THE GREAT STAR MAP 439 



the circling movement of the earth ; but as year follows year the 

 successive steps add together and the cumulative effect becomes 

 ultimately perceptible even to very distant stars. Now all 

 the stars are moving in this way — persistently in the same 

 direction — year after year. Hence, though the movement in 

 one year may be imperceptible, by waiting ten years or twenty 

 or a century we ultimately perceive the movements of many 

 of them. The more distant require even longer — how much 

 longer we cannot yet say ; this is one of the questions on which 

 we hope to get some light by the work on the Great Star Map 

 and its successors. Our knowledge will grow : we shall find 

 that after ten years a certain percentage of the stars have 

 moved ; after twenty, new movements previously undetected or 

 uncertain, will be added ; after thirty, more still, and so on ; 

 and by watching the run of the sequence we may even be able 

 to predict what will happen in longer periods not yet reached, 

 though this extrapolation has its risks. 



We cannot, of course, afford to repeat the whole map every 

 ten years ; we must be satisfied with samples made as repre- 

 sentative as possible. We have already obtained some samples 

 at Oxford and the following results will serve to illustrate our 

 expectation. Plates have been repeated after intervals varying 

 from ten years to seventeen and the measures compared. If 

 the measures of any star in either co-ordinate differed by more 

 than i"'2 (the angle subtended by an inch at a distance of 

 2^ miles) they were carefully repeated. In a number of cases 

 the discordance was found to be due to some mistake or a care- 

 less measure. (It must be remembered that many thousands 

 of measures were made in all, and that occasional mistakes are 

 inevitable.) But in the majority the difference was confirmed 

 as due to a real movement of the star. Such movements were 

 nevertheless very rare — on the average less than 2 per cent, of 

 all the stars examined. 



The percentage was higher for the longer intervals somewhat 

 as follows : 



After 10 years, i per cent, had moved appreciably. 



I) ^2 )» ^'2 >) » )> 



» '4 » 2 ,, ,, ,, 



But this method of statement is defective by reason of a most 

 significant fact brought to light by the results themselves. 



