SCIENCE OF COMMON THINGS. 251 



How we estimaUthe size and position of distant objects. 



and from C and 7>, and meeting at the eye, forms an angle at the point 

 of intersection, which is the eye. This angle is the angle of vision. As 

 the inclination of the ines proceeding from A and B, and from C and Z>, 

 is the same, the angles will be equal, and the man and the bird will ap- 

 pear of the same size. 



1660 Why does a man on the top of a mountain or church-spire seem 

 ti be no larger than a crow t 



Because the angle made in our eye by the perpen- 

 dicular height of the man at that distance is no larger 

 than that made by a crow close by. 



Let A B (Fig. 59) be a man on a distant mountain or spire, and C I) A 

 crow close by, the man will appear only as high as the line C D, which 

 is the height of the crow. For the same reason the trees and houses far 

 down a street or avenue appear smaller than those near by. 



1661 Why does the moon appear to us so much larger than the stars, 

 though, in fact, it is a great deal smaller ? 



Because the moon is very much nearer to us than any 

 of the stars. 



Fig. 60. 



Let A B represent a fixed star, and C D the moon. The angle of 

 vision, A G B, which the fixed star, A B, makes with the eye is evidently 

 less than the angle of vision, G G D, which the moon makes with the eye. 

 But we judge of the size of a body by the size of the angle, and therefore 

 the moon, which is nearest and makes the greatest angle of vision, ap- 

 pears the largest. A B, though much the larger body, will appear no 

 bigger than E F; whereas the moon (CD) will appear as large as the 

 line, D, to the spectator, G. 



The moon is 240,000 miles from the earth, not quite a quarter of a 

 million of miles. The nearest fixed stars are 20,000,000,000,000 (that is, 

 twenty billions). 



1663 Why does the moon (which is a sphere) appear to be a fiat sur- 

 face f 



Because it is so far off that we cannot distinguish any 

 difference between the length of the rays issuing from 

 'the edge and those which issue from the centre. 



