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espoused by Aristarchus, from the island of Samos in Asia Minor. Aristarchus

Very little is known about Aristarchus. According to Hipparchus he observed the time of the summer solstice in 281 BC.

is credited by Archimedes as having postulated that the Earth is not at the centre of the Universe, but that it orbits the Sun. It was Copernicus, in the sixteenth century, who first developed a serious cosmology based on this principle, and so Aristarchus is often referred to as the Copernicus of antiquity. However, the attribution of a fully fledged heliocentric theory to Aristarchus is based actually on extremely scant evidence. Much is made nowadays of the following passage from Archimedes:

But Aristarchus of Samos brought out a book consisting of certain hypotheses, in which the premises lead to the conclusion that the universe is many times greater than that now so called. His hypotheses are that the fixed stars and the sun remain motionless, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same centre 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 centre of the sphere bears to its surface.

Archimedes' work, of which the above quotation forms a part, is a demonstration of his skill in using the Greek numeration system to manipulate very large numbers. Ostensibly, Archimedes was attempting to calculate the number of grains of sand that the Universe could hold and he concluded that this had to be less than the unimaginably large number, 10 63. Clearly, this required a knowledge of the size of the Universe, and Archimedes tried deliberately to overestimate this. He criticized Aristarchus' heliocentric hypothesis because, in order for it to reproduce the observed phenomena; the stars would have to be infinitely far away, contradicting Archimedes' hypothesis that the Universe is finite. The problem is that of stellar parallax: if the Earth is orbiting the Sun, then the observed longitude at which a fixed star is found would vary as the Earth moved around its orbit (see Figure 2.6). Such changes were, however, not observed.

There are many theories, all very speculative, of why the heliocentric theory did not catch on and was superseded so completely by geocentric astronomy. Perhaps the most plausible is the simple fact that the geometrical skill of the Greeks allowed them to devise ingenious constructions that modelled accurately the motions of all the heavenly bodies without having to take the drastic step of removing the Earth from its privileged position at the centre of the Universe.

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