ADVISORY COMMITTEE ON ASTRONOMY 9 1 



based upou this knowledge. There is decided deficiency in deter- 

 minations of the positions of the principal stars at the present time. 

 This work is very exacting and laborious, and, in our opinion, is 

 worthy of ample aid. The extension of such observations to the 

 southern hemisphere deserves the careful attention of the Institution. 



Much work still remains to be done in the deduction upon modern 

 principles of revised results from the older series of observations. 

 This work is of very great importance and is worthy of support. 



Measurement of the parallaxes of stars to ascertain their distances 

 from the earth should be undertaken with powerful instrumental 

 means on broader lines than those hitherto followed. On the results 

 of the investigations mentioned in what precedes, combined with 

 those derived from the spectroscopic measurement of stellar motions 

 in the line of sight, and the determination of the positions of nebulae, 

 the solution of the great problem of the structure of the univer.se 

 must be based. 



The accurac}^ of star positions is connected with an exact knowl- 

 edge of the constant of aberration, which demands further study by 

 new and more perfect methods. This constant has also a physical 

 interest of its own. Equallj' desirable is more precise information 

 regarding the changes in the direction of the earth's axis of rotation, 

 rendered pos.sible through observations of the variation of latitude. 



Many problems relating to the stellar system call for research. 

 The existing uncertainty regarding the motion of the solar system 

 in space should be remedied through a more comprehensive investi- 

 gation than has yet been attempted, an undertaking which is now 

 especially timely and feasible. 



The So/a? System. — The lunar theory requires special attention. 

 Tables in general use are based upon Hansen's investigations pub- 

 lished forty-five years ago, and need thorough reconstruction in 

 order to meet the present requirements of astronomy. Of funda- 

 mental interest is the question whether Newton's law of attraction, 

 according to inverse squares, is exactly true within the limits of error 

 of observation. It is desirable that this problem should receive 

 further investigation. * * * 



The minor planets, nearly 500 of which are known, preseiit a 

 problem of much difiiculty, which demands attention. Some pro- 

 vision should be made for more numerous computations of their 

 orbits. This subject can be effectively handled by means of well- 

 devised cooperation, to which it is very well adapted. 



The computation of the solar parallax from observations of the 



