i68 



Trees, Stars, and Birds 



FIG. no. Determining distances to objects without measuring them. 



amining Figure no you will note that the farther away 

 the tree is the larger are the angles at the two stakes. 



Finding the distances to the heavenly bodies. As- 

 tronomers use the method of the surveyor in finding 

 the distances to the heavenly bodies. Two observers 

 can sight their instruments on the moon from different 

 points on the earth's surface and find the angles which 

 the lines to the moon make with a line connecting the 

 two points on the surface of the earth. They know the 

 distance apart of the observers on the surface of the 

 earth, and they can then calculate the length of the 

 lines which extend to the moon (Fig. 1 1 r) . 



In measuring the distance to fixed stars, which are 

 very distant from us, astronomers take observations at 

 different times of the year and use the distance of the 

 earth from the sun as one side of the triangle. Figure 

 112 will help to make clear how this is done. 



Distances to some heavenly bodies. Suppose that 

 Romulus and Remus, the traditional founders of Rome ; 



