July 1, 1897.] 



KNOWLEDGE. 



165 



this point. I take it that we do not necessarily imply that 

 all the stars are revolving round one another, but that there 

 is in some way a physical connection which differentiates 

 them from other stars. We mean a group ; though not 

 necessarily one separated by wide spaces from all other 

 groups, for it is possible two or more miyht be mixed 

 together and yet really distinct, as shown by their different 

 motions — just as it is possible for two swarms of meteors 

 to move through each other, and yet for each swarm to 

 be quite distinct. 



One of the most important points with reference to the 

 proper motion of these stars is that it is nearly opposite 

 to that which would be caused by the sun's motion. This 

 expression, " the sun's motion," is a very vague one (a 

 fact which is often forgotten) ; but what Proctor appears 

 to mean in the present instance is the motion of the 

 sun with reference to the average of the few thousand 

 stars that are nearest to it. Now, as we have very 

 little idea of the distance of the five stars in question, 

 we cannot tell their speed ; but in any case we see that 

 their motion with regard to these few thousand stars 

 is roughly in the same direction as that of our suu, but 

 swifter. Let us call this their " relative motion.'' Their 

 distance from us is hardly likely to be less than J that of 

 the average of the one-magnitude stars ; and, according to 

 Proctor's map of proper motions, in that case the relative 

 motion of these five stars is twice as great as the sun's, 

 i.e., as that part of the sun's which is at right angles to 

 the line joining the sun with them, and is therefore | of 

 the whole of the sun's relative motion. If they are more 

 distant, their relative motion is swifter. 



In this letter I have dealt only with the motion of the 

 stars at right angles to the line of sight. 



T. W. Backhouse. 

 West Hendon House, Sunderland. 

 May 27, 1897. 



!-♦-• 



To the Editors of Knowledge. 



Sirs, — The question of star systems is one of sufficient 

 interest to justify some further discussion. I am disposed 

 to think the apparent common drift of stars in certain 

 localities (when the direction of the motion is pretty much 

 the same but the amount is different) is mainly due to the 

 motion of the sun in space. Let me take some examples 

 from Prof. Porter's catalogue of stars with a motion of 

 half a second per annum or upwards. This catalogue 

 contains yS, -ij, and ju, Cassiopeise. The position angles are 

 respectively 110-4°, 113-8°, and 114-9^ — a very close agree- 

 ment ; but the quantities are 0-54", 1-19", and 3-73", the 

 last being seven times as great as the first. Can we suppose 

 that these three stars have really a common proper motion, 

 and that the difl'erence in quantity is due to their respective 

 distances from us, (i being seven times as far off as fi ? Or 

 is it not more likely that they are three stars with small 

 real proper motion situated at different distances from us, 

 and whose apparent proper motion is almost entirely due 

 to the motion of the sun ? 



With a view of obtaining some further light on this 

 subject I examined the motions of the other stars in this 

 catalogue whose E.A. is between Oh. and 2h. and Decl. 

 between 45° and 70°. Here is the result : — 



These angles are a little less than those in the case of 

 the three stars in Cassiopeia, but the approximation is 



pretty close, and as regards the quantity of motion they 

 are better agreed, viz., 0-90', 0-69", 0-76", and 0-62". 

 Had I gone on to the third hour of R.A. I would have got 

 122-6° and 131-4- for the next two direction angles, the 

 mean of the six stars being almost the same as that of the 

 three in Cassiopeia, and the difference in no instance being 

 very wide. Further, I found that the position angles for 

 the three stars in Cassiopeia are very near the mean for 

 this Plight Ascension. 



I may note that in this catalogue of three hundred and 

 one stars, taking each hour of R.A. separately, the mean 

 position angle was always less than 180° from Oh. to 6h., 

 from 6h. to 7h. it was almost exactly 180°, then from 7h. 

 to lilh. it was always greater than 180 ', and from 19h. 

 to 24h. it was again less. The maximum deviations from 

 180° on each side occur at about Oh. to Ih., and 12h. to 

 13h. They were not quite equal, however, the former 

 somewhat exceeding the latter. 



I tried the southern stars in the catalogue within the 

 same limits as the foregoing, when the direction angles 

 proved to be 55-9°, 122-0°, G4-5°, 76-2°, and G5-5\ Four 

 out of these five are in very close agreement, and the last 

 three were pretty well agreed in quantity, viz., 100", 

 0*71", and 0-72". But their positions on the sphere are 

 hardly consistent with the assumption that they belong to 

 the same system. 



Between 3h. 30m. and 5h. 30m. of R.A., and 40' and 

 50° N. Decl., there are four stars with proper motion of 

 half a second and upwards. Their direction angles are 

 the following ; 154-2°, 127-2 , 145-1°, and 141-7°. 



On the other hand, I find in several instances two stars 

 with large proper motion situated near to each other in 

 the sphere and yet moving in very different directions. 

 Apparently they do not belong to the same physical system, 

 notwithstanding their comparative nearness to each other. 

 This increases the probability that the apparent common 

 drift of others really results from the sun's motion. Thus 

 Groombridge 2305 and Lalande 29439 are situated much 

 nearer to each other on the sphere than those which I have 

 been considering, and they have nearly the same proper 

 motion in quantity ; but the direction angle in one case 

 is 275-2° and in the other 159-4°. Lacaille 2740 and 

 T Puppis are not very remote on the sphere, and, judging 

 from the amount of their proper motions, are situated at 

 about the same distance from us. The angle in one case 

 is 342-5° and in the other 183-1°. The motion of LacaiUe 

 2957 is more directly opposite to that of 2940, but the 

 distance on the sphere is somewhat greater. ^ and w Herculis 

 both appear in the list. The angle in one case is 811-7° 

 and in the other 172-9°. But it will be noted that all the 

 stars which I have referred to as exhibiting this opposition 

 of directions are situated in a part of the sky where the 

 sun's motion would have little influence. 



My conclusion is that the sun's motion affects the 

 proper motions of the stars to a greater extent than 

 is commonly supposed. When the observed proper 

 motion is very small it is largely affected by errors of 

 observation, and the effects of solar motion are thus 

 disguised. But when the observed motion is so large that 

 errors form but a small proportion of it, the effect of 

 the sun's motion becomes plainly manifest. These latter 

 stars are probably travelling with more than average velocity 

 (in the direction perpendicular to the line of sight), yet 

 their motions appear to be more affected by that of the 

 sun than those of other stars which are moving with less 

 absolute velocity. I can only explain this fact by supposing 

 that the smaller observed proper motions are largely 

 affected by errors. At all events, before we ascribe a 

 common drift to a large number of stars in some particular 



