The Early Greek Astronomers

Thales of Miletus, Philolaus of Croton, Eudoxus of Cnidos

George Pararas-Carayannis

(Excerpts from series of articles published in the Hellenic Free Press, Chicago, 1960)


For thousands of years people looked up to the skies and wondered about the Sun, Moon, planets, stars, and their complex ballet of motions. The first known attempt to explain these apparent motions of celestial bodies in scientific terms, is to be found in Greece, in the fifth century B.C. The Ancient Greeks were the founders of the majority of today's mathematics and astronomy. Early Greek philosophers/scientists/mathematecians laid down the first principles of mathematics. In the time span from 600 B.C. to 250 B.C. more break-through accomplishments were made in mathematics than in any other time span of the same length.

Through their mathematical principles, these early Greeks laid the foundations and invented methods to explain celestial motions. Thales, Euclid, Pethagoras, Anaxagoras, Aristarchus, Aristotle, Apollonius, Archimedes, Aristarchus, Aristotle, Autolycus of Pitane, Callippus, Cleomedes, Conon, Democritus, Eratosthenes, Eudoxus, Eutocius, Geminus, Heraclides of Pontus, Heron, Hipparchus, Hypsicles, Menelaus, Pappus, Plato, Porphyry, Posidonius, Proclus, Simplicius, Theodosius, Theon of Alexandria,Theon of Smyrna , Ptolemy and many others, founded the majority of today's mathematics and astronomy. The best known of these early Greek astronomers and mathematicians are those who contributed to the development of modern scientific results.

The first known attempt to explain the apparent motions of celestial bodies in scientific terms, is to be found in Greece, in the fifth century B.C.


The Pythagoreans - Pythagoras 's followers - held that the earth is a sphere, whereas Thales, who lived 100 years earlier, believed it to be a disk afloat on the ocean. They also thought that all celestial motions are circular, geometric, and of constant speed. However, Philolaus of Croton, a Pythagorean, was the first to discard the view that the earth is the center of the universe, but retained the motion of circular orbits. He assumed that all the known bodies - the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn- revolved around a central fire. As he explained it, the fire could not be seen from Greece because that side of the earth was always away from it. But these ideas were too contrary to common-sense experience to be readily accepted. Philolaus had few followers.



In the fourth century B.C., the standard-bearer of Greek cosmology was, without a doubt, Eudoxus of Cnidos, who has been called the founder of scientific astronomy. The theory of Eudoxus was a complex mathematical attempt to explain the motions of celestial bodies.


Eudoxus was born in Cnidos at the year 408 B.C. He studied geometry and medicine, and became a pupil of Plato at the age of twenty-three. After learning Pythagorean astronomy at the newly founded academy, he traveled to Egypt where he remained for some time. There he found the opportunity to supplement by his own observations (at an observatory near Heliopolis) the already extensive material that he secured from Egyptian astronomers. The geometric interpretation of celestial motions devised by Eudoxus is called the "theory of homocentric spheres". By this theory, he attempted to account for all the peculiarities in the apparent paths of celestial bodies, and particularly for the "loops" in the sidereal motion of planets.

Eudoxus retained the Pythagorean principles of a static earth and of uniformly circular celestial motions. However, since the apparent orbits of planets depart appreciably from a circle, he attempted to explain these vagaries as combinations of circular motions. He devised then the "theory of concentric spheres" giving, thus, a rational explanation of what may be happening. Within the accuracy of observation available in his day, Eudoxus' theory gave a satisfactory account of the apparent motions of Venus, Mars, Jupiter and Saturn with the possible exception of Mars. This planet's orbit, being somewhat more elliptical than those of Jupiter and Saturn, lent itself less readily to a description in terms of circular motions. Eudoxus' system of homocentric spheres gained wide acceptance. In the ensuing centuries, this original system was modified gradually by the introduction of new data and the increasing accuracy of observations. It was adopted by Aristotle without essential alterations and was superseded only by the theories of Hipparchus in the second century B.C.









AristotleAristotle and Plato at The Peripatetic School of Athens (Painting by Raphael)


EuclidThe Great Orion (US Naval Observatory)


Greek Asto Vaticanmebula

Ptolemy's Astrolabe




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