202 



SCIENCE 



[N. S. Vol. XXVI. No. 659 



This at once creates a new difficulty, viz., 

 that of defining an absolute locus in space. 

 The human mind may exhaust itself in the 

 effort, but it can never solve the problem. 

 "We can imagine, for example, the position 

 of the sun at any moment to be defined 

 with reference to any number of surround- 

 ing stars, but by no effort of imagination 

 can we devise means of defining the abso- 

 lute position of a body in space without 

 reference to surrounding material objects. 

 If, therefore, the referring objects have un- 

 known motions of their own, the rigor of 

 the definition is lost. 



What we call the observed proper motion 

 of a star has three possible sources of 

 origin : 



1. The parallactic motion, or the effect 

 of our sun's motion through space, where- 

 by our point of view of surrounding celes- 

 tial objects is changed. 



2. The peculiar or particular motion of 

 the star, i. e., its own absolute motion in 

 space. 



3. That part of the observed or tabular 

 motion which is due to inevitable error of 

 observation. 



In all discussions of the solar motion in 

 space, from that of Herschel down till a 

 recent date, it has been assumed that the 

 peculiar motions of the stars are arranged 

 at random, and may therefore be con- 

 sidered zero in the mean of a considerable 

 number of them. It is then possible to 

 find such a value for the precession, and 

 such a common apex for the solar motion 

 as shall leave the residual peculiar motions 

 of the stars under discussion to be in the 

 mean = zero. That is to say, we refer the 

 motion of the sun in space to the center 

 of gravity of all the stars considered in the 

 discussion, and regard that center of 

 gravity as immovable in space. 



In order to proceed rigorously, and espe- 

 cially to determine the amount as Avell as 



the direction of the sun's motion in space, 

 we ought to know the parallax of every 

 star employed in the discussion, as well as 

 its proper motion. In the absence of such 

 data it has been usual to start from some 

 such assumption as the following : the stars 

 of a particular magnitude are roughly at 

 the same distance ; those of different classes 

 of magnitude may be derived from the 

 hypothesis that on the average they have 

 all equal absolute luminosity. 



The assumption is not a legitimate one— - 



1. Because of the extreme difference in 

 the absolute luminosity of stars. 



2. Because it implies that the average 

 absolute luminosity of stars is the same in 

 all regions of space. 



The investigation has been carried out 

 by many successive astronomers on these 

 lines M'ith fairly accordant results as to the 

 position of the solar apex, but with very 

 unsatisfactory results as to the distances of 

 the fixed stai«." In order to judge how 

 far the magnitude (or brightness) of a star 

 is an index of its probable distance, v/e 

 must have evidence from direct determina- 

 tions of stellar parallax. 



STELLAR PAEALLAX 



To extend exact measurement from our 

 own solar system to that of other suns 



" Argelander, Mem. pr4sentes a I'Acad. Imp. des 

 Sciences St. Petersbourg, tome III.; Lundahl, 

 Astron. Nachrichten, 398, 209; Argelander, 

 Astron. Nachrichten, 398, 210; Otto Struve, Mem. 

 Acad, des Sciences St. Petersbourg, VI' sgrie, 

 Math, et Phys., tome III., p. 17; Galloway, Phil. 

 Trans., 1847, p. 79; Madler, Dorpat Observations, 

 Vol. XIV., and Ast. Nach., 566, 213; Airy, MSm. 

 R.A.S., Vol. XXVIII., p. 143; Dunkin, Mem, 

 B.A.S., Vol. XXXII., p. 19; Stone, Monthly No- 

 tices R.A.8., Vol. XXIV., p. 36; De Ball, inaug- 

 ural dissertation, Bonn, 1877; Eancken, Astron. 

 Nachrichten, 2482, 149; Bischoff, inaugural dis- 

 sertation, Bonn, 1884; Ludwig Struve, M4m. 

 Acad. St. Petersbourg, VII' s6rie, tome XXXV., 

 No. 3. 



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