Astronomy Becomes a Science

After 323 B.C., the year of the death of Alexander the Great, the Centre of Greek learning shifted from Athens to Alexandria. Another change, and a healthy one, was the appearance of a group of men who felt that theory had too greatly out stripped observation. Astronomy depends to a larger extent on the detailed and systematic observation of the movements of the heavenly bodies, a point which earlier Greeks had tended to overlook.

Foremost in the new approach was Aristarchus of Samos, who about 260 B.C. attempted to determine the sizes and distances of the sun and the moon. He also anticipated Copernicus by suggesting that the sun and celestial sphere were at rest while the planets, the earth included, revolved around the sun. But since his scheme was far removed from the earth-centered of commonsense view, it received little or no support.

The practical approach was also favoured by Eratosthenes, a younger contemporary of Aristarchus and head of the great library at Alexandria. He was the first to measure the diameter of the earth by a sound method based on observation of the sun`s height in the sky, and his result, 7,850 miles, was only 50 miles short of the actual diameter.

About 150 B.C. in Alexandria, the Greek astronomer Hipparchus used measuring instruments to determine the places of objects in the sky with an accuracy greater than ever previously obtained. Altogether he recorded the positions of 1,080 stars and grouped them into six magnitudes according to their brightness. By comparing his own observations with others made about 150 years earlier he discovered the precession of the equinoxes, although in this he was preceded by Kiddinu, a Babylonian astronomer.

Evening Skies

The Evening Sky. Photo: Megan Jorgensen