ASTRONOMY. 201 



therefore not directed to the point from which the 

 ray comes, and the line of collimation (the line in 

 the tube along which the object is viewed) must 

 be inclined, so as to have the direction of the path 

 of the ray ; or of the diagonal just mentioned. 



The angle thus contained between the line drawn to 

 the object, and the line of the tube in which the 

 object is seen, is called the Aberration. 



210. When the motion of the tube, or of the 

 earth, is perpendicular to the motion of the raj, 

 the aberration is a maximum, and is measured 

 by the arch which the earth describes in its or- 

 bit, in the time which light takes to come to it 

 from the sun. 



In 8' 13", light moves over the radius of the earth's 

 prbit : and, in the same time, the earth moves over 

 an arch of 20".23'2, which is therefore the maxi- 

 mum of the aberration ; and this is found by ob- 

 servation to be accurately true. 



211. Every star appears to describe an ellipsis 

 in the heavens, of which the true place of the 

 star is the centre ; the semi-transverse axis, 

 20 V .232, in the direction of a tangent to the pa- 

 rallel of latitude of the star ; and the semi-con- 

 jugate axis, 20". 232 x sin Lat. of the star. 



SIMPSON'S Essays, 1740, p. 1. &c. 



If 



