Ancient astronomers gave considerable thought to the structure of the universe and to the laws governing the motions of heavenly bodies, looking both for convenient models for the purpose of calculation, as well as deeper explanations of the underlying p rinciples. One debate concerned the determination of the center of the universe. The "universe," or cosmos, of the ancients was similar to what we now think of as the solar system, except that the "fixed stars" were supposed to lie at its outer edge. The Greek philosopher Aristotle (ca. 350 BC) attempted to supply a scientific foundation for the generally accepted, common-sense theory of a geocentric, or earth-centered, universe. A Greek astronomer, Aristarchos of Samos (ca. 330 BC), put forth the first heliocentric, or sun-centered, theory of the universe [4, p.85]. Accompanying this debate over a sun-centered vs. an earth-centered universe was the problem of understanding or predicting the actual movements of the planets around the center, or from the perspective of the earth: the shapes of the paths they followed, their velocities, and their distances from the center. The adoption of a geocentric theory considerably complicated the task of understanding the motion of the planets around this supposed center. Aristotle believed the universe to be built in concentric spheres over a spherical Earth-the sphere being the most "perfect" solid figure, and circular motions being a distinguishing characteristic of perfection and, therefore, befitting descriptions of the perfect heavens.
The Greek astronomer Ptolemy (AD 100-170) was concerned with both understanding the observed planetary motions and allowing them to be calculated accurately and ensured the survival of Aristotle's universe for centuries by fitting to it a sophisticated mathematical model. Today, planetary orbits are known to be elliptical, but for a long time, largely due to Aristotle's influence, there existed a dogma that everything must be explained...