THE GREAT STAR MAP ^ 

 II. STAR COUNTING 



By H. H. turner, D.Sc, D.C.L., F.R.S., 



Savilian Professor of Astronomy in the University of Oxford 



The map is to be a record of the positions of all stars 

 brighter than a certain standard and will indicate the approxi- 

 mate brightness of each star. Before following the history of 

 the project further, it is desirable to consider what are the 

 problems which may be solved by the accumulation of such 

 information and cannot be solved without it. 



What can we learn, it may well be asked, of the great 

 universe of stars from observations made under the severe 

 restrictions which limit astronomers ? We are permanently 

 bound to a small satellite attendant upon one of the stars ; 

 other stars are at distances so vast that their movements are 

 only discernible with difficulty : can we learn anything at all 

 about their arrangement in space ? 



At first sight the inquiry might seem well-nigh hopeless 

 but with a little persistence we find that the chances of 

 learning some essential facts are not to be despised : some, it 

 is true, can be learnt only after centuries of labour but there 

 are one or two which have been established without very 

 much trouble. For instance, it does not take long to satisfy 

 ourselves that the stars are not scattered simply at random 

 through space : it may take a long time to find out what their 

 particular arrangement is but we feel confident that there is 

 some arrangement for reasons which may be indicated as 

 folloVk^s. 



The stars have been divided into classes according to their 

 brightness, such that each class (or "magnitude") is fainter 

 than the one above it in a constant ratio. There is of course 

 no sharp distinction obvious in the sky between one class 

 and the next : the brightnesses vary by imperceptible steps, 



' The previous article appeared in Science Progress, 1910, v. pp. 1-14. 



240 



