lO CARNEGIE INSTITUTION 



(^) An investigation upon the motions of the bodies asso- 

 ciated together in the solar system. 



The problems encountered in these two lines of research are of 

 singular fascination, and they are capable of development in essen- 

 tial points upon the basis of observations that we now regard as 

 comparatively crude. The element of time was favorable. Obser- 

 vations extending over relatively few years are sufficient to develop 

 the facts of planetary astronomy in their broad outlines. It was 

 natural that these subjects should have received the first attention 

 of astronomers. The number of separate objects concerned was 

 small, so that the work of observation did not bear a relatively large 

 proportion to that of mathematical development of them. 



But the time came, during the last third of the nineteenth cen- 

 tury, when astronomers began to feel satiated with their conquests 

 in the terrestrial and planetary fields. Facts and observations rela- 

 tive to the astronomy of the stars had been slowly accumulating. 

 Plans were made and carried out to increase the extent and value of 

 these observations. The 7ion possumus of the more conservative 

 astronomers of the old school fell upon less and less willing ears. 

 The influence of the ideas which led to the establishment of the 

 great Pulkova Observatory, sixty years ago, began to be more and 

 more felt. On all sides it is perceived that the sidereal problem is 

 to be the astronomical problem of the twentieth century, as the 

 planetary problem was the characteristic problem of the eighteenth 

 century. 



Thus a great movement has been inaugurated for the accumula- 

 tion of facts and observations in sidereal astronomy. The scope 

 and meaning of the movement may be learned from a brief review 

 of a part of the work accomplished or in progress : 



(i) We are witnessing now the greatest activity ever known in 

 the history of astronom}- for the accurate, systematic, and extensive 

 measurement of the positions of the principal fixed stars at succes- 

 sive epochs. 



(2) We have seen the plan of the Astronomische Gesellschaft, for 

 the accurate observation of all stars down to the ninth magnitude 

 in the northern heavens, brought to the point where its success is 

 fully assured. This is by far the most extensive work in astronomy 

 ever attempted, previous to the project for the Astrographic Chart 

 next to be mentioned. 



(3) We are now watching the successful prosecution of observa- 

 tions for the Astrographic Chart — the attempt to measure from 

 photographs the accurate positions of all stars down to the eleventh, 



