Oenopides

Oenopides of Chios (Greek: Οἰνοπίδης) was an ancient Greek mathematician (geometer) and astronomer, who lived around 450 BCE. He was born shortly after 500 BCE on the island of Chios, but mostly worked in Athens.

Astronomy

The main accomplishment of Oenopides as an astronomer was his determination of the angle between the plane of the celestial equator, and the zodiac (the yearly path of the sun in the sky). He found this angle to be 24°. In effect this amounted to measuring the inclination of the earth axis. Oenopides's result remained the standard value for two centuries, until Eratosthenes measured it with greater precision.

Oenopides also determined the value of the Great Year, that is, the shortest interval of time that is equal to both an integer number of years and an integer number of months. As the relative positions of the sun and moon repeat themselves after each Great Year, this offers a means to predict solar and lunar eclipses. In actual practice this is only approximately true, because the ratio of the length of the year and that of the month does not exactly match any simple mathematical fraction, and because in addition the lunar orbit varies continuously.

Oenopides (crater)

Oenopides is a lunar crater that is located near the northwestern limb of the Moon, and so appears foreshortened when viewed from the Earth. This formation lies due south of the prominent crater Pythagoras, and is attached to the southwest rim of Babbage E. The southwest rim of Oenopides is part of the northern edge of Oceanus Procellarum. To the south is Markov.

This formation is an old crater that has been heavily eroded by subsequent impacts, leaving a low outer rim that is generally hilly and contains a few clefts. There is a gap in the southeastern rim, and the level interior is attached to the lunar mare to the south. There are several small craters lying near the eastern rim, and the remainder of the floor is marked by tiny craterlets.

To the southwest is the remnant of Oenopides R, of which only parts of the rim are still protruding above the surface and the southern rim is completely missing.

Satellite craters

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Oenopides.