23.2 Mars and Jupiter

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, Jupiter and Saturn much bigger. Earth is a planet too and others exist as well (too faint to be seen without a telescopes), all orbiting 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.