556 SCIENCE PROGRESS 



was faced, along with many others, with great courage and 

 patience, with the ultimate result that, in the spring of 1909, 

 Mr. Hinks was able to announce to the Paris Academy of 

 Sciences a most satisfactory result for the distance of the 

 planet and, by implication, for that of the sun and of the 

 other members of the solar system. 



He was also able to add a value for the mass of the moon. 

 It may seem strange that this altogether different measurement 

 of mass is to be deduced from the figures which give us a 

 measure of length ; but the fact is that we measure the mass of 

 the moon by noting a certain length, namely, the distance by 

 which it pulls the earth from side to side as it waltzes round 

 with it. The earth and the moon may be compared to a pair of 

 partners dancing round a ballroom ; if they were of equal size, 

 they would swing equally to right and left of their average path ; 

 but the moon is much the smaller and only pulls the earth a very 

 little way from side to side. Nevertheless the oscillation is per- 

 ceptible and it alters the aspect of the planet Eros in the same 

 kind of way as the oscillation of a telescope from one side of the 

 earth to the other. Indeed the two oscillations are combined 

 together in the measures and we only separate them by the 

 fortunate circumstances that one takes place in a day and the 

 other in a month. 



Before leaving the conclusion of this great problem of the 

 planetary distances, which has come down to us through the 

 ages, a word or two may be devoted to its history. The Greeks 

 made attempts to determine the sun's distance but they were 

 very crude : for example Aristarchus of Samos made it only 

 nineteen times the distance of the moon or about 4^ million 

 miles ; and it was long before anything like the true value (about 

 93 millions) was arrived at. In the middle of the nineteenth 

 century the margin of doubt was some millions of miles, but it 

 was expected that the transits of Venus in 1874 and 1882 would 

 reduce this margin within narrow limits. The observations 

 made at these famous transits were however very disappointing, 

 and even before the second of them was due, some astronomers 

 had already turned to other methods for finding the sun's dis- 

 tance, especially the observation of the planet Mars and later of 

 one or other of the small planets. The best determination of 

 this kind, previous to the Eros determination, was that by Sir 

 David Gill at the Cape of Good Hope about 1889, who obtained 



