58 PLEASANT WAYS IN SCIENCE. 



distance had been found, and the only difficulty in applying 

 the method to the sun or to any planet consisted in the 

 delicacy of the observations required. The earth being the 

 only surveying-ground available to astronomers in dealing 

 with this problem (in dealing with the problem of the stars' 

 distances they have a very much wider field of operations), 

 it was necessary that a base-line should be measured on this 

 globe of ours, large enough compared with our small selves, 

 but utterly insignificant compared with the dimensions of the 

 solar system. The diameter of the earth being less than 

 8000 miles, the longest line which the observers could take 

 for base scarcely exceeded 6000 miles ; since observations 

 of the same celestial object at opposite ends of a diameter 

 necessarily imply that the object is in the horizon of both the 

 observing stations (for precisely the same reason that two 

 cords stretched from the ends of any diameter of a ball to a 

 distant point touch the ball at those ends). But the sun's 

 distance being some 92 millions of miles, a base of 6000 

 miles amounts to less than the i5,oooth part of the distance 

 to be measured. Conceive a surveyor endeavouring to de- 

 termine the distance of a steeple or rock 15,000 feet, or nearly 

 three miles, from him, with a base-line one foot in length, 

 and you can conceive the task of astronomers who should 

 attempt to apply the direct surveying method to determine 

 the sun's distance, at least, you have one of their difficulties 

 strikingly illustrated, though a number of others remain 

 which the illustration does not indicate. For, after all, a 

 base one foot in length, though far too short, is a convenient 

 one in many respects : the observer can pass from one end 

 to the other without trouble he looks at the distant object 

 under almost exactly the same conditions from each end, 

 and so forth. A base 6000 miles long for determining the 

 sun's distance is too short in precisely the same degree, but 

 it is assuredly not so convenient a base for the observer. A 

 giant 36,000 miles high would find it as convenient as a sur- 

 veyor six feet high would find a one foot base-line ; but 

 astronomers, as a rule, are less than 36,000 miles in height 



