must be on (or very near) the ecliptic.

Stars produce their own light independent of the Sun’s location and they are very far away. The brightness of the planets does depend on the Sun’s location. In comparison to the stars, they are near to Earth.


Image: Martin Powell.