ARISTARCHUS, one of the most celebrated Greek astronomers, was born at Samos, and flourished about the year 281. According to Vitruvius, (Architect. lib. 1, cap. 1.) Aristarchus was one of the most ingenious and able men of his time, and was the author of many useful inventions.
The Pythagorean philosophers had already determi ned the distance of the sun from the earth, from prin ciples incapable of conducting them even to an approx imate result. Aristarchus, unfettered by these opinions, discovered an ingenious method for solving this impor tant problem. He perceived that when the moon is dichotomised, or when the obscure and enlightened parts of her disc are equal, and are separated by a straight line, a line joining the centres of the sun and moon, will be at right angles with a line joining the centres of the earth and moon, and that, when the cen tres of the sun and earth are joined by a third straight line, a triangle will be formed, the smallest of whose sides is the distance of the moon from the earth, and the great est the distance of the sun from the earth. By observing, therefore, the arch of the heavens intercepted between the sun and moon, all the angles of the triangle become known, and, therefore, the distance of the sun from the earth might be found, in terms of the earth's distance from the moon. Aristarchus found that the arch inter cepted between the sun and moon, when the disc of the lattliwas dichotomised, was 87'; and hence he conclu ded that the sun's distance from the earth was 18 or 20 times the moon's distance from the earth, that is about 4,800,000 miles. This result is only about the 20th part of the sun's distance, as ascertained by more accurate observations ; but the error arises, not from the method itself, but from the imperfections of the instruments with which the Samian philosopher must have determi ned the angles of the triangle.
Aristarchus also found, that the diameter of the earth was to that of the moon, in a ratio greater than that of 108 to 43, and in a ratio less than that of 60 to 19, a re sult sufficiently exact. He estimated the diameter of the sun at the 15th part of a sign, or the 180th part of the zodiac ; but some very considerable error must have been committed in his observation as it is only about the 60th part of a sign, or the 720th part of the zodiac, a result which was obtained even by Archimedes.
Aristarchus had the great merit of reviving the Py thagorean opinion respecting the motion of the earth. He maintained that the sun was immoveable in the cen tre of the universe, and that the orbit of the earth was a mere point, compared with the distance of the fixed stars. In consequence of this opinion, Cleanthes re marked, that he ought to have been charged with im piety ; but it does not appear from the passage in Plutarch, in which this fact is preserved, whether Cleanthes spoke seriously or in jest.
We are informed by Vitruvius, that Aristarchus inven ted a kind of sun-dial, called scaphe, which sheaved the time of the day by means of a stile.
The treatise of Aristarchus on the magnitudes and distances of the sun and moon, was first published by Vallus, at Venice, in 1498 ; afterwards, with the notes of Commandine and Wallis, at Oxford, in 1687 ; and then in Wallis's works, tom. iii. See Vitruvius, ?irchitect. lib. i. cap I. lib. ix. cap. 9. Pappus's Mathemat. Col lect. lib. vi. p. 135. Plutarch De facie in orbe Luna. ; Fabricii Biblioth. Gret'. lib. iii. cap. 5 ; and Montucla's ,list. des Mathcinat. tom. i. p. 228. See also ASTRONO M Y. (0)