THE GREAT STAR MAP 

 III. STAR POSITIONS 



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



Savilian Professor of Astronomy i?t the University of Oxford. 



In the last article ^ it was shown that from the mere counting 

 of the stars (taking note of their brightness) it is possible 

 to infer some important facts about the nature of the universe 

 in which we are placed : such as the existence of a cluster of 

 stars to which our Sun belongs ; and the existence of an 

 extremely tenuous "fog" in the depths of space. But we can 

 always learn much more from prolonged inspection than from 

 a mere glance. A single photograph of a scene tells us only 

 the actual situation at a given moment. It may suggest that 

 changes are going on ; but to be sure of these changes we 

 must have another photograph taken later ; and if we can get 

 a whole series taken consecutively, as in a kinematograph, 

 we may get a complete history of the changes. The Great Star 

 Map in process of construction is only the first picture ; others 

 must follow it if we are to study the motions of the stars, and 

 our knowledge will grow with each repetition. What revela- 

 tions the future may have in store for us we cannot at present 

 even guess, though it is not too soon to be learning something. 

 The important point to be remembered — and its importance 

 cannot be too strongly emphasised — is that the main purpose 

 of the present project is to provide a basis for these future 

 discoveries, by fixing the present places of the stars with such 

 accuracy that movements can be detected readily. It is only 

 by keeping this fact in mind that we can understand the reasons 

 for the great labour which is being undertaken so cheerfully. 

 A much less laborious project would tell us a great deal ; thus 

 nearly all the knowledge about the number of the stars of a 

 given magnitude, which we considered in the last article, could 

 be gathered from photographs taken on a very much smaller 

 scale. There is for instance a very handy map of the complete 

 sky published by the Harvard University Observatory, con- 

 ^ Science Progress, 1910, v. pp. 240-55. 

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