1 article for "Aristarchus"

**Aristarchus**[Astro*Index]

(320?-250? BC) Greek astronomer. Born on Samos; died in Alexandria.

The planetary theory of Heracleides held that some of the planets revolved about the sun; Pythagoras held that the Earth also moved. Aristarchus united these theories into a complete heliocentric model for planetary motion. His book on the subject has not survived, but it is mentioned in the works of Archimedes. That these sun-centered views of Aristarchus were known to Copernicus is clear, as the latter mentioned them in a passage which he later eliminated. Other writings of Aristarchus have survived, from which we learn of his theoretical methods for the determination of the size and distance of the moon and sun. When the moon's disk is half-illuminated, the earth, sun, and moon are located at the vertices of a right triangle, and knowing the angle between the sun and moon, the ratios of the lengths of the sides can be determined. Then, if the absolute length of one side is determined by other means, the remaining sides can be computed. He also showed how the actual size of the moon could be determined from the size of the shadow cast by the earth during a lunar eclipse. Unfortunately, he lacked the instruments necessary to accurately measure angles and thus his numerical results were flawed.

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