STAR POSITIONS AND GALACTIC CO-ORDINATES. 



By Robert T. A. Innrs, F.R.A.S. 



The Galactic circle, the great circle to which the course of the Via 

 Lactea most nearly conforms. Every subject has its technical or conven- 

 tional terms, by whose use circumlocution is avoided and ideas rendered 

 definite. This circle is to sidereal what the invariable ecliptic is to 

 planetary astronomy, — a plane of ultimate reference, tlie ground-plane of 

 the sidereal system." — Sir J. Herschel, " Outlines of Astronomy," 1849, 

 P- 533- 



in nearly every case concerning the inotions of the heavenly 

 bodies, the astronomer has to refer their places to the centre 01 

 the Stm ; so that if an observer could be heliocentric, much labour 

 of reduction would be saved. Let us trv and ima.a:ine how a 

 heliocentric observer, with all our notions upon astronomy, would 

 start to work. His first object would be to form a catalog^ue of 

 the stars, and to do so, he would have to decide on the planes of 

 reference to which their places should be referred. If a terres- 

 trial friend shoidd sug^g-est that the jjlane of the Earth's equator 

 should be the fttndamental plane, and that the direction of the 

 intersection of the Earth's orbit with its equator should be the 

 initial point, I think our heliocentric astronomer would be sur- 

 prised. He mig^ht say : '" but both the Earth's orbit and its equator 

 are changing constantly, and the stars are virtually fixed ; would 

 I not by choosing such co-ordinates involve myself in endless cal- 

 culations? Why should I not choose soine plane and starting 

 point nearly invariable and related in .some way to the stars ? " 

 The terrestrial astronomer could only reply that his predecessors 

 used the Earth's equator, etc, and that the theory of meridian 

 instruments which they used was based on the fact that the Earth 

 rotates. But what, the heliocentric astronomer might ask, has 

 the Earth's rotation to do with the places of the stars; would not 

 the stars .still be in their places if the Earth ceased to rotate? 

 Why not fix the jilaces of your stars by photography? It gives 

 results more precise than any meridian instrument, and from 

 these precise places, yoti may. if vou wish, find out the positions 

 of the Earth's equator and orbit ; l)irt do not mix y(mr ideas, and 

 put the cart before the horse ! 



How is it that sidereal astronomy has got so involved? I 

 think the difttculty grew^ in this way: Precise astronomv com- 

 menced about 1750. It is interesting to note that the oldest 

 catalogue that the astronomer keeps on his working shelf is for 

 the epoch 1750. and was compiled from Lacaille's observations 

 made in Strand Street. Capetown, with a ^-inch telescope. 

 The other star catalogues between 1750 and 1800 are due to the 

 labours of Bradley, Mayer, and Laiande. But the work of 

 these four astronomers is remarkable in this, that although they 



