683 



Astronomy. — "On the Infiueiice exercised by the systematic 

 connection between the parallax of the stars and their apparent 

 distance from the galactic plane ■ upon the determination of the 

 precessional constant and of the systematic proper motions of 

 the stars." By Prof. E. F. van dk Sande Bakhuyzen and C. deJong. 



(Communicated in the meeting of Sept. 25, 1915.) 



Since tlie researches made by Kapteyn, it may be regarded as 

 an estabbslied fact, tliat stars of a given magnitude are at a greater 

 mean distance from us, in proportion as they are nearer to the 

 galactic plane. At the galactic poles the mean parallax is found 

 to be about one and a half limes as great as in the »alactic plane 

 itself. As in the researches so far undertaken concerning the pre- 

 cessional constant and the systematic proper motions of the stars 

 this connection had not been taken into consideration, it is obvious 

 that the determination of these (iiiantities may be affected by syste- 

 matic errors. 



For some time it had l)een the intention of one of us to institute 

 a nearer investigation of this matter, all the more because it might 

 throw light upon a ditferenoe, found by Newcomb, between the values 

 of the precession-constant, as deduced on the one hand from Right 

 Ascension- and on the other one from Declination-observations. Later on 

 it was noticed, that Nkwcomb himself had indicated tiie possibility of 

 such an explanation of the difference, [Prec. Cons/, p. 67 and 73) and 

 also that Eddington in his well known monograph published last year, 

 "Stellar movements and the structure of the universe", in pointing- 

 out the desirability of taking the differences of distance into consi- 

 deration, had already made a beginning in this direction. At the 

 same time, he only deals with the influence of the inequality of the 

 distance upon the determination of the apex of the Parallactic 

 motion (p. 81 — 83), and only develops it in the case of the inves- 

 tigation being based upon stars which are evenly distributed over 

 the entire celestial sphere. 



A new research, therefore, embracing the whole question, was by 

 no means superfluous. We have undertaken it, and in the following 

 paper we communicate our results. The term "Systematic proper 

 motions" is here taken in a somewhat limited sense; it includes 

 oidy those motions which are functions of the spherical place of 

 the star, although the coefficieiits may still be dependent upon 

 their distance from us, and perhaps also upon the spectral type, 

 (we leave that here out of account). Systematic movements which 



