NEW WA YS OF MEASURING THE SUN'S DISTANCE. 57 



system. "We know only," they might have urged, "the 

 distance of the moon, our immediate neighbour, beyond 

 her, at distances so great that hers, so far as we can judge, 

 is by comparison almost as nothing, lie the Sun and Mer- 

 cury, Venus and Mars; further away yet lie Jupiter and 

 Saturn, and possibly other planets, not visible to the naked 

 eye, but within range of that wonderful instrument, the 

 telescope, which our Galileo and others are using so success- 

 fully. What hope can there be, when the exact measurement 

 of the moon's distance has so fully taxed our powers of 

 celestial measurement, that we can ever obtain exact infor- 

 mation respecting the distances of the sun and planets ? By 

 what method is a problem so stupendous to be attacked ? " 

 Yet, within a few years of that time, Kepler had formed 

 already a rough estimate of the distance of the sun ; in 1639, 

 young Horrocks pointed to a method which has since been 

 successfully applied. Before the end of the seventeenth 

 century Cassini and Flamsteed had approached the solution 

 of the problem more nearly, while Halley had definitely for- 

 mulated the method which bears his name. Long before 

 the end of the eighteenth century it was certainly known 

 that the sun's distance lies between 85 millions of miles and 

 98 millions (Kepler, Cassini, and Flamsteed had been unable 

 to indicate any superior limit). And lastly, in our own time, 

 half a score of methods, each subdivisible into several forms, 

 have been applied to the solution of this fundamental problem 

 of observational astronomy. 



I propose now to sketch some new and very promising 

 methods, which have been applied already with a degree of 

 success arguing well for the prospects of future applications 

 of the methods under more favourable conditions. 



In the first place, let us very briefly consider the methods 

 which had been before employed, in order that the proper 

 position of the new methods may be more clearly recognized. 



The plan obviously suggested at the outset for the solu- 

 tion of the problem was simply to deal with it as a problem 

 of surveying. It was in such a manner that the moon's 



