Planetary Spheres and the Size of the Universe

Babylonian sky watchers were aware of wandering celestial objects (or planets; a wider concept than today). Of these, the Sun always moves eastward among the stars following the ecliptic, its yearly path through the Zodiacal constellations. The Moon stays fairly close to the Sun’s path taking about a month to circle the sky with respect to the stars. Most of the time the other planets also move slowly toward the east, keeping close to the ecliptic. It takes a certain time for the planet to make a full circle around the sky from a constellation of the Zodiac back to the same place (its sidereal period). However, planets other than the Sun and Moon slow down, stop, and go backward for some time and then resume their normal motion (Fig. 3.1). There is regularity in this odd retrograde motion. Each planet has a synodic period, the time between successive reversal loops. The synodic period differs from the sidereal, and so successive stops happen at different constellations of the Zodiac. Table 3.1 gives the synodic and sidereal periods of several planets (of which Uranus, Neptune, and Pluto were not known in antiquity).