Astronomer; b. c. 310 BC, d. c. after 230 BC (Athens)
Aristarchos was born on the Ionian island of Samos but moved to Alexandria. Only a short work of his entitled "On the Sizes and Distances of the Sun and Moon" survives. His ideas are known through the record of others, mainly Archimedes (who was one of his students) and Plutarch.
Aristarchos was the first Greek astronomer known to have stated that the Earth rotates around its axis and revolves around the Sun. His advanced ideas caused the ire of some of his contemporaries, who suggested that he ought to be brought before a court and tried for blasphemy.
Among Aristarchos' observational achievements was an improved value for the length of the year. His other main observational work, the evaluation of the sizes and distances of Sun and Moon, was less successful, although his measurements were well designed. He used geometry to obtain his values, but faulty observations made them inaccurate:
When the Moon is at dichotomy, ie in its first or third quarter, the angle Earth-Moon-Sun is 90°. Measuring the angle Moon-Earth-Sun from the Sun's position at that moment gives the shape of the triangle formed by the three bodies. Measurement of one of its sides will then determine its size.
Aristarchus obtained the distance and size of the Moon by measuring the time it takes the Moon to pass through the Earth's shadow during a lunar eclipse. His geometrical determinations led him to conclude that the distance Sun-Earth is 19 times the distance Earth-Moon and that the Sun is 19 times the size of the Moon and 7 times the size of the Earth. (Today's value for the ratio of the Sun's size against the Moon is approximately 393:1.)
The errors result mainly from the difficulty to determine the precise moment of the dichotomy of the Moon. Aristarchus' angle measurement of 87° is too small by 2.5°. Thales of Miletus had measured the same angle 300 years earlier to within 0.5° of the correct value. Fragments suggest that Aristarchus came to a similar value but was unsure and wanted to err on the safe side.