Aristarchus of Samos
c. 310-c. 230 B.C.
Greek Astronomer and Mathematician
Aristarchus is famous for developing the first heliocentric planetary theory. For this, he has come to be known as the "Copernicus of antiquity." He also made the first rational estimates of the distance to the Sun and Moon as well as the size of those bodies.
Very little is known of Aristarchus's personal life. He was born on the Aegean island of Samos sometime around 310 B.C. He made his way to Alexandria sometime before 287 B.C. There he studied under Strato of Lampsacus (d. c. 270 B.C.). His only surviving work is On the Size and Distances of the Sun and Moon. The details of his heliocentric theory were preserved by Archimedes (287-212 B.C.) in The Sand-Reckoner.
Aristarchus was first to attempt a determination of astronomical distances and dimensions by geometrical analysis. The basis of his method was the realization that at the Moon's quadrature—when exactly half-illuminated by sunlight—the Sun (S), Moon (M), and Earth (E) occupy the apices of a right triangle. Angle SME is a right angle, and Aristarchus believed angle MES could be observationally determined. From this information, angle ESM could then be deduced as well as the ratios of the solar and lunar distances.
Though Aristarchus's mathematical reasoning was flawless, the necessary observational techniques did not exist. First, he had no way of determining the precise moment of quadrature. Second, no instrument was capable of measuring angle MES with sufficient accuracy. Small errors in either value would result in seriously inaccurate results. In fact, his conclusion that the solar distance was between 18 and 20 times greater than the lunar distance was approximately 200 times too small.
In On the Size and Distances Aristarchus also attempted to determine the diameters of the Sun and Moon. By noting the size of Earth's shadow cast during an eclipse of the Moon, he determined the lunar diameter to be one-third that of Earth. Though his geometrical argument was again sound, inaccurate measurements meant this estimate was slightly too large. However, his estimate that the Sun's diameter is seven times that of Earth's was grossly in error—the actual value is closer to 100 times. Nevertheless, the fact that the Sun was larger than Earth may have suggested to him the possibility that Earth traveled about the Sun.
The groundwork for such an idea had been prepared by Pythagorean philosophers. Philolaus of Crotona (fl. 440 B.C.) postulated a universe of concentric spheres at the center of which was a central fire. Earth, an anti-Earth, and the other heavenly bodies, including the Sun, all moved in circular orbits about this central fire. Furthermore, Hicetas of Syracuse (fl. fifth century B.C.) attributed an axial rotation to Earth.
Aristarchus combined these ideas into a true heliocentric model. His universe was spherical with a stationary Sun at its center and the stars fixed at the periphery. Following Hicetas, he had Earth rotate about its axis. He then introduced the revolutionary concept of Earth traveling in a circular orbit about the Sun.
Earth's orbital motion implied solar and stellar parallax. Aristarchus argued, respectively, that Earth's orbital radius was so small in comparison with the Sun's distance and the distance of the stars so great that neither effect was large enough to observe. Though indeed prescient, Aristarchus's theory failed to explain the inequality of the seasons and other phenomena better handled with epicycles in a geocentric model. Thus, the heliocentric hypothesis attracted little attention, that is, until Nicolaus Copernicus (1473-1543) reinvented it 18 centuries later.
Vitruvius (fl. c. 25 B.C.) credited Aristarchus with inventing the widely used skaphe sundial. Aristarchus also developed the first geometric procedure for approximating the sine of small angles.
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