December 9, 1897] 



NATURE 



127 



LETTERS _ TO THE EDITOR 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he taideitake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communicaitons.'\ 



Astronomical Constants and the Paris Conference. 



Many objections have recently been raised — above all, in 

 America — to the decisions of the International Conference that 

 met in May 1896, at Paris, with the object of choosing a uniform 

 system of fundamental stars and astronomical constants for the 

 four great ephemerides of Berlin, London, Paris and Washing- 

 ton. The matter is of the greatest importance, inasmuch as 

 it refers to the bases of precise astronomy, together with a 

 general tendency of all science to a method of international 

 discussion that, leaving free and autonomous all personal and 

 local initiatives, bring workers in such agreement as is necessary 

 for nullifying discrepancies and contradictions. Of such ten- 

 dency we have the most comforting manifestations in the 

 International Geodetical Association, in the Commission (also 

 international) for unifying weights and measures, and in the 

 similar meetings of scientific men of every nation for the 

 electrical measurements, for meteorological services, for the 

 photography of the heavens. Astronomy is, perhaps, among 

 all physical sciences the one destined by its historical tradition, 

 no less than by its present and future necessities, to second — 

 nay, to promote and develop — the cosmopolitan tendency. The 

 grand spectacle of the face of the heavens, ever before the eyes 

 of all ; the difference of phenomena according to the horizons, 

 which carries with it the need of co-operation between the 

 observers diversely situated with regard to the celestial sphere ; 

 in fine, the high and significant moral education that comes to 

 astronomers from the continual contrast between the immensity 

 of the heavens and the miserable narrowness of the limits 

 traced out conventionally on the globe between one country and 

 another ; here are the causes through which a spirit superior to 

 any narrow nationalism was soon breathed into our souls. Tycho 

 Brahe, the proud Danish patrician, the founder of practical 

 astronomy in the Renaissance, sings sternly — 



Omne solum forti patria est, coelumque 

 Undique supra. . . . 



And his name, with those of Copernicus, Kepler, Galileo and 

 Newton, form a constellation that shines not more for the sky 

 of Denmark, than for that of Germany, Italy, or England ! 



On the other hand, on the sentimental considerations of the 

 solidarity of the human race, to which a great weight must 

 always be given, other arguments of a more modest and more 

 positive character rest and flourish; these comfort, and, if I do not 

 deceive myself, put out of all reasonable doubt the necessity of 

 imprinting on our science an international character. From 

 the day in which Frederic William Bessel solidly constructed, 

 on the admirable observations of James Bradley, the "Found- 

 ations of Astronomy," enriching with his teutonic analysis the 

 product of the patient British ingeniousness, a new current of 

 collaboration among astronomers of every country was opened. 

 The works of the said Bessel, of Struve, of Argelander, and 

 lately those of Auwers and Newcomb, set out by admitting that 

 the modern era of precise astronomy opens with Bradley, as the 

 era of the Renaissance began with Tycho Brahe, and the 

 Hellenic with Hipparchus. Each of those three great observers 

 had his precursors, whose work seems to establish a mysterious 

 continuity with the astronomy of the civilisation immediately 

 preceding it. Aristillus and Tymocharis, the first Alexandrian 

 observers, are connected with the astronomy of the Oriental 

 nations ; King Alfonso of Castile, Paolo Toscanelli and Regio- 

 montanus with that of the Arabians ; Hevelius, Cassini, Flam- 

 steed with that of the Renaissance. But as in the great 

 catalogue of Hipparchus we have the measure of what the 

 genius of the Greeks could give when applied to observations 

 of the heavens, as only in the celestial places determined by 

 Tycho Brahe we have the material for the new planetary theories 

 of Kepler, which led to the discoveries of Newton, so to Bradley 

 (and to Bradley alone) we must refer the rational application of 

 these delicate means of astronomical observation that, revealing 

 the most minute corrections to be applied to the places of stars, 

 led to the foundation of a sidereal astronomy. The precession 

 of the equinoxes more rigorously determined, the nutation of the 

 terrestrial axis and the aberration of the fixed stars discovered 



and measured in their effects ; the places of 3222 stars exactly 

 settled in regard to the fundamental circles of the sphere, with 

 a precision unknown till now, and comparable only to that of 

 the modern meridian observations : these are the fruits of the 

 long, fatiguing and sapient vigils of the Royal Astronomer in 

 that Greenwich Observatory whence astronomy issued trans- 

 formed one hundred and fifty years ago. 



But if Bradley merited in the competent and severe judgment 

 of Bessel the glorious epithet of vir incotnparabilis, if Lindenau 

 and Struve had been able to say that astronomy had been 

 renovated at Greenwich and that the work of the great English 

 observatory surpassed at the end of the eighteenth century all 

 remaining astronomical productions, if, after the exhaustive 

 discussion of Bessel {Fundamenta- Astronomies), Bradley's work 

 has still been good enough for a more refined elaboration in the 

 last quarter of our century, by the merit of Auwers, who has 

 given it all the value of which it was intrinsically susceptible, 

 it is no longer allowable nowadays to put entirely on one side 

 all the material that has been accumulated from 1750 till now, 

 and which ought to concur with Bradley's observations in fixing 

 with all possible approximation the values of the fundamental 

 constants and the coordinates of the leading stars. As to what 

 refers more especially to the precession of the equinoxes and 

 to the proper motions of the stars, it is undeniable that the 

 value of Bradley's observations is notably augmented by their 

 antiquity : the greater the length of time that has elapsed, the 

 more is manifested the influence of corrective terms, in which 

 time itself is multiplied by the coefficient of the precession or 

 by the proper motion on the one or the other coordinate. But 

 to render such influence really efficacious in the calculation of 

 the factors that enter into the same terms, it is necessary that 

 the lapse of time should be so great as to carry the terms that 

 contain it to a higher order of term (unknown) that represents 

 the accidental and systematic errors of the places settled by 

 Bradley. Now no one could affirm that the century and a half 

 that has passed between Bradley and our time be sufficient for 

 this ; on the contrary, indications are not wanting that authorise 

 an opposite opinion. Shall we, for this, resign ourselves to a 

 perfectly passive indifference, allowing time to mature the terms 

 still too restricted ? For the accidental errors, there is nothing 

 else to be done ; but they are not the most dangerous, and, on 

 the other hand, the abundance of Bradleyan observations would 

 allow us in many cases to rely upon fortuitous compensations. 

 For the systematic errors, however, the case is different We can 

 and we ought, with all the means of investigation with which the 

 improved state of the science furnishes us, to seek out those 

 systematic errors that still remain in Bradley's Catalogue, even 

 after the acute revision of Auwers. Such a work would in no 

 wise be irreverent to the labours of the talented Berlin astronomer, 

 who, for the first, has given us precious materials for investiga- 

 tions of that sort, by his new and masterly discussion of the 

 observations that Tobias Mayer made at Gottingen, con- 

 temporaneously with those of Bradley. It is necessary in a 

 special manner to ascertain (as my revered master Schiaparelli 

 suggests to me) whether and how far it is possible to render 

 independent of Bradley the enormous amount of observations 

 made by Piazzi at Palermo between 1792 and 1814. To such 

 research I am now attending, while at the New York Observatory 

 (through Dr. Davis) and at Turin the elements are being 

 prepared i<i accord for a new reduction of the Palermitan 

 Catalogue. Until such a reduction be completed, and until 

 analogous researches have been instituted on all the observations 

 that can lend themselves to the solution of the problem, and 

 that were executed in the first half of the century, it will be 

 vain to impose by international agreement the values for the 

 precession and for proper motions that should be not provisory. 

 The antiquity of the equinoxes observed by Bradley is not 

 sufficient title to make us exclude ii priori equinoxes more recent 

 but more precise, those observed in Germany especially. For 

 tl e needs of astronomy until now Bradley's work has rendered 

 incalculable service ; but what Bradley could by himself give, 

 he has given. It is perhaps illusory to believe that a deeper 

 and minuter discussion (as is undoubtedly Newcomb's last, com- 

 pared to those of the Struves) can lead us to a more intimate 

 knowledge of the truth. 



Bat it is not alone in the exclusion of intermediary observa- 

 tions, of those in a special manner belonging to the first half of 

 our century, that the conclusions of the Paris Conference find 

 serious objections, for what refers to the precession and to proper 

 motions. As Prof. Lewis Boss notes in a deep and animated 



NO. 1467, VOL. 57] 



