SCIENCE 



NEW YORK, NOVEVIBER 6, 18 



THE SUN'S MOTION IN SPACE/ 



Science needed two thousand years to disentangle the 

 earth's orbital movetnent from the revolutions of the other 

 planets, and the incomparably more arduous problem of dis- 

 tinguishing the solar share in the confused multitude of 

 stellar displacements first presented itself as possibly tracta- 

 ble little more than a century ago. In the lack for it as yet 

 of a definite solulion there is, then, no ground for surprise, 

 but much for satisfaction in the large measure of success at- 

 tending the strenaous attacks of which it has so often been 

 made the object. 



Approximately correct knowledge as to the direction and 

 velocity of the sun's translation is indispensable to a profita- 

 ble study of sidereal construction; but, apart from some ac- 

 quaintance with the nature of sidereal construction, it is dif- 

 ficult, if not impossible, of attainment. One, in fact, pre- 

 supposes the other. To separate a common element of 

 motion from the heterogeneous shiftings upon the sphere of 

 three or four thousand stars is a task practicable only under 

 certain conditions. To begin with, the proper motions in- 

 vestigated must be established with general exactitude. The 

 errors inevitably affecting them must be such as pretty 

 nearly, in the total upshot, to neutralize one another. For 

 should they run mainly in one direction, the result will be 

 falsified in a degree enormously disproportionate to their 

 magnitude. The adoption, for instance, of a system of dec- 

 linations as much as 1" of arc astray, might displace to the 

 extent of 10° north or south the point fixed upon as the apex 

 of the sun's way (see L. Boss, Astr. Jour., No. 213). Risks 

 on this score, however, will become less formidable, with 

 the further advance of practical astronomy along a track 

 definable as an asymptote to the curve of ideal perfection. 



Besides this obstacle to be overcome, there is anothei' 

 which it will soon be possible to evade. Hitherto, inquiries 

 into the solar movement have been hampered by the neces- 

 sity for -preliminary assumptions of some kind as to the rela- 

 tive distances of classes of stars. But all such assumptions, 

 especially when applied to selected lists, are highly insecure; 

 and any fabric reared upon them must be considered to 

 stand upon treacherous ground. The spectrographic method, 

 however, here fortunately comes into play. " Proper 

 motions" are only angular velocities. They tell nothing as 

 to the value of the perspective element they may be supposed 

 to include, or as to the real rate of going of the bodies they 

 are attributed to, until the size of the sphere upon which 

 they are measured has been otherwise ascertained. But the 

 displacements of lines in stellar spectra give directly the ac- 

 tual velocities relative to the earth of the observed stars. 

 The question of their distances is, therefore, at once elimi- 

 nated. Now the radial component of stellar motion is mixed 

 up, precisely in the same way as the tangential component, 

 with the solar movement; and since complete knowledge of 

 it, in a sufficient number of cases, is rapidly becoming ac- 

 cessible, while knowledge of tangential velocity must for a 



' A. M. Gierke in Nature of Oct. 15. 



long time remain partial or uncertain, the advantage of re- 

 placing the discussion of proper motions by that of motions 

 in line of sight is obvious and immediate. And the admira- 

 ble work carried on at Potsdam during the last tliree years 

 will soon afford the means of doing so in the first, if only 

 a preliminary investigation of the solar translation based 

 upon measurements of photographed stellar spectra. 



The difficulties, then, caused either by inaccuracies in star- 

 catalogues or by ignorance of star-distances, may be over- 

 come; but there is a third, impossible at present to be sur- 

 mounted, and not without misgiving to be passed by. All 

 inquiries upon the subject of the advance of our system 

 through space start with an hypothesis mosi unlikely to be 

 true. The method uniformly adopted in them — and no 

 other is available — is to treat the inherent motions of the 

 stars (their so-called motus peculiares) as pursued indiffer- 

 ently in all directions. The steady drift exfricable from 

 t'nem by rules founded upon the science of probabilities is 

 presumed to be solar motion visually transferred to them in 

 proportions varying with their remoteness in space, and 

 their situations on the sphere. If this presumption be in any 

 degree baseless, the result of the inquiry is pro tanto falsi- 

 fied. Unless the deviations from the parallactic line of th& 

 stellar motions balance one another on the whole, their dis- 

 cussion may easily be as fruitless as that of observations 

 tainted with systematic errors. It is scarcely, however,, 

 doubtful that law, and not chance, governs the sidereal rev- 

 olutions. The point open to question is whether the work- 

 ings of law may not be so exceedingly intricate as to produce 

 a grand sum total of results which, from the geometrical 

 side, may justifiably be regarded as casual. 



The search for evidence of a general plan in the wander- 

 ings of the stars over the face of the sky has so far proved 

 fruitless. Local concert can be traced, but no widely-dif- 

 fused preference for one direction over any other makes it- 

 self definitely felt. Some regard, nevertheless, must be paid 

 by them to the plane of the Milky Way. since it is alto- 

 gether incredible that the actual construction of the heav- 

 ens is without dependence upon the method of their revolu- 

 tions. 



The apparent anomaly vanishes upon the consideration of 

 the profundities of space and time in which the fundamental 

 design of the sidereal universe lies buried. Its composition 

 out of an indefinite number of partial systems is more than 

 probable; but the inconceivable leisureliness with which 

 their mutual relations develop renders the harmony of those 

 relations inappreciable by short-lived terrestrial denizens. 

 "Proper motions," if this be so, are of a subordinate kind; 

 they are indexes simply to the mechanism of particular ag- 

 gregations, and have no definable connection with the 

 mechanism of the whole. No considerable error may then 

 be involved in treating them, for purposes of calculation, as 

 indifferently directed; and the elicited solar movement may 

 genuinely represent the displacement of our system relative 

 to its more immediate stellar environment. This is perhaps 

 the utmost to be hoped for until sidereal astronomy has 

 reached another stadium of progress, unless, indeed, effect 

 should be given to Clerk Maxwell's suggestion for deriving' 



