THE GREAT STAR MAP ^5; 



a result closely like that at which Mr. Hinks arrived twenty 

 years later. But there are other measurements which bear an 

 interesting relation to this direct measure of distance, especially 

 those which compare the velocity of the earth in its revolution 

 round the sun with the velocity of light. This comparison can 

 be made in two entirely independent ways which take account 

 of two entirely independent movements of a star, one directly 

 in the line of sight and the other at right angles to it. Until the 

 invention of the spectroscope, the latter was the only movement 

 of a star, real or apparent, of which we could take account. It 

 was discovered by Bradley early in the eighteenth century that 

 by noting the apparent changes in direction of any star, we could 

 find the ratio of the earth's velocity to that of light; for the 

 aberration, as he called it, was due to the relation of these two 

 velocities. Hence if we can find the velocity of light by indepen- 

 dent means, we can deduce the velocity of the earth and from 

 this the length of its path during one year ; from this the radius 

 of its orbit can be found, which is the quantity we seek. 

 Now the velocity of light has been successfully measured by 

 terrestrial experiments with sufficient accuracy, so that the dis- 

 tance of the sun can be deduced from measures of aberration. 

 But curiously enough the value so found does not quite accord 

 with that given by Sir David Gill and confirmed by Mr. Hinks. 

 The discrepancy has been rendered more remarkable within the 

 last year or two by the successful measurement of aberration by 

 the other method, using the spectroscope, which enables us to 

 measure the velocity of a star in the line of sight. The velocity 

 thus found is partly that of the star and partly that of our earth ; 

 in many cases (though not in all) we may consider that of the 

 star as steady but that of the earth varies during the year, being 

 sometimes towards a particular star and sometimes directly away 

 from it. By comparing the observations on these two occasions, 

 we can eliminate the steady velocity of the star and deduce the 

 velocity of the earth alone, from which we get, as before, the 

 distance of the sun. 



Now measures made recently on this plan have given a result 

 in satisfactory accordance with that of Gill and Hinks, and have 

 thus rendered the isolation of the other result from aberration 

 the more remarkable. There are some who think that the dis- 

 crepancy will ultimately lead us to the discovery of some new 

 phenomenon about which we are at present entirely in the dark. 



