The New International Encyclopædia/Aristarchus of Samos
AR'ISTAR'CHUS (Gk. Άρίσταρχος, Aristarchos) of Samos. A celebrated ancient astronomer of the Alexandrian School, who made his observations about B.C. 280-264. All his writings have perished, excepting a short essay on the sizes and distances of the sun and the moon. In this he shows the method of estimating the relative distances of the sun and moon from the earth, from the angle formed by the two bodies at the observer's eye when the moon's phase reaches exactly the first or third quarter; i.e., when we see a half moon.
Remembering that the moon's light is simply reflected solar light, it is easy to see from the annexed figure that the three bodies must then form a right-angled triangle, with the moon at the right angle. The angle MES being then observed, we can readily calculate the ratio EM to ES. This is quite correct in theory; but the impossibility of determining when the moon is exactly half illuminated renders the method inaccurate in practice. Besides, in the days of Aristarchus there were no instruments for measuring angles with anything like accuracy. Aristarchus estimated the angle at E at 83° and determined EM to be one-twentieth of ES, the truth being that the angle at E differs only by a fraction of a minute from a right angle, and that EM. the distance of the moon from the earth, is about 1·400 of ES, the distance of the sun. According to some accounts, Aristarchus held, with the Pythagorean School, that the earth moves around the sun. Vitruvius speaks of Aristarchus as the inventor of a kind of concave sun-dial. His essay was first published in Latin (Venice, 1498), then in Greek (Oxford, 1688). and it has since been republished.