148 DISCOVERY CH. 



The fundamental principles of the Copernican system 

 are : (1) That the apparent daily motion from east to 

 west of the celestial sphere and the bodies in it, is due 

 to the daily rotation of the earth on its axis from west 

 to east ; (2) That the earth is one of a family of planets 

 revolving round the sun. Neither of these propositions 

 were established by conclusive proof until long after 

 the time of Copernicus. It was understood that if the 

 earth moves around the sun, the stars ought to undergo 

 an apparent annual displacement of position on account 

 of being viewed from different positions. Tycho Brahe 

 endeavoured to detect such a displacement, but unsuc- 

 cessfully, and on that account he rejected the doctrine 

 of the earth's motion. Great improvements had to be 

 made in astronomical instruments before the difference 

 of position of a star as seen from opposite sides of the 

 earth's path around the sun could be measured. Not 

 until about 1840 were observations of sufficient accuracy 

 made to establish this kind of displacement, from which 

 also the distance of a star could be determined. 



It is nob strange that the displacement should have 

 escaped detection for so long now that we know how 

 very minute is the difference of position due to the earth's 

 annual motion. Imagine a circle an inch in diameter 

 to be drawn somewhere on an extensive plain ; let this 

 circle represent the path which the earth traverses 

 around the sun annually ; and let there be a luminous 

 point representing a star four miles away from it. 

 What the astronomer has to measure is comparable to 

 the difference of the apparent position of this point of 

 light as viewed from different parts of the one-inch 

 circle four miles distant. Suppose two lines were 

 drawn from the light to the circle at points as far apart 

 as possible ; the angle between these lines represents 



