RESULTS OF BRITISH TRANSIT EXPEDITIONS. 59 



ground in our books of astronomy as the true distance of 

 the sun, was replaced for a while by an estimate of about 91^ 

 million miles, which has in turn been displaced for an es- 

 timate of about 92^ million miles, it has been said that astro- 

 nomy has very little claim to be called the exact science. It is 

 even supposed by some that astronomy is altogether at sea 

 respecting the sun's distance which, if the estimates of 

 astronomers thus vary in the course of three-quarters of a 

 century, may in reality, it is thought, be very different from 

 any of the values hitherto assigned. Others suppose that 

 possibly the sun's distance may vary, and that the diminution 

 of three or four million miles in the estimates adopted by 

 astronomers may correspond to an approach of the earth 

 towards the sun by that amount, an approach which, if con- 

 tinued at the same rate, would, before many centuries, bring 

 the earth upon the surface of the sun, to be consumed as 

 fuel perhaps for the warming of the outer planets, Mars, 

 Jupiter, and the rest. 



All these imaginings are mistaken, however. The exact- 

 ness of astronomy, as a science, does not depend on the 

 measurement of the sun's distance or size, any more than 

 the accuracy of a clock as a timekeeper depends on the 

 exactness with which the hands of the clock are limited to 

 certain definite lengths. The skill with which astronomy has 

 dealt with this particular problem of celestial surveying has 

 been great indeed ; and the results, when considered with 

 due reference to the conditions of the problem, are excel- 

 lent : but in reality, if astronomers had failed utterly to form 

 any ideas whatever as to the sun's distance, if for aught they 

 knew the sun might be less than one million, or more than a 

 million millions of miles from us, the exactness of astronomy 

 as a science would be no whit impaired. And, in the second 

 place, no doubts whatever need be entertained as to the 

 general inference from astronomical observations that the 

 sun's distance is between 92 and 93 millions of miles. All 

 the measurements made during the last quarter of a century 

 lie between 90 and 95 millions of miles, and by far the 



