Motions of the planets

The planets. [NASA]

The planets. [NASA]

Sun. This explains why the orbits of the planets are almost in the same plane and why the orbital motions are in the same direction, preserving the angular momentum from the molecular cloud

Photograph of Halley's comet. [NOAO/

AURA/NSF]

When we look at the night sky, it is clear that most of the objects maintain their relative positions. These are the stars. However, apart from the Sun and Moon, a small number of objects move against the background of fixed stars. These are the planets. The study of the motions of the planets has occupied astronomers for centuries. These motions do not appear simple. The planets occasionally seem to double back along their paths, as shown in Fig. 22.3. This doubling back is known as retrograde motion. Historically, any explanation of the motions of the planets had to include an explanation of this retrograde motion.

The earliest models of our planetary system placed the Earth at the center. This idea was supported by Aristotle in approximately 350 bc. His view was that the planets, the Sun and the Moon move in circular orbits about the Earth. Even though there is now ample evidence against this picture, one can see how placing the Earth at the center was a naturally simplifying assumption. The picture was modified by Claudius Ptolemy, in Alexandria, Egypt, around 140 ad. In order to explain retrograde motion, he added additional

The apparent motion of Mars against the fixed background of stars.The loop when it doubles back is called retrograde motion.

Photograph of Halley's comet. [NOAO/

AURA/NSF]

The apparent motion of Mars against the fixed background of stars.The loop when it doubles back is called retrograde motion.

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