ON THE HISTORY OF ASTRONOMY. $93 



the zodiac. Aristarchus of Samos was the next; he imagined a method of 

 finding the sun's distance, by observing the portion of the moon's disc that, is 

 enlightened, when she is precisely in the quadrature, or 90° distant from the 

 sun; and although he failed in his attempt to determine the sun's distance 

 with accuracy, yet he showed that it was much greater than could at that 

 time have been otherwise imagined; and he asserted that the earth was but 

 as a point in comparison with the magnitude of the universe. His estimation 

 of the distance of the sun is made by Archimedes the basis of a calculation 

 of the number of grains of sand that would be contained in the whole 

 heavenly sphere, intended as an illustration of the powers of numerical 

 reckoning, and of the utility of a decimal system of notation, which was the 

 foundation of the modern arithmetic. 



Eratosthenes, the successor of Aristarchus, is known by his observation 

 of the obliquity of the ecliptic, and his measurement of a certain portion 

 of the earth's circumference ; the whole of which he determined to 

 be 250 000 stadia; but the length of his stadium is uncertain. Pto- 

 lemy, calculating perhaps from the same measures, or from some others 

 still more ancient, calls it 180 000; which, if the stadium is determined from 

 the Nilometer at Cairo, and from the base of the pyramid, is within one 

 thousandth part of the truth, the length of the base of the pyramid being equal 

 to 400 Egyptian cubits, or to 729 feet 10 inches English. 



Hipparchsu of Bithynia flourished at Alexandria about the year 140 before 

 Christ. Employing the observations of Timocharis, and comparing them 

 with his own, he discovered the precession of the equinoxes. He also 

 observed that the summer was 9 days longer than the winter, and that the 

 solstices divided each of these seasons a little unequally. In order to explain 

 this, Hipparchus supposed the sun to move uniformly in an eccentric circle, 

 the distance of its centre from that of the earth being -^ "of the radius, and 

 placed the apogee in the sixth degree of gemini. Probably the annual equa- 

 tion of the moon, which has some influence on the time of eclipses, was the 

 cause of his making the eccentricity too great; had he assumed it but one 

 fifth part less, the supposition would have represented the sun's place with 

 tolerable accuracy. Hipparchus appears to have been the first that employed 



