VENUS ON THE SUN'S FACE. 71 



for then, if we make the least error in observing the 

 bearings of the object, we get an important error in 

 the resulting determination of the distances. The 

 reader can easily 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 were 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 dis- 

 tance of five miles. Would its length be appreciable, 



