Aristarchus of Samos
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- For other uses of this name, including the grammarian Aristarchus of Samothrace, see Aristarchus
Aristarchus (310 BC - c. 230 BC) was a Greek astronomer and mathematician, born on the island of Samos, in ancient Greece. He is considered the first person to propose a scientific heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe (hence he is sometimes known as the "Greek Copernicus"). He was influenced by his teacher, the pythagoréan Philolaus of Kroton, but in contrast to Philolaus he had both identified the central fire with the Sun, as well as putting other planets in correct order from the Sun. His astronomical ideas were not well-received and were displaced by those of Aristotle and Ptolemy, until they were successfully revived and developed by Copernicus nearly 2000 years later.
The Aristarchus crater on the Moon was named in his honour.
The only work of Aristarchus which has survived to the present time, On the Sizes and Distances of the Sun and Moon, is based on a geocentric world view. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes another work by Aristarchus in which he advanced an alternative hypothesis of the heliocentric model. Archimedes wrote: (translated into English)
- "You King Gelon are aware the 'universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface."
Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than was assumed in ancient times, which is why stellar parallax is only detectable with telescopes. But the geocentric model was assumed to be a simpler, better explanation for the lack of parallax. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):
- "Cleanthes [a contemporary of Aristarchus] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis."[citation needed]
His hypothesis no longer fit the Greek conceptual system, which had left the speculative period and entered a more authoritative phase. He was struck by a problem that would trickle through the entire history of philosophy – the tension between conservatism and revolution, between what is regarded as certain knowledge and what one cannot know. The most serious discussions along these lines within modern epistemology have been entertained by Thomas Kuhn, Karl Popper, Imre Lakatos, Paul Feyerabend and lately also by the Swedish philosopher Sören Halldén (2005). Aristarchus' ideas fell into oblivion because they led away from the main-stream that had already been laid down by the school forming giants of philosophy: Plato and Aristotle.
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[edit] Distance to the Sun
Aristarchus argued that the Sun, Moon, and Earth form a near right triangle at the moment of first or last quarter moon. He estimated that the angle was 87°. Using correct geometry, but insufficiently accurate observational data, Aristarchus concluded that the Sun was 20 times farther away than the Moon. The true value of this angle is close to 89° 50', and the Sun is actually about 390 times farther away. He pointed out that the Moon and Sun have nearly equal apparent angular sizes and therefore their diameters must be in proportion to their distances from Earth. He thus concluded that the diameter of the Sun was 20 times larger than the diameter of the Moon; which, although wrong, follows logically from his data. It also leads to the conclusion that the Sun's diameter is almost seven times greater than the Earth's, which can be taken to support the heliocentric model: the volume of Aristarchus's Sun would be almost 300 times greater than the Earth, and it therefore would seem illogical that a Sun so large would revolve around a so much smaller Earth. The opposite would more likely be the case.
[edit] External links
- O'Connor, John J..; Edmund F. Robertson "Aristarchus of Samos". MacTutor History of Mathematics archive.
- Biography: JRASC, 75 (1981) 29
[edit] References
- Heath, Sir Thomas. Aristarchus of Samos - The Ancient Copernicus, A history of Greek astronomy to Aristarchus together with Aristarchus' treatise on the sizes and distances of the sun and moon, a new Greek text with translation and notes. (ISBN 0-486-43886-4)
[edit] Further reading
- Stahl, William. (1970). "Aristarchus of Samos". Dictionary of Scientific Biography 1: 246-250. New York: Charles Scribner's Sons.
