i2z A Short History of Astronomy [Cn. iv. 



observation were sufficiently improved to measure with some 

 accuracy the apparent sizes of the sun and moon, and so 

 check the variations in their distances. But any variation 

 in the distance of the earth from the sun would affect not 

 merely the distance, but also the direction in which a planet 

 would be seen ; in the figure, for example, when the planet 

 is at P and the sun at s, the apparent position of the planet, 

 as seen from the earth, will be different according as the 

 earth is at E or E'. Hence the epicycles and eccentrics of 

 Coppernicus, which had to be adjusted in such a way that 



E E' S 



FIG. 49. The alteration in a planet's apparent position due to an 

 alteration in the earth's distance from the sun. 



they necessarily involved incorrect values of the distances 

 between the sun and earth, gave rise to corresponding 

 errors in the observed places of the planets. The obser- 

 vations which Coppernicus used were hardly extensive or 

 accurate enough to show this discrepancy clearly ; but a 

 crucial test was thus virtually suggested by means of which, 

 when further observations of the planets had been made, 

 a decision could be taken between an epicyclic representa- 

 tion of the motion of the planets and some other geometrical 

 scheme. 



91. The merits of Coppernicus are so great, and the part 



