204 



SCIENCE 



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



which moves with a velocity of 80 miles 

 per second at right angles to the line of 

 sight (we do not know with what velocity 

 in the liae of sight). It is at about the 

 same distance from us as Sirius, but it 

 emits but one ten-thousandth part of the 

 lightenergy of that brilliant star. Sirius 

 itself emits about thirty times the light- 

 energy of our sun, but it in turn sinks into 

 insignificance when compared with the 

 giant Canopus, which emits at least 10,000 

 times the light-energy of our sun. 



Truly "one star differs from another 

 star in glory." Proper motion rather 

 than apparent brightness is the truer indi- 

 cation of a star's probable proximity to the 

 sun. Every star of considerable proper 

 motion yet examined has proved to have a 

 measurable parallax. 



This fact at once suggests the idea. Why 

 should not the apparent parallactic mo- 

 tions of the stars, as produced by the sun's 

 motion in space, be utilized as a means of 

 determining stellar parallax? 



SECULAR PARALLACTIC MOTION OF STABS 



The strength of such determinations, un- 

 like those made by the method of annual 

 parallax, would grow with time. It is true 

 that the process can not be applied to the 

 determination of the parallax of individual 

 stars, because the peculiar motion of a par- 

 ticular star can not be separated from that 

 part of its apparent motion which is due to 

 parallactic displacement. But what we 

 specially want is not to ascertain the 

 parallax of the individual star, but the 

 mean parallax of a particular group or 

 class of stars, and for this research the 

 method is specially applicable, provided we 

 may assume that the peculiar motions are 

 distributed at random, so that they have no 

 systematic tendency in any direction; in 

 other words, that the center of gravity of 



any extensive group of stars will remain 

 fixed in space. 



This assumption is, of course, but a 

 working hypothesis, and one which from 

 the paper on star-streaming communi- 

 cated by Professor Kapteyn, of Groningen, 

 to the Johannesburg meeting of the Asso- 

 ciation two years ago we already know to 

 be inexact.^^ Kapteyn 's results were quite 

 recently confirmed in a remarkable way by 

 Eddington,^'' using independent material 

 discussed by a new and elegant method. 

 Both results showed that, at least for ex- 

 tensive parts of space, there are a nearly 

 equal number of stars moving in exactly 

 opposite directions. The assumption, then, 

 that the mean of the peculiar motions is 

 zero may, at least for these parts of space, 

 be still regarded as a good working hypo- 

 thesis. 



Adopting an approximate position of the 

 apex of the solar motion, Kapteyn resolved 

 the observed proper motions of the Bradley 

 stars into two components, viz., one in the 

 plane of the great circle passing through 

 the star and the apex, the other at right 

 angles to that plane. ^^ The former com- 

 ponent obviously includes the whole of the 

 parallactic motion; the latter is inde- 

 pendent of it, and is due entirely to the 

 real motions of the stars themselves. From 

 the former the mean parallactic motion of 

 the group is derived, and from the com- 

 bination of the two components, the rela- 

 tion of velocity of the sun's motion to that 

 of the mean velocity of. the stars of the 

 group. 



As the distance of any group of stars 

 found by the parallactic motion is ex- 

 pressed as a unit in terms of the sun's 

 yearly motion through space, the velocity 



'^Rep. Brit. Assoc, 1905, p. 257. 

 " Monthly Notices R.A.8., Vol. LXVII., p. 34. 

 ^''Publications Astron. Laboratory Ch-oningen, 

 Nos. 7 and 9. 



