Astronomy Hypertext

Greek Geocentric and Heliocentric Cosmologies

Latest Modification: July 22, 2002

As early as 2000 B.C. the Babylonians had begun recording the movements of planets, and a thousand years later the Greeks had inherited much of that knowledge. The Greeks, who to a great extent are responsible for developing geometry and trigonometry, sought a geometrical explanation of planetary motions rather than the simply numerical relationships found by the Babylonians. For them to see the Earth, which was not in their minds a planet, as the center of a world system was not unreasonable, even though they were aware of the difficult concept of a spherical Earth. A geocentric (Earth-centered) cosmology certainly seems more intuitively obvious than does, say, a heliocentric (Sun-centered) cosmology. For centuries, various Greek schools of philosophy proposed, debated over, and elaborated on several geocentric theories.

Any conceptual scheme had to explain the observed motions of the Sun, Moon, and five naked-eye planets, all of which seemed to wander among the stars. Never changing their course, the Sun and Moon move eastward on the celestial sphere in a somewhat steady fashion relative to the stars. The inferior planets, however, move eastward through the background stars of the zodiac for a time and then undergo retrograde motion, moving westward. The superior planets generally follow an eastward path relative to the background stars with only a brief period of retrograde motion westward. All the planets further confound an understanding of their motion by moving swiftly some times and slowly at other times while displaying noticeable variations in brightness.

Greek philosophers, with their aesthetic taste for symmetry, reasoned that nature arrayed her celestial bodies on the perfect geometric figure, the sphere, and moved them in the perfectly symmetric plane figure, the circle. Beginning with Plato and possibly earlier, generations of astronomers thought that planetary movements must be accounted for by combinations of uniform circular motions with the Earth at the center. With this geocentric concept, they tried a variety of conceptual models to account for the observed motions. The ultimate product of geocentric cosmology was the Ptolemaic system.

The heliocentric concept, which followed the geocentric one, did not originate with Copernicus. He became aware that in the third century B.C. the Greek natural philosopher Aristarchus had proposed the Sun as the center of planetary motion. In his treatise, On the Sizes and Distances of the Sun and Moon, Aristarchus estimated that the Sun is 20 or so times farther from the Earth than the Moon (the actual value is about 400), and since both have approximately the same angular size, the Sun must be 20 times larger than the Moon or, he reasoned, about 7 times the Earth's diameter (the actual value is almost 109 times). From these estimates he apparently thought it natural to put the largest and only self-luminous body in the Solar System, the Sun, at the center of the system. Additionally, Aristarchus attributed the daily movement of the heavens to the rotation of the Earth on its axis. Annual changes in the sky and the planet`s motions could be explained if they and the Earth then revolved about the Sun. Even prior to Aristarchus, the Greeks were aware that the Moon, and possibly the planets, "shine" by reflecting sunlight, a notion they probably came to by observing lunar eclipses. It is also possible that Aristarchus recognized that the stars were self-luminous and conceivably like the Sun only farther away, but this is speculation.

Aristarchus's contemporaries objected to the heliocentric concept for several reasons: If the Earth does move, why don't we feel its motion, which must be exceedingly fast? Moreover, if the Earth moves, why don't we see every unattached object on the Earth's surface sailing swiftly past us in the opposite direction? However, what seems to have been the strongest argument against the heliocentric concept was the failure to see a shift (which would be due to the Earth's orbital motion) in the apparent position of nearby stars relative to more distant stars, a phenomenon known as parallax. We know today that nearby stars do appear to change place, but they are so far away from us that the apparent angular displacement, or parallactic shift, is extremely small, even for the closest stars, and can be measured only with a large telescope.

The fact that the parallactic phenomenon could be adduced as a criticism suggests that the Greeks possibly were aware that the stars are not all at the same distance from the Earth--a point they may have arrived at by arguing that all the stars have about the same brightness and that therefore the fainter ones are the more distant ones. However, even though they may have suspected that the stars are not all at the same distance from us, they apparently did not realize that even the nearest stars are too incredibly distant to reveal parallactic shifts to the naked eye.

Within a couple of centuries after Aristarchus, the heliocentric cosmology had lost out to the geocentric until its revival by Copernicus. Several generations of astronomers at the Museum in Alexandria set for themselves the goal of removing discrepancies between the geocentric concept and observed planetary motions. They felt that their revised geocentric concept must retain the circular motions and uniform rates appearing in such theories as Aristotle's geocentric system. They fashioned their geocentric model on the basis of several earlier natural philosophers, especially Heraclides (388?-?315 B.C.), Apollonius (261?-?190 B.C.), Hipparchus (190?-120 B.C.), and a later one of their own, Ptolemy (A.D. 100?-?170). The resulting system had a number of combinations of circles and off-center motions. To be an acceptable conceptual scheme, the geocentric theory had to represent more accurately the many small cyclic changes and large general motions of the world system. In this they succeeded, and the final version, which dominated philosophical thought for 13 centuries until the time of Copernicus, was published around A.D. 150 in the Syntaxis of Astronomy, an astronomical encyclopedia compiled by the last of the great Alexandrian astronomers Claudius Ptolemy.

© 1995, J. C. Evans
Physics & Astronomy Department, George Mason University
Maintained by J. C. Evans;