130 A SHORT HISTORY OF SCIENCE 



Thus in the figure, C is the centre of the circular deferent, E the 

 earth and E' the equant. The center A of the epicycle travels 

 at such a rate that the line E'A has uniform 

 angular velocity. The planet J travels in 

 an epicycle about A. These assumptions 

 afforded the needful freedom for a fairly close 

 approximation to observed planetary motions, 

 the mathematical computations involved be- 

 coming naturally quite elaborate. Ptolemy 

 disclaimed the power of determining the distances or even the 

 order of the planets. 



That the system as a whole deserves our admiration as a ready 

 means of constructing tables of the movements of sun, moon, and 

 planets, cannot be denied. Nearly in every detail (except the varia- 

 tion of distance of the moon) it represented geometrically these move- 

 ments almost as closely as the simple instruments then in use enabled 

 observers to follow them, and it is a lasting monument to the great 

 mathematical minds by whom it was gradually developed. 



To the modern mind, accustomed to the heliocentric idea, it is 

 difficult to understand why it did not occur to a mathematician like 

 Ptolemy to deprive all the outer planets of their epicycles, which were 

 nothing but reproductions of the earth's annual orbit transferred 

 to each of these planets, and also to deprive Mercury and Venus of 

 their deferents, and place the centres of their epicycles in the sun, as 

 Heraclides had done. . . . The system of Ptolemy was a mere geo- 

 metrical representation of celestial motions, and did not profess to 

 give a correct picture of the actual system of the world. . . . For 

 more than 1400 years it remained the Alpha and Omega of theoretical 

 astronomy, and whatever views were held as to the constitution of 

 the world, Ptolemy's system was almost universally accepted as the 

 foundation of astronomical science. Dreyer. 



After Ptolemy we have no record of any important advance in 

 astronomy for nearly 1000 years. 



In reviewing Greek astronomy Berry says, 



The Greeks inherited from their predecessors a number of observa- 

 tions, many of them executed with considerable accuracy, which were 



