54 LIGHT SCIENCE FOR LEISURE HOURS. 



convince himself of this by drawing an illustrative case 

 or two on paper. 



The astronomer has to take his base-line for deter- 

 mining the sun's distance, upon our earth, which is 

 quite a tiny speck in comparison with the vast distance 

 which separates us from the sun. It had been found 

 difficult enough to determine the moon's distance with 

 such a short base-line to work from. But the moon is 

 only about a quarter of a million of miles from us, while 

 the sun is more than ninety millions of miles off. Thus 

 the problem was made several hundred times more 

 difficult or, to speak more correctly, it was rendered 

 simply insoluble unless the astronomer could devise 

 some mode of observing which should vastly enhance 

 the power of his instruments. 



For let us consider an illustrative case. Suppose 

 there was a steeple five miles off, and we had a base- 

 line only two feet long. That would correspond as 

 nearly as possible to the case the astronomer has to deal 

 with. Now, what change of direction could be observed 

 in the steeple by merely shifting the eye along a line of 

 two feet? There is a ready way of answering. Invert 

 the matter. Consider what a line of two feet long 

 would look like if viewed from a distance of five miles. 

 Would its length be appreciable, to say nothing of its 

 being measurable ? Yet it is just such a problem as the 

 measurement of that line which the astronomer would 

 have to solve. 



But even this is not all. In our illustration only 

 one observer is concerned, and he would be able to use 



