48 STAR POSITIONS AND GALACTIC CO-ORDINATES. 



above given -f- 0.05" means anything, because, after all, it depends 

 Oti the position of the equinox, a very elusive point indeed. 



I have laid the advantage of using ( ialactic Co-ordinates 

 before astronomers in Circulars Xos. 2, 5 and 6, issued by the 

 Union Observatory. In Circular No. 2 the necessarv formulae 

 are developed, and tables given for the conversion of mean 

 equatorial co-ordinates to invarial>le galactic co-ordinates for any 

 year from 1750 to 1950, with a special table for the conversion 

 of true equatorial co-ordinates to invariable galactic co-ordinates 

 or ricc-Tcrsa for the current \'ear igi^. The processes are illus- 

 trated 1)v numerical examples. The most thorough investigation 

 •of the position of the Cialactic Plane was made by' Newcomb, and 

 published in [904 under the title '" On the f^oslfion of the Galactic 

 and other Principal Planes tozvards -ichich the Stars tend to 

 crowd." Unfortunately the ])rincipal galactic ])lane cannot be 

 found verv preciselv because the Milk\- W'av i'. a verv irregular 

 aggregation of stars, throwing out wisps and branches, whilst the 

 zone of brightest stars is considerablv inclined to it. Xewcomb 

 gives the following poles : — 



R.A. Dec. 



Galactic Plane (omitting branch) .. .. 192.8° +27.2° 

 Galactic Plane (including branch ) .. .. 191. i 26.8 



Plane of fifth type stars '<p.9 26.7 



It is known that star> of the hfth type (bright line spectra) 

 are all close to the princijial ])lane, but their number is small. It 

 has further been assumed that the solar system is in the principal 

 plane. But it is not a matter of great importance if the assumed 

 position of the plane is in error so long as it is nearly correct, 

 as any small error can easily be allowed for when the progress of 

 astronomy requires. The total change in many thousands of 

 years will certainly be less than tlie change in all the present 

 star-catalogues every fifty years caused by the fictitious precession 

 of the stars. Besides the position of the plane, it is necessary 

 to adopt a departure point from which longitudes are to be 

 counted, and it is obvious that this must be a fixed point. The 

 point actually chosen is that one which in this age will make 

 the longitude of the apex of solar motion, — the direction to- 

 wards which the Sun is moving — equal to o'^. This point is 

 chosen because a considerable portion of the proper motions of 

 the stars is actually due to the solar motion through space, and 

 it renders the efifect of this motion in a uniform manner, in that 

 generally all proper motions so far as they are caused by the 

 Sun's motion, tend towards 180' of galactic longitude. Here, 

 again, a compromise has had to be made, because the apex of solar 

 motion depends on the class of stars to which it is referred. I 

 have adopted for 1900 the position of the apex recommended bv 

 Dr. Campbell, of the Lick Observatory, namely. R.A. i8h. 

 Dec. -f- 30"^. and for the plane. Newcomb's determination includ- 

 ing the branch, so that we have 



