THE DISTANCES OP THE STARS. 



291 



If we try to measure the distance of the sun in this way, we 

 can do it, but not very accurately, for the distance of the sun 

 is so great compared to the distance between Greenwich and 

 the Cape that the unavoidable errors in measuring our angles 

 would seriously vitiate the results ; we might get a result 

 within perhaps 5 per cent, of the truth. 



But if we tried to measure the distance of a star in this way 

 we should come to grief entirely, for the unavoidable errors in 

 measuring the angles would be a million times as great as the 

 small angle A 0 B on which the distance essentially depends. 

 The fact is that the base-line between Greenwich and the Cape 

 is so short compared with the distance of the star that the star 

 appears to be in the same direction as seen from both places. 



B 



Thus we cannot measure the distance of a star by using two 

 places on the earth as the ends of a base line, the earth is so 

 incomparably small compared with the distance we wish to 

 determine. 



The problem of measuring the distances of the stars took on 

 a new aspect when it was shown that the earth moved round 

 the sun. Copernicus, in his book, De EevohUionibus, published in 

 1543, showed that the movements of the planets in the sky and 

 the annual recurrence of the seasons were more simply explained 

 if it were admitted that the earth travelled round the sun each 

 year. It was of course a great effort of imagination to conceive 

 of the earth moving in this way, and his views were not readily 

 admitted. They were, however, reinforced very powerfully by 

 Galileo after the discovery of the telescope ; among other things 

 he actually saw Jupiter's moons revolving around Jupiter. He 

 removed many of the difficulties in the way of accepting the 

 Cop ernican system, and in 1632 established the fact that the 

 ear % moved round the sun. There was, however, one real 

 diffi culty which he did not remove, and that was one connected 

 with the distances of the stars. His opponents said: If the 

 earth moves round the sun, then at opposite times of the year, 

 say in January and July, it will be in such widely different 



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