48 HISTORY OF SCIENCE. 



with the race of Egyptian monarchs, but from the circumstance of his 

 having been a native of Ptolemais in Egypt. He revived the study 

 of astronomy at Alexandria, where little advance in the science had 

 UeerTmade since the time of Hipparchus. He formed the design of 

 collecting all the materials which his predecessors had left behind, 

 and embodying the existing astronomical knowledge into a regular 

 and complete system. The publication of Ptolemy's work, entitled 

 Meya\r) Swrats, " The Great System," forms an epoch in the history of 

 science. This work constituted the basis of all the astronomy of the 

 Middle Ages and, for a considerable time, of that of modern Europe. 

 It is a matter of regretful speculation to consider how much more 

 rapid might have been the progress of astronomy had Ptolemy accepted 

 the Pythagorean opinion, which places the sun and not the earth in 

 the centre. In extenuation of this error, it may be remembered that 

 the ancients were not in possession of so many proofs of the earth's 

 motions as are familiar to us ; yet the arguments which had been ad- 

 vanced by Aristarchus and others ought to have decided the point, if 

 they had been considered in a truly philosophic spirit. The argument 

 advanced by Ptolemy against the earth's motion round the sun, on the 

 ground of the invariable aspect of the fixed stars, had been refuted, as 

 we have seen, by Aristarchus four centuries before. Ptolemy also 

 reasoned from the erroneous and arbitrary theories of motion assumed 

 by the Aristotelean philosophers, contending that if the earth were 

 really in motion, it would leave behind it, riding on the air, all the 

 loose bodies we see on its surface, because they are so much lighter 

 than the earth. This shows how little the Greeks were acquainted 

 with the facts relating to the motions of bodies, and how their best 

 geometers could arrive at false conclusions on such subjects, by reason 

 of assuming false premises. They accepted it as a truth that a heavy 

 body must move faster than a lighter one, whereas now everybody who 

 has heard of the guinea and feather experiment knows that this is not 

 the case. Ptolemy repeats the same argument in opposing the doctrine 

 of the earth's diurnal rotation, which appears to him as ridiculous as 

 the former ; for the earth rapidly revolving from west to east would 

 leave behind it the clouds, the birds in the air, and, in general, all 

 objects not solidly attached to it. A stone thrown eastwards would 

 not, he thought, advance at all, but the earth would gain upon it by 

 reason of its greater velocity. 



Ptolemy therefore pronounces that the earth is fixed and motion- 

 less in the centre, and that the planets revolve round it in the follow- 

 ing order, according to their distances from it: first the Moon, then 

 Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and beyond these the 

 sphere of the fixed stars. 



To account for the observed motions of the moon and planets, 

 Ptolemy adopted and extended the theory of epicycles. Hipparchus 

 had explained the sun's motions by supposing that the sun revolved 



