62 Britain's Heritage of Science 



star examined described an elliptic curve similar to that 

 observed with Molyneux's telescope, but the difference? 

 in size and shape did not agree with the hypothesis he had 

 formed. At last the true explanation occurred to him. 



Owing to the fact that light is not transmitted instanta- 

 neously, a star is not actually seen in the direction in which 

 it would appear if light took no time in its passage to the 

 earth The cause of this curious effect may be illustrated by 

 a familiar analogy. A person driving in a carriage during a 

 shower of rain on a windless day, though the drops fall 

 down vertically will feel them striking against his face, as if 

 he were meeting the wind. Hence, holding up an umbrella 

 to shield himself, he would have to tilt it forwards and if 

 he were unaware of his own motion, he would believe that the 

 drops fall at an angle slightly inclined to the vertical. Sub- 

 stituting Newton's corpuscles of light for the drops of rain, 

 it becomes clear that the velocity of the earth affects the 

 angle at which the light coming from a star seems to reach 

 us. This effect is called the " aberration of light." As the 

 earth's velocity changes in direction while it revolves round 

 the sun, a star, though stationary, will appear to describe 

 a closed curve. From the known velocity of the earth, and 

 the extent of a star's apparent motion, the velocity of light 

 may be calculated, and Bradley found it to agree closely 

 with that which had been calculated by Roemer from the 

 eclipses of Jupiter's satellites. The accuracy of Bradley 's 

 observations may be appreciated by noting that if the star's 

 position in the sky be such that it appears, owing to the 

 aberration of light, to describe a circle, the angular diameter 

 of the circle is about that of a halfpenny piece placed at a 

 distance of 420 feet; the dimensions of the curve described 

 by the star were measured by Bradley with an accuracy of 

 about two per cent. 



After Bradley had established himself at Greenwich 

 Observatory, he continued his observations, and found that 

 the stars after a year's interval did not return to the same 

 position, as they ought to do if the aberration of light were 

 the only cause of their apparent displacement. Returning 

 to his original idea of a small change in the inclination of 

 the earth's axis, he then found it to account satisfactorily 



