It is commonly believed that science operates entirely by the test of observation: theories explain observations, but observations are trumps and can override any theory -- even when no new theory is available to explain the observations. This is a myth, since most observations require a theoretical framework or conceptual model to make sense, and some observations can be explained by more than one theory or model. For this reason, scientists are reluctant to accept models -- or even observations -- that fly in the face of an accepted theory, especially if that theory is well supported.
In ancient times, it was obvious that the Moon went around the earth. This was obvious because it is true.
The ancients also noticed the Sun moving around the earth -- a perfectly reasonable explanation of its apparent motion, and the simplest one available. When other moving heavenly bodies -- as distinct from the "fixed stars" -- were discovered, it seemed obvious that they, too, went around the earth; this is the geocentric model of the universe. The ancients had deduced the correct shape of the earth by about 700-600 B.C., and the size of the earth was correctly estimated by Eratosthenes of Alexandria in about 500 B.C.
There was a physical theory to explain both the geocentric model and the spherical Earth. By 500 B.C. or so, the most widely accepted scientific ideas held that the world below the Moon was made of four elements (earth, air, water, fire), which "gravitated" toward the center. It was pretty obvious that if you let go of anything except air or fire, it fell. Furthermore, fire rose through air, so air must naturally tend to be lower than fire. This meant that the earth was made of, in ascending order, a sphere of earth, a sphere of water, a sphere of air, and a sphere of fire -- which was blamed for the "char marks" on the moon. On the other hand, the heavenly bodies did not fall. Therefore, they must be made of something different (the fifth element or "quintessence"). Because the heavenly bodies were observed to move around the earth, this fifth element must have the property of moving in circles.
In the second century A.D., Claudius Ptolemy organized astronomical and physical knowledge into what is now known as the Ptolemaic system (Figure 1). The earth was at the center, because that is what both observation and theory indicated. Heliocentric (sun-centered) models had been proposed for religious reasons (Sun God), but they were not supported by the evidence or by theory. Ptolemy's model stood the test of naked-eye observation for many centuries.
By the sixteenth century A.D., naked-eye observations had improved, and problems were becoming apparent in the geocentric system. Because everybody assumed that heavenly bodies moved in perfect circles, observational discrepancies were explained by adding epicycles, or circles on circles. Some versions of the Ptolemaic system had three or four levels of epicycles, and Nicolaus Copernicus (1473-1543) realized that a heliocentric system (Figure 2) could eliminate one or two, but not all, levels of epicycles.
Thus, a heliocentric universe was inferred from the facts that the geocentric system was becoming unwieldy and that a heliocentric model provided a simpler explanation. There was no observational evidence to unequivocally prove, or even to indicate, that the earth moved around the Sun! …