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ARISTARCHUS of Samos was one of the principal astronomers of Alexandria, in the generation before Archimedes and Apollonius. One of his works, entitled Magnitudes and Distances, has come down to us. In this work a most remarkable attempt was made to calculate the distance of the earth from the sun in terms of the earth's distance from the moon. Aristarchus observed that when the moon was in quadrature--i.e. at the moment of half-moon--the observer was in the plane of the circle separating the illumined from the dark portion of the surface. He goes on to maintain that, at this moment, the angle made by lines drawn from the sun and the moon to the earth was 87°. As the angle at the moon of lines drawn from the earth and the sun was at this time 90°, it followed that the remaining angle of the triangle, the angle at the sun, was 3°. The lunar distance being taken as known, the other sides of the triangle could now be calculated. There was no trigonometry in those days, and the solution of triangles was a cumbrous process; but Aristarchus arrived at the conclusion that the sun's distance from the earth was greater than eighteen lunar distances and less than twenty.
The method followed was strictly scientific; but, in the first place, the difficulty of ascertaining the precise moment of the moon's quadrature was too great for the observers of that time; and a still more serious error was made in estimating the angle made at this moment by the position of the sun and moon as seen from the earth. Since the true value of this angle is not 87°, but more than 89° 50', the result of the calculation was of course wide of the mark. Nevertheless the attempt was of the highest importance.
Aristarchus sustained the Pythagorean view of the earth's motion, as we know from a letter of Archimedes to King Gelon, in which the following passage occurs:-- "You are aware that by most astronomers the universe is looked upon as a sphere, of which the radius reaches from the centre of the earth to the centre of the sun. This view was contested by Aristarchus of Samos, who brought forward hypotheses from which it would follow that the universe is many times as great as that which is now supposed. He imagined the fixed stars and the sun to be motionless. He regards the sphere of the fixed stars to be of such magnitude that the whole orbit of the earth compared with it is a mere point."
Thus Aristarchus anticipated the most serious objection made then and long afterwards to the theory of the earth's annual revolution; namely, that the aspect of the heavens remained the same from every part of the orbit: that there was no annual parallax.
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| This biography is
reprinted from The New Calendar of Great Men. Ed. Frederic
Harrison. London: Macmillan and Co., 1920. |
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