XXI.] THE SUN. 361 



the diameter of one inch. Hold the disc, or the coin, at 

 such a distance from the eye that it just covers the solar 

 disc; keeping the object, of course, square to the line of 

 sight. It will be found that the required distance is about 

 nine feet. Now on reference to Fig. 117 it will be seen 

 that the object A B has exactly twice the height of the 

 object C D, and that it is also placed at exactly twice the 

 distance ; under these circumstances the two objects appear 

 to the eye at O to have precisely ihi same height. To speak 

 generally, the actual heights of two bodies which have 

 the same apparent height, are directly proportional to their 

 distances. Hence, the distance of the halfpenny bears to 

 the distance of the sun just the same proportion that the 

 actual diameter of the halfpenny bears to the actual diameter 

 of the sun. The actual diameter of the sun is therefore 

 found by a simple nile-of three sum ; ^ provided, of course, 

 that the sun's distance be known. Astronomers have 

 measured this distance, by methods too complicated to be 

 described here, and have found it to be rather more than 

 ninety-one million miles.^ Hence, it follows, that the diameter 

 of the sun — that is to say, the distance measured from 

 side to side through the centre of the sun — is about 852,900 

 miles. The sun's diameter is therefore more than 107 times 

 as great as the earth's diameter. 



This comparison refers only to diameters. If sections of 

 the sun and of the earth were made exactly through the 

 centre of each, the aj-ea of the sun's section would be 107 



1 It will be understood that th'S rough method is simply introduced to 

 illustrate \.\\e. principle on. which such measurements are made. 



2 As the earth is nearer to the sun at one season than at another (p. 

 354), the jneait, -or average distance, may be taken. The sun's greatest 

 distance from the earth is 92,963,000 miles, and its least distance 

 89,897,000 miles; hence the mean is 91,430,000 miles, or about 107 

 diameters of the sun. 



