ADVISORY COMMITTEE ON ASTRONOMY 9 1 



based upon this kuowledge. There is decided deficiency in deter- 

 laiaacicns 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 amuje aid. The extension of such obser\'^ations 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 iy 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 si>ectroscopic measurement of stellar motions 

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

 the solution of the great poroblem of the structure of the universe 

 must be based. 



The accuracy 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. Equally desirable is more precise information 

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

 rendered possible through observations of the variation of latitude. 



Manv' problems relating to the stellar sy.stem call for r(^earch. 

 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 Solar 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 obseivatiou. It is desirable that this problem should receive 

 further investigation. * * * 



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

 problem of much difficulty, 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 



