STAR POSTTIONS ANl") (lALACTIC CO-ORDIN' ATES. 4? 



made the obser\ations, thev did not reduce tlieni. The prison- 

 house of variable rigfht ascensions and declinations was not yet 

 readv. But early in the nineteenth century. IJessel g^ot all into 

 shipshape order. Bessel's aptitude for rll the problems of 

 spherical astroncunv was marvellous, so that to a subject which 

 seemed unwield>- in its chunsiness. he managed to fit formulas 

 of — when the complexitv of the subject is considered — remark- 

 able neatness. The relief was so threat that astronomers accepted 

 the fetters gleefulh'. Bessel's star-reduction numbers, which are 

 published in ever\- astronomical ephemeris. provide for the 

 aberration of light, nutation in latitude and longitude, and the 

 precession of the equinoxes. Tlius encouraged, astronomers soon 

 reduced the observations of I.acaille. Bradley, Mayer, and 

 Lalande to mean epochs. So long as the stars concerned are 

 not ver\- near the ])oles of rlie rotating- sky and the period does 

 not exceed one century, the precession formulae are not too un- 

 wieldy, one has to calculate for both right ascension and 

 declination the first term of the precession, which will be multi- 

 plied by / the time elapsed, then the secular variation which is 

 to be multii)lied bv f^/200, then the third term of the precession, 

 •which will be midtiplied l)v t^. Tt is true that for many thousands 

 of stars these precessional terms are already calculated, and all 

 that remains is for the user of a catalogue to do the multiplica- 

 tions by t, t-/200 and t^. But the labour of making all these 

 calculations is prodigious, and it is all done to impress an 

 imaginary motion to the fixed stars. This motion makes it 

 difficult to compare the places of stars in dififerent catalogues. 

 Thus we have in catalogues the following positions of e Orion: — 



Catalogue. Epoch. 



Lalande 1800 



Brit. Assoc 1850 



Cape 1900 



indicating change in one century of 



R.A. + 5m. 4.70s. Dec^ + 4' 30.7". 



whereas the real movement of the star in that time has only been 

 11.66" in all,- — the rest is fictitious. 



The use of the moving or equatorial co-ordinates of Right 

 Ascension and Declination is inevitable for some purposes, such 

 as in determining the places of the Sun, the interioi planets 

 Mercury and \'enus. and the clock-stars; for finding with an 

 equatorial telescope, rough equatorial co-ordinates such as can 

 be read off a good star map by inspection are all that are required. 

 Why should we, therefore, impose moving co-ordinates on 

 millions of fixed stars because we require moving co-ordinates 

 for the Sun and a few |)lancts and clock stars? 



