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Motion of the planets

starsandplanetsImage: This 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.




planet loop animationImage: Martin Powell








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.




The Dance of the Planets

planets today labels
The Sun and the five brightest naked-eye planets are shown here in continuous motion from January 2000 to December 2015.

The 'invisible path' along which the Sun appears to move - and which the Moon and planets follow very closely - is known as the ecliptic.

Mercury and Venus are closer to the Sun than Earth and these planets always remain close to the Sun, their positions switching between East and West of it (i.e. between evening and morning "stars").

The outer planets (i.e. from Mars outwards) move well clear of the Sun (up to 180º away) and describe looping motions at much slower speed . The further out the planet is, the slower we see it move in the sky. Note that Mars moves steadily through the constellations while Saturn seems to move at a snail's pace.

This animation has been obtained from Martin J. Powell's web site, and was produced and is kindly made available for public use by David Colarusso .






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