Overview

Much of the history of mathematical and physical thought was inspired by curiosity about the motion of the planets the very same laws that govern the motion of spacecraft. The first observations of the celestial bodies predate recorded history. The inertial position of the vernal equinox vector was observed and recorded in stone constructions Stonehenge, for example as early as 1800 B.C. Written evidence of stellar observations was left by the Egyptians and the Babylonians from about 3500 years ago. (The Babylonians of this era divided time into 60 even units, a tradition that survives to this day. ^[1])

In about 350 B.C. Aristotle explained the wandering motion of the planets by proposing that the universe was composed of 55 concentric rotating spheres centered in the Earth. The outermost sphere contained the fixed stars; its rotation is a very adequate explanation of the observed motion of stars in the night sky and the irresistible image of a celestial sphere. The rotation of the inner sphere containing the moon was also a simple, descriptive idea. The motion of the planets, however, was much more difficult. Not only were the observing instruments crude and the mathematical tools nonexistent, the Earth is a singularly poor observation post for heliocentric motion. Usually the planets move slowly eastward across a background of fixed stars; however, at times they reverse direction and move westward. The retrograde loop of Mars is renowned. To explain this motion, Aristotle invented the remaining 53 concentric spheres. Each planet was located in...