ALEXANDRIAN SCIENCE. 49 



in a circle, but the position of the earth was not at the centre of this 

 arch, but nearer to one part of the circle than to another. This is the 

 theory of excentrics. The sun's motions 

 admit also of the following explanation, 

 which was that advanced by Ptolemy. 

 Suppose E (Fig. 17) to be the earth's 

 position, and that the sun revolves about 

 it, not simply in the circle A B c D, which 

 is the path described by the centre c of 

 a smaller circle in which the sun revolves, 

 while the small circle is carried uniform - 

 ly round the larger one A B c D. The 

 smaller circle is called an epicycle. Now, 

 the moon's motion is much more com- F 



plex than that of the sun, and one of 

 the inequalities (due to the form of its 



orbit) had been discovered by Hipparchus. Ptolemy detected another, 

 called the evection, of a more complicated kind, and depending on the 

 angular distance between the sun and moon. To explain these in- 

 equalities in the moon's motion, Ptolemy was obliged to resort to a 

 very complex combination of eccentrics and epicycles, and when he 

 came to deal with the movements of the planets, an increased com- 

 plexity in the systems of excentrics, epicycles, and deferents was 

 necessary. 



This extremely complicated system arose from Ptolemy's binding 

 himself by the dogma of the celestial motions being always circular 

 and uniform. It was not unnatural that in the infancy of astronomical 

 science all the movements of the heavenly bodies should have been 

 conceived to take place in circles at an unvarying velocity, and such 

 a theory would doubtless offer facilities for calculations which woulct 

 yield results sufficiently corresponding with observation to meet the 

 requirements of that time. But the error which had so long retarded 

 the progress of science, and which lay in mistaking gratuitous and 

 arbitrary assumption for the laws of nature, still showed itself in Pto- 

 lemy's accepting Aristotle's fancies about the perfection of circular 

 motions, incorruptibility of the heavens, etc. Yet the Ptolemaic system 

 of the universe attained the ascendency, and prevailed until modern 

 times, in spite of the increasing perplexities in which every new dis- 

 covery of a hitherto unobserved inequality involved its followers, by. 

 requiring additional machinery, until they had 



the sphere 



With centric and excentric scribbled o'er, 

 Cycle and epicycle, orb in orb. 



Ptolemy has given an account of the instruments used for astro-; 

 nomical observations at Alexandria. The use of the gnomon, a ver- 

 tical pillar by the shadow of which the sun's altitude may be deter^ 



4 



