Advances in Hellenistic astronomy were made when the ancient Greeks considered theories of an Earth-centered and a Sun-centered universe; the geocentric epicycle-on-deferent system proved to explain the most observations.

That Earth was spherical was known to learned Greeks of the fourth century b.c.e. by the shape of its shadow on the moon during a lunar eclipse. The accepted view of the universe, however, was that Earth remained unmoving at its center, while around it in concentric spheres moved the seven planets of the ancient world: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. About 340 b.c.e. at Athens, Heraclides of Pontus postulated that the earth rotated daily on its axis and that the Sun and the other planets revolved around the Earth. His work “On Things in the Heavens” is lost, so modern scholars do not know how he arrived at these conclusions. Heraclides of Pontus
Aristarchus of Samos
Apollonius of Perga
Eratosthenes of Cyrene
Hipparchus
Ptolemy

This theory was the most advanced position taken by Greek astronomers by the time of Alexander the Great’s conquest of Persia, which opened up a new world to scientists. At Babylon, Uruk, and Sippar, in Mesopotamia, fairly accurate observations of the movements of the heavenly bodies had been recorded and kept for centuries. Part of this mass of new knowledge became known to Greek scientists in the third century b.c.e. The Greeks also had their own means of acquiring data, for among the wonders of the new museum established in Alexandria as a sort of university was an observatory, a simple tower whose only instrument was a device without lenses for measuring the azimuth and angle of height of a star or planet.

A diagram illustrating Aristarchus’s observations and calculations regarding the Sun, Moon, and Earth.

(Library of Congress)

From these small beginnings, Greek astronomers reached astonishing conclusions. Aristarchus of Samos, invited to Alexandria, showed by the use of observations and of plane geometry that the Sun was some three hundred times larger than Earth. This estimate was a considerable improvement over the fifth century b.c.e. estimate that the Sun was about the size of the Peloponnesus. Aristarchus demonstrated his findings through geometrical proofs in his extant treatise Peri megethon kai apostematon heliou kai selenes (c. early third century b.c.e.; On the Size and Distance of the Sun and the Moon, 1913). Having established this fact to his own satisfaction, Aristarchus went on to deduce that the Sun, apparently because it was so much larger than Earth, must itself be the unmoving center of the cosmos, with Earth and the other planets revolving about it in circles, the Moon about Earth, and Earth rotating on its axis. The unmoving fixed stars were at an infinite distance. The book in which he explained his reasons for holding these bold hypotheses is lost and, because his system violated ancient authority and common sense and predicted a shift in the position of the stars that was actually too small to be observed at that time, his ideas were not widely accepted.

Apollonius of Perga, on the other hand, made adjustments to the Earth-centered system that Greeks found to be more reasonable. He proposed the theory that the planets moved in epicycles around imaginary points on spheres called deferents. The points were also supposed to move in spherical orbits around Earth, but their centers were not Earth itself. The complex scheme accounted for variations observed in the speeds of the planets and their distances from Earth. It also explained why a planet sometimes seemed to be moving backward and why that “retrograde” motion coincided with the planet’s brightest appearance.

Meanwhile, at Alexandria and Syene, Eratosthenes of Cyrene conducted an imaginative experiment during which he measured the circumference of Earth to within perhaps less than 2 percent. He noticed that at Syene on the Nile River (modern Aswān) at noon on the summer solstice, the Sun was exactly overhead. His proof began with the observation that then a vertical pole cast no shadow and the bottom of a deep well with vertical sides was completely illuminated. He arranged for an assistant at Alexandria to measure the angle cast by a vertical pole there at the same time on the same day. This angle measured one-fiftieth of a complete turn (7 degrees, 12 minutes), so he concluded that the distance between Syene and Alexandria was about one-fiftieth of the circumference of the earth. Determining this land distance, Eratosthenes then calculated the circumference of the earth as 250,000 stadia. This is an error of only about 250 miles (403 kilometers) according to some scholars’ estimates of the length of a stade. He later changed his estimate to 252,000 stadia, although it is not known on what basis.

Eratosthenes actually made two mistakes: He wrongly assumed that Alexandria and Syene were on the same great circle, and his measurement of the distance between the two cities was inaccurate. Fortunately the two errors tended to cancel each other out, and his method was otherwise sound. Because he also knew that the distance from Gibraltar to India was only some sixty-nine thousand stadia, he made the remarkable prediction that another continental system would be found at the Antipodes by sailing west into the Atlantic Ocean or east into the Indian Ocean, an opinion held later by Christopher Columbus (1451-1506).

Like most of these astronomers, Hipparchus of Nicaea said that he acted “to save the phenomena.” Theoretically, he accepted the geocentric system, but he is most noted for numerous observational contributions. He measured the length of the solar year to within 6 minutes, 14.3 seconds, discovered the precession of the equinoxes, and cataloged more than 850 fixed stars together with their magnitudes into an accurate star map. He estimated the mass of the Sun as 1,800 times that of Earth and its distance as 1,245 Earth diameters, improvements on those of Aristarchus, whose system had otherwise faded away.

The theories of the Hellenistic astronomers reached their culmination in Ptolemy. He added circular orbits and the concept of an equant point to the epicycle-on-deferent model of Apollonius, in part to resolve difficulties raised by the observations of Hipparchus. The equant point was as far from the true center of the universe as was the earth. A planet in orbit swept out equal areas of its circle around the earth in equal times with respect to the equant point. This system, which admittedly involved some complicated mathematics, remained influential into the Renaissance.

The researches of the Hellenistic astronomers laid the groundwork for modern astronomy, and also severed the association between astronomy and religion for the first time. Their investigations were intended to discover how the natural world worked for its own sake rather than to predict and interpret astronomic events as signs of a deity’s intentions or wrath. While the hypothesis of a heliocentric universe was dismissed, the fact that such a hypothesis could be proposed, and that it was one of several competing hypotheses, indicates the radical change in worldview that had taken place within Greek science.

• Aaboe, Asger. Episodes from the Early History of Astronomy. New York: Springer Verlag, 2001. Chapter 2 of this short and accessible book is concerned with the Greek geometrical theories of planetary movement.
• Evans, James. The History and Practice of Ancient Astronomy. New York: Oxford University Press, 1998. This unusual book teaches astronomy by explaining ancient theories and then setting readers experiments to carry out themselves, in order to understand exactly what the ancients were doing and thinking in elaborating their theories.
• Gingerich, Owen. The Eye of Heaven: Ptolemy, Copernicus, Kepler. New York: Springer Verlag, 1993. Gingerich places the Hellenistic astronomers in relation to these three; he also introduces the reader to notable themes and scholars in the history of science.
• Jacobsen, Theodor S. Planetary Systems from the Ancient Greeks to Kepler. Seattle: University of Washington Press, 1999. Reviews the astronomical theories and knowledge of all major Greek astronomers, mathematicians, and philosophers.
• Toulmin, Stephen, and June Goodfield. The Fabric of the Heavens: The Development of Astronomy and Dynamics. Chicago: University of Chicago Press, 1999. Close to half of this history of astronomy is devoted to ancient Greek theories of the universe. Illustrations and index.

Anaximander; Apollonius of Perga; Aristotle; Empedocles; Eratosthenes of Cyrene; Eudoxus of Cnidus; Hipparchus; Nabu-Rimmani; Ptolemy (astronomer); Pythagoras; Sosigenes; Thales of Miletus. Astronomy;Hellenistic