IV.

Early 16th-Century Cosmology

The cosmology that was eventually replaced by Copernican theory postulated a geocentric universe in which the earth was stationary and motionless at the center of several concentric, rotating spheres. These spheres bore (in order from the earth outward) the following celestial bodies: the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn. The finite outermost sphere bore the so-called fixed stars. (This last sphere was said to wobble slowly, thereby producing the precession of the equinoxes; see Ecliptic.)

One phenomenon had posed a particular problem for cosmologists and natural philosophers since ancient times: the apparent retrograde (backward) motion of Mars, Jupiter, and Saturn. From time to time the daily motion of these planets through the sky appears to halt and then to proceed in the opposite direction. In an attempt to account for this retrograde motion, medieval cosmology stated that each planet revolved on the edge of a circle called the epicycle, and the center of each epicycle revolved around the earth on a path called the deferent (see Ptolemaic System).