January 5, 191 1] 



NATURE 



325 



rating in our solar system, of which the visible star 

 ns a constituent member. 



2) Changes of position due to orbital motion in binary 

 multiple stars. Where both components of a binary 

 : are visible, these changes readily admit of direct 

 isurement. In other cases the existence of a com- 

 ion is inferred to account for regular periodic changes 

 position of the visible component, though this com- 

 ion cannot be seen either on account of intrinsic want 

 light or on account of its close proximity to the primary, 

 1 the consequent incapacity of our telescope to render 



two visually distinct. 

 1 hese changes are of interest as affording evidence of 

 validit)" of the Newtonian laws in systems other than 

 solar system. 

 1 he changes to which I have so far referred are changes 

 ch affect isolated stars or groups of stars, but which 

 not occur, at least to a sensible extent, in the generality 

 of stars. 



I come now to the changes of position due to the earth's 



' iral motion which, on the other hand, may be expected 



.nfluence all stars in common. Even where, as in the 



- s I have already spoken of, their influence is obscured 



by orbital motion within the system, when once this 



orbital motion has been thoroughly examined, its laws 



deduced, and due allowance made for it by computation, 



we might expect to find the effects of the earth's motion 



still apparent. 



The earth in its orbit round the sun approximately 



H-i^ribes a circle of 186,000,000 miles in diameter, and its 



;essive positions in space at intervals of six months 



r separated from one another by this extent. But 



experience has shown that recurring changes in the 



relative positions of the stars, as viewed at intervals of 



six months — that is to say, from two different points of 



the universe separated by this vast distance — can only be 



f1-rected in the case of a limited number of stars, and then 



by the application of the most delicate methods of 



asurement spyeciallv designed to bring these changes to 



light. 



To the Cape Observatory and its former director, 



Henderson (1832-4), belongs the credit of first producing 



trustworthy evidence of the existence of any fixed star, for 



which these changes could be unmistakably detected, and 



which, therefore, was not too remote fr<Mn the solar system 



to permit of its distance being at least roughly determined 



comparison with the diameter of the earth's orbit. 



derson's discoven,- has since been fully confirmed by 



larer obser\ers, and other stars likely to yield tangible 



results have now been examined. As illustrative, how- 



'•vpf, of the evasiveness of the quantities sought, and the 



ssive labour by which only they can be derived, though 



problem of stellar distance has always been in the 



forefront of astronomical interest, and has attracted the 



attention of several able observers, the number of stars 



for which well-determined parallaxes have been published 



up to the present day does not exceed some 400. This 



number is quite insignificant in comparison even with the 



^ber of stars visible to the naked eye without tele- 



pic-aid. Moreover, the stars investigated have been, in 



general, selected on the grounds of some a priori proba- 



bilit}' of their possessing a measurable parallax, either on 



account of apparent brightness or on account of their 



large apparent motion, and for this reason they can 



scarcely be regarded as typical of the generalitv of stars. 



In order, then, to gauge the depths of the visible 

 universe it would appear imperative that our base-line 

 must in some manner be extended. The distance of 

 186.000,000 miles through which we are carried in the 

 -" rse of a single half-year by the orbital motion of our 

 et round the sun is so small in comparison with inter- 

 .;ar distances as to give rise to changes in the apparent 

 relative positions of stars which, except in the most pro- 

 nounced instances, are so insignificant in amount as to 

 defy detection even by the most refined processes of 

 measurement we possess. 



How, then, can such an extension of our base-line be 

 ined? I have already pointed out that the so-called 

 xed stars " are not truly " fixed," but that on close 

 observation it is found that each star has an apparent 

 motion either peculiar to itself or shared bv other neigh- 

 bouring stars which, with it, constitute an independent 

 NO. 2149, VOL. 85] 



system. I refer primarily to the visible motion transverse 

 to the line of sight. 



If then our sun, as we maj- reasonably suppose, is itself 

 a member of the stellar universe, it may be anticipated 

 that it too will not be at rest, but will be moving forward 

 in space, and the visible motions will be those due to the 

 combined effects of the motion of the sun and stars. 



That the apparent motions of the stars were not entirely 

 fortuitous, but that they could, at least partially, be co- 

 ordinated throughout the sky as the visible manifestations 

 of a single phenomenon, viz. a translatory motion of the 

 sun with its system of planets through interstellar space, 

 was first pointed out by Sir William Herschel, who further 

 indicated that the point of space to which this motion was 

 directed was situated in the constellation " Hercules." 



Before proceeding to the further consideration of this 

 solar motion, I wish first to point out to you how its 

 existence at once suggests a means of " extending our 

 base-line " for the purpose of gauging these interstellar 

 depths. I have refrained from any numerical estimates of 

 the amount of this motion, as this involves philosophical 

 questions into which I do not desire to enter to-night ; 

 but in order to fix our ideas it is necessary for me to give 

 you some notion, at least, of the order of magnitude. It 

 is now possible to state with some certainty that the speed 

 of the sun's motion relatively to the stars as a whole 

 amounts to about 20 kilometres per second, and that the 

 space traversed in a single day therefore amounts to rather 

 more than 1,000,000 miles, that in a year to about 

 400,000,000 miles. Thus the stars, as seen on two 

 occasions a year apart, may be considered as viewed from 

 two points in space separated by this length, and it only 

 requires lapse of time in order to increase the length to 

 an almost indefinite extent. 



The great scheme for the photographic mapping of the 

 heavens at present being carried out on an extensive scale 

 by means of the cooperative efforts of the leading obser\'a- 

 tories of the world will shortly furnish a highly accurate 

 delineation of the skies as seen at the commencement of 

 the twentieth century. This alone has called for con- 

 centrated effort extending over some twelve years at least, 

 while it would even now scarcely be safe to say that 

 another ten years will see its completion. An immediate 

 repetition is scarcely to be contemplated, though a sub- 

 sequent repetition at some future epoch, which may be 

 agreed on by astronomers, forms an essential part of the 

 programme as originally introduced. 



When this scheme is completed in its entirety very 

 ample data will be av-ailable for the discussion of stellar 

 distribution by the methods I have suggested to you. 



In the meantime, however, in such tentative attempts 

 as have been made to fathom the secrets of the universe 

 by means of the study of stellar proper motion, it has 

 been necessary to rely on early recorded exact observa- 

 tions. It will be clear from what I have already explained 

 to you that it is the earliest trustworthy records in com- 

 parison with the most up-to-date available which will vield 

 the greatest length of base-line, and consequently the most 

 trustworthy results. For this reason the majority of the 

 discussions hitherto attempted have been based on the 

 catalogue of Bradley, dependent on observations made by 

 him at Greenwich between the years 1750-62. This cata- 

 logue contains the places of some 3000 stars observed 

 with a precision far surpassing any similar previous 

 observations, and comparing favourably with the best 

 modern catalogues. The stars selected by Bradley are 

 fairly uniformly distributed over the portions of the skv 

 accessible to him, viz. from the North Pole to 30° south 

 of the equator. 



Unfortunately no early catalc^fue of stars of even 

 approximately similar precision exists for the remaining 

 region of the sky between 30° S. dec. and the South Pole, 

 and the absence of exact knowledge of these regions for 

 the earlier epochs has always hampered these discussions. 



The discussions I refer to have generally had as their 

 immediate objective : — 



(i) The determination of the precessional constant, i.e. 

 the annual amount by which the earth's axis of rotation 

 changes its position in space, and 



(2) The determination of the speed of the solar motion 

 and the position of the solar apex, i.e. the point in the 

 heavens towards which the sun's motion is directed. 



