THE.SUN’S MOTION IN SPACKE.* 
By AGNES M. CLERKE. 
Science needed two thousand years to disentangle the earth’s orbital 
movement from the revolutions of the other planets, and the incompar- 
ably more arduous problem of distinguishing the solar share in the 
confused multitude of stellar displacements first presented itself as 
possibly tractable, little more than a century ago. In the lack as yet of 
a definite solution for it, there is then no ground for surprise, but much 
for satisfaction in the large measure of success attending the strenuous 
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 profitable study of sidereal 
construction; but apart from some acquaintance with the nature of 
sidereal construction, it is difficult, 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 3,000 or 
4,000 stars is a task practicable only under certain conditions. To be- 
gin with, the proper motions investigated must be established with 
general exactitude. The errors inevitably affecting them must be sueh 
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 adop- 
tion, for instance, of a system of declinations as much as 1” of are 
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 another which it will 
soon be possible to evade. Hitherto, inquiries into the solar movement 
have been hampered by the necessity for preliminary assumptions of 
some kind as to the relative distances of classes of stars. But all such 
assumptions, especially when applied to selected lists, are highly inse- 
cure; 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 veloci- 
* From Nature, October 15, 1891, Vol. xitv, pp. 572-574. : 
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