SEQUEL TO THE EPOCH OF HIPPARCHUS. 169 



Sect. 5. Measures of the Earth. 



THERE were, as we have said, few attempts made, at the period of 

 idiich we arc speaking, to improve the accuracy of any of the deter- 

 minations of the early Alexandrian astronomers. One question nat- 

 urally excited much attention at all times, the magnitude of the earth, 

 its figure being universally acknowledged to be a globe. The Chal- 

 deans, at an earlier period, had asserted that a man, walking without 

 stopping, might go round the circuit of the earth in a year ; but this 

 might be a mere fancy, or a mere guess. The attempt of Eratosthenes 

 to decide this question went upon principles entirely correct. Syene 

 was situated on the tropic ; for there, on the day of the solstice, at 

 noon, objects cast no shadow ; and a well was enlightened to the bot- 

 tom by the sun's rays. At Alexandria, on the same clay, the sun was, 

 at noon, distant from the zenith by a fiftieth part of the circumference. 

 These two cities were north and south from each other : and the dis- 

 tance had been determined, by the royal overseers of the roads, to be 

 5000 stadia. This gave a circumference of 250,000 stadia to the earth, 

 and a radius of about 40,000. Aristotle" says that the mathematicians 

 make the circumference 400,000 stadia. Hipparchus conceived that 

 the measure of Eratosthenes ought to be increased bv about one-tenth. 30 



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Posiclonius, the friend of Cicero, made another attempt of the same 

 kind. At Rhodes, the star Canopus but just appeared above the hori- 

 zon; at Alexandria, the same star rose to an altitude of Jg-th of the 

 circumference ; the direct distance on the meridian was 5000 stadia, 

 which gave 240,000 for the whole circuit. We cannot look upon 

 these measures as very precise ; the stadium employed is not certainly 

 known ; and no peculiar care appears to have been bestowed on the 

 measure of the direct distance. 



When the Arabians, in the ninth century, came to be the principal 

 cultivators of astronomy, they repeated this observation in a manner 

 more suited to its real importance and capacity of exactness. Under 

 the Caliph Almamon, 31 the vast plain of Singiar, in Mesopotamia, was 

 the scene of this undertaking. The Arabian astronomers there divided 



o 



themselves into two bands, one under the direction of Chalid ben Ab- 

 dolmalic, and the other having at its head Alis ben Isa. These two 



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parties proceeded, the one north, the other south, determining the dis- 

 \ance by the actual application of their measuring-rods to the ground, 



= a >t C<do, ii. ad tin. 30 Plin. ii. (cviii.) S1 Montu. 3") 7. 



