The Planets and Their Motions

Abstract

Ancient astronomy probably began when the earliest observers of the sky recognized the difference between what the Greeks called the fixed stars and the wandering stars or planets. It was probably easy enough to discover that the sun and the moon shift their apparent positions with respect to the constellations, but rather difficult to find that five other celestial bodies—Mercury, Venus, Mars, Jupiter, and Saturn, the naked eye planets—do so as well. This discovery probably came with the observation that the two brightest planets, Venus and Jupiter, shift their positions with respect to the sun quite differently from the way the stars do. It probably was also noted that the light from Venus and Jupiter is quite steady whereas the light from the stars varies continuously (the twinkling of the stars). These phenomena must have aroused considerable curiosity and wonder among the ancient observers and led to careful observations of the apparent motions of the naked eye planets and to the development of the earliest models of the solar system.

The snow had fallen many nights and days;

The sky was come upon the world at last,

Sifting thinly down as endlessly

As though within the system of blind planets

Something had been forgot or overdriven.

—GORDON BOTTOMLEY, The End of the World