520 



NATURE 



[March 29, 1888 



OUR ASTRONOMICAL COLUMN. 

 The Pulkowa Catalogue of 3542 Stars for 1855. — Dr. 

 Backlund has given in the " Melanges Mathematiques et Astrono- 

 miques " of the St. Petersburg Academy, tome vi., pp. 563-99, 

 a comparison of the star places of the Pulkowa Catalogue of 

 3542 stars for 1855, with those of the other Pulkowa star 

 catalogues, including the unpublished one for 1875 by Herr 

 Romberg, as well as with the, catalogues of Becker, Boss, and 

 Resphigi. This catalogue, which forms part of vol. viii. of the 

 Pulkowa observations, contains the mean places of 3542 stars 

 observed with the meridian-circle of that Observatory during the 

 years 1840-69. It includes all the Bradley stars north of S. 

 Decl. 15°, with the exception of the Pulkowa fundamental stars, 

 which have not been included because the catalogue-places 

 depend on their positions as determined with the transit-instru- 

 ment and vertical circle. And as the definitive positions of the 

 present catalogue depend on the two catalogues of fundamental 

 stars for 1845 and 1865, it appears that the system of the 

 catalogue for the epoch 1855 will be practically identical with 

 that of the mean of the two catalogues above mentioned. Dr. 

 Backlund's comparisons give a very favourable view of the 

 accuracy with which the relative positions of the stars have been 

 determined, the probable error of an R.A. in the 1855 catalogue 

 being ± o'"034 for a star south of N. Decl. 30°, and of a declina- 

 tion ± o"-30 ; but they also show that the problem of the 

 determination of absolute positions is in a far less satisfactory 

 state. A further comparison of the Pulkowa 1855 catalogue has 

 been made by Herr Seyboth {Astr. Nacli. 2808), the catalogue 

 with which it has been compared being that of the Cape 

 Observatory for 1880. The agreement appears to be as close as 

 could be expected considering the unfavourable position at either 

 Observatory of several of the stars used ; the Pulkowa star places 

 south of the equator show, however, some discordance from 

 those obtained at the Cape as also from those of Boss's standard 

 catalogue. From a comparison of the fundamental stars only, 

 Herr Seyboth finds for the probable error of the Cape places 

 ± o'02is. in R.A,, and ± o'38s. in Decl. 



The Constant of Precession and the Proper Motion 

 OF THE Solar System. — The completion of Auwers' re-reduc- 

 tion of Bradley's observations, and of the definitive catalogue 

 based thereupon, taken together with the catalogue of 3542 stars 

 and the two fundamental catalogues referred to in the preceding 

 note, has furnished M. L. Struve with the means for a very im- 

 portant investigation, which he has recently published in vol. 

 XXXV. of the Memoires of the Imperial Academy of St. Peters- 

 burg. The epoch for Bradley's catalogue being 1755, ^^^^ t^^ 

 mean date of the three catalogues just mentioned, 1855, M. Struve 

 had at his disposal the places of all Bradley's stars north of S. 

 Decl. 15° for two epochs a century apart. The differences of 

 these places would be due to a combination of three causes : the 

 actual proper motions of the stars, the movement of the solar 

 system, and the error in the constant of precession employed ; and 

 they therefore furnish the means of determining both the true 

 value of the precession constant and the direction and rapidity 

 of the motion of translation of the solar system. In his discus- 

 sion of his materials, M. Struve has followed the method adoped 

 by Sir G. Airy in his treatment of the same problem (Memoirs 

 R.A.S., vol. xxviii.), and determined all his unknowns at the 

 same time. As after excluding all those stars which rest upon 

 but one observation of Bradley, together with a few others omitted 

 for special reasons— seven for their large proper motion, — there 

 still remained 2509 stars, with 2181 proper motions in R.A., and 

 2345 in Decl., without some method of grouping, the equations 

 of condition would have been too numerous for manipulation. 

 M. Struve has therefore marked off seven zones, each 15° in 

 breadth, and divided these by lines of right ascension into 120 

 spherical trapezia of nearly equal areas. Each group has been 

 weighted according to the number of the stars it contains, and 

 also according to their magnitudes, so as to reduce the influence 

 of the brighter stars. The solution of the equations of con- 

 dition by the method of least squares shows very nearly the same 

 correction to the adopted constant of precession, — that of Prof. 

 O. Struve, of 1841, viz. 50" -3 798, —from the proper motions in 

 right ascension as from those in declination, the resulting value 

 for the constant being 5o"'35i4, not so small a value as Nyren's, 

 50" -3269, but smaller than those of Bessel, Dreyer, and Bolte. 

 The equating of X, Y, and Z to zero in the normal equations 

 gives a result substantially the same, and proves that the constant 

 can be considered as independent of the motion of the solar 

 system. 



For the speed of translation of the solar system, M. Struve 

 finds q — + 4" -3642 ; for the co-ordinates of the apex of the 

 motion, A = 273° 21', and D = -f 27° 19'. Comparing the 

 various determinations which have been made by other 

 astronomers, he is disposed to adopt as a mean position 

 of the apex — A — 266° 7, D = -f 3i°-o. For q, the displace- 

 ment of the sun in 100 years as seen from an average star 

 of the sixth magnitude, a number of investigators, O. Struve, 

 Dunkin, Gylden, and Madler, have found values not greatly 

 differing from 5"; but others, Ubaghs, Airy, Rancken, and 

 Bischof, obtain very different results, varying from q = l"'45 by 

 Ubaghs, io q — 49"'S by Bischof. Taking o"-oii as the mean 

 parallax of a star of the sixth magnitude, q — 5" would represent 

 an annual motion of about five radii of the earth's orbit, or a 

 velocity of a little over 15 miles per second. It is clear, how- 

 ever, that we are yet far from being in a position to regard these 

 estimates of velocity as more than provisional. 



The chief difficulty in these investigations lies in our ignorance 

 of the actual distances of the stars from us, and even of their 

 relative distances. M. Struve has assumed the following mean 

 values of p — the distance of the star from the sun — for each order 

 of magnitude, the sixth magnitude being taken as unity : — 

 m. m. 



1 ... 0-13 5 ... 070 



2 ... 0*23 6 ... i-oo 



3 - 0-36 7 ... 1-49 



4 •. 0-51 8 ... 2-25 



and adopting 8" for the secular proper motion of a sixth magni- 

 tude star, the scale represents to some extent the proper motions 

 actually observed. M. Struve has also discussed the question of 

 the rotation of the entire sidereal system in the plane of the 

 Milky Way, but his results do not afford any support to the 

 hypothesis, and it has been neglected in the general investigation. 

 At the end of the memoir M. Struve has given the means for 

 the calculation of the general and planetary precessions, together 

 with their secular variations. An appendix furnishes a list of 

 those Bradley stars for which the Pulkowa catalogues gave 

 proper motions sensibly different from those deduced by 

 Auwers from the Greenwich and Berlin observations. 



Comet 1888a (Sawerthal). — Dr. B. Mattheissen gives 

 {Astr. Nach. No. 2830) the following ephemeris from Finlay's 

 elements for this object : — 



For Berlin midnight. 

 1888. R.A. Decl. Log r. Log ^. Bright- 



h. m. s. o / ness. 



April 3 22 13 45 7 35-9 N. 9-8821 00728 0*83 

 5 20 12 9 407 



7 26 36 n 39-3 9 "9035 00934 0-68 



9 32 55 13 317 



II 39 8 15 18-4 9*9267 0-1134 0-56 



13 45 17 16 59-5 



15 51 21 18 35-7 99507 01328 4-46 



17 22 57 19 20 6-9 



19 23 3 u 21 335 N. 99749 01513 0-38 

 The brightness on February 18 has been taken as unity. 



ASTRONOMICAL PHENOMENA. FOR THE 



WEEK 1888 APRIL 1-7. 



/"pOR the reckoning of time the civil day, commencing at 



^ Greenwich mean midnight, counting the hours on to 24, 



is here employed.) 



At Greenwich on April i 

 Sun rises, 5h. 36m. ; souths, I2h. 3m. 45 'Ss. ; sets, i8h. 32m. : 

 right asc. on meridian, oh. 44 ■9m. ; decl. 4° 49' N. 

 Sidereal Time at Sunset, 7h. 14m. 

 Moon (at Last Quarter April 3, I3h.) rises, 23h. 28m.* j 

 souths, 4h. 2m. ; sets, 8h. 30m. : right asc. on meridian, 

 i6h. 41 'Sm, ; decl. 17° 59' S. 



Right asc. and declination 

 Planet. Rises. Souths. Sets. on meridian. 



h. m. h. m. h. m. h. m. o / 

 Mercury.. 5 o ... 10 24 ... 15 48 ... 23 5-3 ... 7 48 S. 



Venus 5 o ... 10 28 ... 15 56 ... 23 9*1 ... 6 54 S. 



Mars 19 34*... o 59 ... 6 24 ... 13 38-1 ... 7 32 S. 



Jupiter..., 23 25*... 3 38 ... 7 51 ... 16 i8-i ... 20 23 S. 

 Saturn.... 11 26 ... 19 25 ... 3 24*... 8 7-6 ... 20 49 N. 

 Uranus... 18 42*... o 18 ... 5 54 ... 12 57-7 ... 5 25 S. 

 Neptune., 7 22 ... 15 3 .. 22 44 ... 3 44-9 ... 18 8 N. 



* Indicates that the rising is that of the preceding evening and the setting 

 that of the following morning. 



