DISCOVERY 



329 



circle with centre o was named an " epicycle," and 

 the bigger circle upon which o tra\-elled was known 

 as the " deferent." It was further laid down that 

 the line joining o and j must rotate at such a speed 



as to remain always parallel to the line joining E and s, 

 so that J went round o once every time that s went 

 round e, that is, once a year. It is evident that 

 such an arrangement will give rise to the loop-like 

 motion, the forward movement being obtained when 

 J is outside the deferent, the backward when it is 

 inside. 



This explanation holds for Mars, Jupiter, and 

 Saturn, which the ancients regarded as being outside 

 the orbit of the sun. The case of Mercury and Venus, 

 however, is somewhat different. These bodies were 

 recognised to lie between the earth and the sun, 

 and to explain the observed fact that they were never 

 seen very far from the latter, never, for instance, 

 being in opposition, Ptolemy said that the line ov 

 need not be parallel to es — the condition laid down 

 in the case of the outer planets — but that, instead 

 of it, the centre of the \'enusian epicycle, o, must 

 always lie on the line es (see Fig. 2). Precisely 

 the same explanation applied to Mercury, and the 

 Ptolemaic system is completed when we add that the 

 moon was supposed to revolve round the stationary 

 earth in a simple circle. 



The origination and building up of such a system 

 was a veritable triumph of the human mind. Ptolemy's 

 scheme, indeed, was far more ingenious and complex 

 than the simple arrangement of Nature. This is 

 not infrequently the case with human hypotheses, 

 for Nature is at once immensely great and intenselv 

 simple. In the early da3-s of astronomv the Ptolemaic 

 explanation of the movements of the planets fitted 

 very well with what could be observed and with the 

 measurements that had been made. But as time went 

 on and observation became more accurate, slight 

 irregularities were noticed that could not be explained 

 by the epicycle rotation. The original theory was 

 therefore modified ; it was stated that the earth was 

 not quite at the centre of the concentric circles, nor 

 was o quite at the centre of the epicycle. Thus the 



ciicles were now eccentric, and the movements were 

 eccentric, precisely the same kind of thing as may be 

 seen in that part of an engine to which we give the name 

 " eccentric." The Ptolemaic system, thus adjusted, 

 accounted for what could then be measured, but 

 observations were always becoming more exact and 

 further irregularities were always being detected. To 

 meet these new ditficulties, the Arabian astronomers 

 introduced new epicycles, and superposed them on the 

 old ones, and kept on adding more and more until the 

 system became exceedingly complicated and abso- 

 lutely out of harmony with the fundamental simplicity 

 of Nature. 



A humorous story is told of King Alphonso of Spain 

 in this connection. He had given orders that tables 

 should be compiled to show the motions of the planets. 

 When the astronomers had finished their work and had 

 taken the tables to the King, they looked so very 

 complex that the monarch is reported to have said, 

 " I wish I had been present at the Creation, I would 

 have given some good advice." 



A great change came over the spirit of science with 

 the Renaissance at the beginning of the sixteenth 

 century, and a corresponding change was seen in its 

 statements and results. The ideas of Ptolemy that 

 had flourished so long were rejected altogether by 

 Copernicus, who showed that the complicated explana- 

 tion involving epicycles and deferents was quite 

 unnecessarv, and that for it there could be substituted 



Fig. 3. 



a much simpler one, if it were assumed that the 

 planets went round the sun and that the earth rotated 

 on its axis once a day. These two assumptions 

 completely revolutionised astronomy and laid the 



