128 



NA TURE 



[December 9, 1897 



paper in the Astrono7nical Journal, two great astronomical un- 

 dertakings adapted to throw new light on the controversy await 

 their fulfilment : the zones of the Astrononiische Gesellschaft and 

 the Catalogue of the Paris Observatory. Notwithstanding the 

 differential character of the first, and the heterogeneous origm of 

 the second, Prof. Boss thinks that we cannot estimate from these 

 two copious springs of information on the proper motions, in 

 which the abundance of data compensates (in certain limits at 

 least) their dependence upon the errors of the fundamental 

 systems adopted. 



Allow me to add another consideration to those put forward in 

 all competence by the American astronomer. The problem of 

 the precession is indivisible, as we have said, from the problem 

 of the proper motions : the problem of proper motions may 

 depend in its turn on the displacement of the solar system in 

 space. Now there is no doubt that a solution, fit to be con- 

 sidered definitive for a long series of years, is still wanting as to 

 the last problem. And the reason of it is clear : i t has been till 

 now attacked on different sides, but always with individual 

 method, inherent (we must say) to the state of science, that has 

 not yet mobilised all its forces for a simultaneous attack. The 

 classical methods based on confronting between stellar co- 

 ordinates observed at many years' distance, and on subtracting 

 in the observed displacements the parallactic part from that due 

 to the real motion of single stars, are singularly convenient to de- 

 termine the directions of the translatory movement of the solar 

 system. To determine its velocity it is instead more convenient 

 to use spectroscopic methods, with which the velocity of each 

 star is measured in the direction of the visual ray. The very 

 brief time that has elapsed since the day that Vogel, at Potsdam, 

 with such ability rendered possible the measurement on the 

 photographs of the slender and nearly imperceptible displace- 

 ments of the spectrum lines, has not yet allowed this method to 

 be applied in all its potentiality, the fecundity of which Secchi 

 had already divined, as many worthy observers, such as Pickering 

 and Maunder, had attempted to practically employ it. When 

 the new photographic equatorial (to be mounted at Potsdam) has 

 given the means of studying a large number of stars inferior to 

 the second magnitude (which is the limit till now of the instru- 

 ment used by Vogel), when the rigorous proceedings of the illus- 

 trious German spectroscopist become familiar to the American 

 astronomers, who until now are perhaps the only persons to 

 whom the question of expense presents no difficulty, when a new 

 installation is to be made, then, and only then, the moment for 

 approaching the solution of the problem of the solar translation in 

 space by the medium of a rational combination of astronomical 

 and spectroscopical methods will be arrived. 



Once the movement of our system is known with sufficient 

 exactitude, it will be easy to eliminate its effect from the proper 

 motions of the stars, which can then be reduced, free from all 

 systematic part, to the merely accidental variations of the co- 

 ordinates, and can therefore be treated as accidental errors, and 

 submitted to calculation, together with the eventual correction 

 of the constant of precession. This will only gain in precision 

 the longer its deduction is delayed, which, for obvious reasons, 

 may be afterwards made definitive for a long series of years. It is 

 exactly for such a consideration that I think it right that all the 

 equinoxes determined after Bradley's, and which are not affected 

 (as those of Piazzi are) by incurable errors, should be used in the 

 new determination of precession, it being possible, only, from 

 the comparison of these equinoxes, to rise again by extrapolation 

 to Bradley's epoch, with data adapted to render clear the law of 

 the systematic errors remaining in the revision of Auwers. 



As to the nutation, i: does not seem that the value 9" '21 of 

 this constant, adopted by the Conference, has yet given rise — at 

 all events, in public — to disputes and oppositions. Anyhow, it 

 will not be useless to remember that fresh rigorous determin- 

 ations of it were by several authorities demonstrated to be 

 necessary in recent years, and that especially the persistent 

 uncertainties have their being through the disagreement, not 

 yet well explained, existing between the value of the terrestrial 

 flattening given by the mechanical theory of the nutation and 

 that given by geodetical measures. Harkness has sustained the 

 need of a new series of observations, tending to furnish a new 

 value of the constant of nutation, more precise than that of 

 Peters. While such a series is projected and executed (and it 

 will be necessary to prolong it at least for one of the periods of 

 nineteen years that reconduct the node of the moon's orbit 

 to the equinox), the discussion of the enormous geodetical 

 material that is being accomplished will furnish us more secure 



NO. 1467, VOL. 57] 



facts as to the flattening and to the moments of inertia of the 

 earth, thanks to the comparison between the values given by 

 the arcs of meridian and by the measured lengths of the pen- 

 dulum for seconds. Hence a correction (that can be but 

 arbitrary) seems premature as to the value already assigned by 

 Peters, whose exactitude, according to Harkness, is remarkable. 



For the solar parallax, the Conference thought it opportune 

 to fix a value of 8" 'So, by an act of authority unjustified by 

 sufficient explanations. This appears to many astronomers to 

 be an acceptable mean among the most discordant values that 

 laborious researches have furnished in the last twenty years. 

 The lamented Tisserand found no other motive for being 

 satisfied with this choice, except its approach at less than 

 a hundredth part of a second to the value fixed by Laplace. 

 But the example of the famous mistake of Encke, not too oppor- 

 tunely quoted by him, should have admonished him to use 

 greater prudence. We have, it is true, two distinct groups of 

 results in these last years ; some approximate to the value of 

 8"75, the others about 8"'85. Are we to conclude that the first 

 defect o"'5, the others exceeding as much? Or is it not — 

 given the great intrinsic accordance of the single determinations 

 on which every value reposes — more legitimate to suspect that 

 some series are affected by constant errors relatively enormous, 

 and such to take weight from them all in the definitive discus- 

 sion ? The fact is that, notwithstanding the transits of Venus 

 and the oppositions of Mars, notwithstanding the photographs 

 and the heliometers, or the combined forces of men of value like 

 Gill, Auwers, and many others, the problem of the parallax 

 remains more open than ever. It is true that, as Boss remarks, 

 one active period for this research seems at an end ; but it is 

 true also that, on the contrary, the question of the aberration is 

 more lively than ever ; and that, in the present state of 

 astronomy, it would be absurd to settle the value of the former 

 just when the greatest efforts are concentrated on the latter. 

 To avoid contradictions, I see nothing better at present than to 

 adopt (which, for the immense majority of astronomers, would 

 simply mean to preserve) the constant of aberration given by 

 Nyren, and the corresponding parallax, as it is calculated with 

 the last value of the velocity of light, determined by Michelson 

 at Breteuil. In a few years the material for a new calculation 

 of the aberration will not be wanting ; each of the series in- 

 stituted lately for the study of the variations of latitude at 

 short periods will furnish a useful element for discussion. 



We see from this rapid analysis how all fundamental problems 

 of astronomy, even being connected with each other in a single 

 problem, by effect of the equations of condition that unite the 

 single constants, can, for simplicity of investigation, be gathered 

 in diverse groups, according to the more intimate dependence 

 of some constants upon others. We see also that the gigantic 

 general combination, attempted by Harkness a few years ago, is 

 really premature, but has notable importance, as the first essay 

 of the method which sooner or later must be used. Harkness' 

 work in every way teaches us the inutility, or at least the small 

 efficacy, of isolated researches, that are not sought to be co- 

 ordinated with the analogous researches on elements of the same 

 group. If it be too bold to establish the solar parallax on several 

 dozen physical constants, mechanical or astronomical, bound 

 more or less with it, it would be too timid and behindhand not 

 to take into account with it the velocity of light, the stellar and 

 planetary aberration, and the variation of the geographical lati- 

 tudes. The nutation joins with the constants that define the 

 figure of the globe and the law of the distribution of matter in the 

 internal strata of it. Finally, the precession is united to the 

 proper motions of the stars, and to the translation of the solar 

 system in space, elements which in their turn depend on the 

 systematic corrections of the stellar catalogues. 



A general renovation, then, of fundamental astronomy must 

 precede a new and authoritative definition of the numbers that 

 the twentieth century must accept as the more probable values 

 of the constants. The compilation of a fundamental catalogue, 

 which must be the consequence of all this work, cannot be 

 subordinated to considerations of opportunity, and much 

 less to what Dr. Chandler shrewdly calls the "sentimental 

 association vulgarly attaching to round figures." If, for 

 reasons foreign to science, the editors of the Connaissance 

 des Temps and of the Nautical Almanac will not accept the 

 Newcomb's or the Auwers' stars, let them keep their own ; 

 the slight residuary discrepancy between the ephemeridse will 

 certainly be less harmful than agreement on a new pro- 

 visional system, that, not having more value than the existing 



