176 ELEMENTS OF SCIENCE 



see of them we only see what existed more than one 

 hundred years ago. 



This great and uniform speed of light combined with 

 the motion of the earth, causes what is known as the 

 aberration of light.* It is common to all the heavenly 

 bodies, and causes them all to appear a little out of their 

 true place. While the light is travelling from any star 

 towards an observer, the observer himself is being simul- 

 taneously carried along with great rapidity by the earth, 



The result is that if he directs his telescope exactly 

 towards a star, the light which enters his instrument 

 will strike the side of it before it can reach his eye. 

 This has therefore to be allowed for, and the instrument 

 accordingly directed a little in advance of the object 

 observed, just as a sportsman has to shoot in front of a 

 running hare, to allow for its change of place during the 

 passage of a shot. 



The di&tances and sizes of the nearer heavenly bodies, 

 and the size of the earth itself, have been ascertained by 

 the application of geometrical principles, and mainly 

 that one which teaches us that two triangles, however 

 different in size, are in other respects exactly similar to 

 each other provided the angles of one are the same as 

 the angles of the other. 



Thus if we require to know the exact distance of some 

 object on that side of a wide river which may be oppo- 

 site to us, we may ascertain it as follows : We must 

 select two spots A and B on our side, and measure the 

 length of the straight line between them. Then plant- 

 ing ourselves at A we must observe, with an instrument 

 made for that purpose, a distant object, 0, and ascertain 

 the angle formed by the line AB with that which 



* Before referred to, p. 103. 



