Geminus

Geminus of Rhodes (Greek: Γεμῖνος ὁ Ῥόδιος), was a Greek astronomer and mathematician, who flourished in the 1st century BC. An astronomy work of his, the Introduction to the Phenomena, still survives; it was intended as an introductory astronomy book for students. He also wrote a work on mathematics, of which only fragments quoted by later authors survive.

Life

Nothing is known about the life of Geminus. It is not even certain that he was born in Rhodes, but references to mountains on Rhodes in his astronomical works suggests that he worked there. His dates are not known with any certainty either. A passage in his works referring to the Annus Vagus (Wandering Year) of the Egyptian calendar of 120 years before his own time, has been used to imply a date of c. 70 BC for the time of writing, which would be consistent with the idea that he may have been a pupil of Posidonius, but a date as late as 50 AD has also been suggested.

The crater Geminus on the Moon is named after him.

Astronomy

Geminus (crater)

Geminus is a lunar impact crater that is located near the northeast limb of the visible Moon. In this position the crater appears oval in shape due to foreshortening, but it is actually more nearly circular in form. Notable nearby craters include Messala to the northeast, Bernoulli due east, and Burckhardt and Cleomedes to the south.

The circular rim of Geminus contain a number of outward notches, particularly to the north and east. The crater ejecta is still visible in the rough surroundings beyond the rim, but any rays deposited during the impact have long since been worn away by space weathering. The inner wall is wide and extensively terraced, although these features are now somewhat muted due to impact erosion. There are no notable impacts on the interior floor, but there is a long, slender central ridge located at the midpoint and a pair of readily observed clefts.

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 Geminus.