178 ELEMENTS OF SCIENCE 



dimensions of the two lines AO and BO and so the 

 distance of the object 0. 



By an extension of this principle to the observation of 

 the angles formed by lines passing from any one spot 

 on the earth's surface to some fixed star (a process which 

 cannot be explained here), it has been ascertained that the 

 earth is a globe with a circumference of 24,840 miles. 

 We can ascertain this because the distance of the fixed 

 stars is, as we have seen, so enormous that they remain 

 for our observations always practically in one relative 

 place. 



Having ascertained the earth's dimensions we can 

 treat its diameter as we before supposed the two ends of 

 the measured line AB to be treated; and so ascertain 

 the angles formed by it with lines passing from its 

 extremities to the moon, and when once the distance 

 of a body is known, we can readily find its size and vice 

 versa, by the simplest application of the before-mentioned 

 principle of " similar triangles." 



By careful observations of the apparent path of the 

 planet Venus across the sun's disc (in what is known as 

 the transit of Venus), as seen from two spots on the 

 earth's surface, it was determined in 1761 and 1769 that 

 the sun's distance from the earth was from 93,274,000 

 to 96,432,000 miles. This being ascertained, it became 

 easy to ascertain the size of the earth's orbit, which then 

 provided an enormously larger and more useful base 

 for triangular measurements. Our measured line AB 

 might now be taken as 190,000.000 miles (such being 

 the diameter of the earth's orbit), observations from the 

 opposite extremities of which, as to the angles formed 

 by it with more distant heavenly bodies, served for 

 further investigation of dimensions and distances. But 

 so distant are the fixed stars that even with this 



