23.2: Mars and Jupiter

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The other three planets visible to the eye can be seen anywhere along the ecliptic -- even at midnight, directly opposite the Sun, which was when they appear brightest. Mars seems to move the fastest, Jupiter next, and Saturn the slowest. But all exhibit that puzzling quirk -- near the point of their celestial path exactly opposite the Sun (“opposition"), their motion among the stars temporarily turns around.

Today we understand all that very well (see image above). Planets are spherical objects like Earth -- Venus, Mercury, and Mars are smaller, while Jupiter and Saturn are much bigger. Earth and other planets (too faint to be seen without a telescopes) all orbit the sun on or near the plane of the ecliptic. Their speed, however, varies -- the closer to the Sun, the faster (see http://www.phy6.org/stargaze/Skeplaws.htm and in particular Kepler's third law). Therefore, when the three outer planets are near opposition, the earth, orbiting closer to the Sun, overtakes them, and they seem to move backwards.

The retrograde motion of the two inner planets has a similar cause. Being closer to the Sun, they overtake the earth in their motion.