Oenopides

490?-420? b.c.

Greek Philosopher

One of the many Greek natural philosophers who benefited from the flowering of culture that accompanied Pericles's reign, Oenopides is perhaps best known for his contributions to mathematical astronomy. Among his accomplishments are determining the angle of Earth's axis relative to the plane of the ecliptic, accurately reporting the length of the lunar month, and calculating the length of the "Great Year," during which the Sun, Moon, and planets all return to the same relative locations in the sky.

Almost nothing is known about Oenopides's early life except that he was born on the island of Chios. He may have visited Athens at some point, but even this is not certain. However, knowledge of his scientific contributions is more certain.

Perhaps Oenopides's best-known contribution to science is his estimate of the angle at which Earth's axis is tilted with respect to the plane of the solar system, also called the ecliptic. Simply looking at the night sky, it is not obvious that all the planets orbit the Sun in a common plane. In fact, this is the case, and this plane is roughly the same as the plane of the Sun's equator, projected into space. Early astronomers tracked the positions of the planets in the sky for centuries, and they noted that the planets always appeared in a relatively narrow swathe of sky. It is this swathe that holds all the constellations of the zodiac and, indeed, these constellations were given special meaning because they are the only ones in which the planets appear.

Looking into the sky (or at a star chart), one can see that these constellations make an arc across the sky. As mentioned above, astronomers determined early on that the planets appeared only along that arc. It wasn't really until Oenopides that the significance of this arc was understood. The arc showed the plane in which the planets were confined, and its elevation in the sky showed how much that plane was tilted with respect to Earth. In fact, Oenopides determined this angle to be about 24°, only slightly more than today's accepted value of 23.5°.

Two of Oenopides's other contributions to science—determining the exact length of the lunar month and the length of the Great Year—are somewhat linked. There is still some debate over whether Oenopides intended his Great Year to refer to the time in which all celestial bodies would return to the same relative positions, or if his observations were limited to the Sun, Moon, and the inner planets. In any event, he was able to show that a Great Year lasted 59 years, and the lunar month was 29.53013 days long (as compared to a modern value of 29.53059 days). With respect to Oenopides's meaning of a Great Year, it certainly included the motions of the Sun, Moon, Venus, and Mercury. Mars and Jupiter also return to nearly the same position, with only Saturn out of place, though by only a few degrees. However, although the planets all have the same positions relative to each other every 59 years, they do not return to the exact same position in the zodiac, suggesting that Oenopides did not intend to chart their motions. In any case, his calculations for the lunar month and the length of the Great Year are impressively accurate.

Oenopides was also important in determining the rules under which the Greeks approached some types of mathematical problems, particularly those involving ruler and compass constructions. He also is known to have espoused the concept of Earth as a living being, with God as its soul. He died around 420 b.c., and was later mentioned in some of the works of Plato (427?-347 b.c.) and other philosophers.

P. ANDREW KARAM