46 STAR POSITIONS AND GALACTIC CO-ORDINATES. 



I think that the reason astronomers have kept to moving 

 co-ordinates was a beHef that the use of Galactic Co-ordinates, 

 to coin a term suggested not only by the nature of the problem, 

 but by Sir John Plerschel, would lead to formulae of even gf.reater 

 complexity than those given by Bessel. When I proposed the 

 use of Galactic Co-ordinates, a well-known astronomer wrote me 

 that the idea was attractive, but that no one would look further 

 unless numerical examples showing how it worked in practice 

 were forthcoming. I must confess, than when I took up this 

 challenge I was not too confident that the f ormulje would work ; 

 in the result I was agreeably surprised, — the resulting expressions 

 even in the most disadvantageous cases are hardly longer than 

 the old methods, and in all others they are shorter and sometimes 

 very much shorter. If this was the only gain, it would be con- 

 siderable, but the real gain is behind this, the positions furnished 

 by the Galactic Co-ordinates are final, comparisons betwixt star 

 catalogues will become immediate. To work out a proper 

 motion of a star to-day is hours, if not days, of work, because 

 of the fictitious movements impressed upon it ; with galactic co- 

 ordinates, the comparison will be the work of minutes, and will, 

 moreover, yield the proper motion referred to its natural i)lafie. 



One cannot but regret that in planning the great Carte-du- 

 Ciel the advantages of a proper system of co-ordinates were 

 ignored. Let us look at the difficulty which the system adopted 

 leads to. To take, as an example, the Cape Cartc-du-Ciel 

 Catalogue, we learn from Mr. Hough's last report that it will fill 

 eleven quarto volumes and give the positions of about 990,000 

 stars in all. At the same rate the complete sky Cartc-du-Ciel 

 catalogues will furnish the places of some 13,500,000 stars. 

 These are all to be referred to the mean equator and mean equinox 

 of 1900. If the catalogue is repeated, as is the implied intention 

 in another century (or less as I hope), and it is referred to its 

 mean epoch, any comparison between the two catalogues will be 

 extremely laborious because the efifect of precession on the so- 

 called standard co-ordinates is com])lex. But let us assume that 

 instead of i3-i million stars there are only 8 million catalogued, 

 and that each star can be effectively compared in ten minutes of 

 time, then the complete comparison will occupy one man's full 

 time for 555 years, and at the moderate salary of £200, cost 

 i 110,000. I fear that there is a danger here in that the Cartc-du- 

 Ciel scheme is strangling itself so far as useful work goes by 

 adding so enormously to its load of inertia. Had the places been 

 referred to the galactic system by the u.se of galactic plate centres 

 and secular co-ordinates, comparisons would be practically 

 instantaneous. There is, however, a saving clause : when the 

 Cartc-du-Ciel scheme was started in the 1880-1890's, astronomers 

 tacitly assumed (forgetting Proctor's work) that each star moved 

 on its own— was an individuality distinct from its neighbouring 

 stars — an assumption engendered by the known motions of the 



