Dante and the early astronomers . the summer solstice of the year 281, andthis tells us the date at which he flourished. He wasrenowned for a very ingenious method by which he ^ It is also mentioned in a comiDilation of philosophers opinions,probably made in the fifth century by Stobaeus, who is very likelyquoting Plutarch. 107 108 ARISTARGHUS. tried to discover how much further from us the sunis than the moon. When the moon is half full theangle sun-moon-earth is a riglit angle, and if the anglesun-earth-moon be measured, by pointing the astrolabefirst to sun and then to moon, the t
Dante and the early astronomers . the summer solstice of the year 281, andthis tells us the date at which he flourished. He wasrenowned for a very ingenious method by which he ^ It is also mentioned in a comiDilation of philosophers opinions,probably made in the fifth century by Stobaeus, who is very likelyquoting Plutarch. 107 108 ARISTARGHUS. tried to discover how much further from us the sunis than the moon. When the moon is half full theangle sun-moon-earth is a riglit angle, and if the anglesun-earth-moon be measured, by pointing the astrolabefirst to sun and then to moon, the third angle, at thesun, may be computed, and then the ratio sun-earthto moon-earth will be known. The method is perfecttheoretically, and if the sun were comparatively near,say about ten times the moons distance, it would bepracticable; but the distance is really so much greaterthat the angle at the sun almost vanishes, and a verysmall error in estimating it causes an error equal tomany millions of miles in the result. It is also. Fig. 20. Method of Aristarchus for finding the distance of the sun. impossible to determine from looking at the moonthe exact time when the division between light anddark is a straight line. Aristarchus made the angleat Earth 87° instead of 89° 50, and this gave the suna distance of about 19 times as far as the moon, insteadof 400 times, which is the true value. The foolish fad (as they thought it) about Earthsmotion, held by this otherwise great man, is describedquite clearly by the two writers above-mentioned. Hesuggested that the stars might be immoveable, and Earthbe turning on her axis at the same time that she moves ina circle round the sun. Moreover realizino^ all that thisimplieswith regard to the immensedistanceof thestars,he ARISTARGHUS. 109 said that the circle in which Earth revolves roundthe sun, compared with the sphere of the stars, isas the centre of a sphere compared with its is, not only Earth, but Earths whole or
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