STAR POSTTION.S AiND GALACTU CO-ORDINATES. 49 



- 6.88" 7"-' + 0.186" 73 



- 4.56 7- - 0.145 T^ 



\- 22.46 7'-' - 0.128 73 



in which 7 indicates solar centuries after k)00, assimiing as 

 correct Newconib's variations of the Earth's equator and orbit. 



In Circular No. 5 the advantages of using a fixed system of 

 co-ordinates in planetary theories is insisted on, and for this 

 fixed .system there is no reason why the Galactic system should 

 not be used. If anything, the simplifications which would thus 

 be introduced into the planetary theories, are more considerable 

 than those already dealt with. At its simplest, the motions of 

 planets and comets are complicated, but this comi)lication is 

 vastly increased by adding on to their motions — both in their 

 orbits and of their orbits — other imaginary motions, as is done 

 at present. 



In Circular Xo. 6 it is shown that if even a precise daily 

 ephemeries of a star in equatorial co-ordinates is required — 

 which appears to be the most unfavourable case for the use of 

 galactic co-ordinates — one can be easily and rapidly computed 

 without any knowledge of the star's imaginary mean right ascen- 

 sion and declination. As an example, an ephemeris of f Orion 

 is computed for the current year ( 1913 ). 



Astronomers have been quite aware of the awkwardness of 

 referring the stars and planets to rapidly-shifting planes, and 

 some attempts to get over the chief difficulties have been made. 

 Thus the late Dr. Ristenpart proposed that the equinox should 

 be changed every 25 years, and he would call 1875, 1900, 1925, 

 etc., normal equinoxes. He has published tables for the normal 

 equinox of 1925. His idea would save some work, but not 

 much — it is, in short, only a palliative. The astronomical 

 ephemerides do something in the same direction. As an 

 example, I will deal with the issue for 191 5 of the celebrated 

 Connaissance des Tciuj^s, which, as it proudly boasts, is the 

 oldest astronomical almanac now published, as it first appeared 

 in 1679, and has never suffered an interruption ; besides this, it 

 has always been brought up to date so punctually that it might 

 also claim to be the youngest. For the Sun, the Connaissance 

 des Temps furnishes the longitude and latitude for date, and 

 for the mean equinoxes and ecliptics of 1915 and icyzo — three 

 where one would do. Similarly for the planets the heliocentric 

 co-ordinates are referred to the Earth's true equinox and ecliptic 

 of date and the mean equinox and ecliptic of 1920. Eor 732 

 minor planets the Gaussian Constants and their variations are 

 given. It seems illogical to speak of constants varving — thev 

 really do not do so — nevertheless, 4,392 annual variations are 

 specially comj)uted for 191.S. If galactic co-ordinates had been 

 introduced, it is only three annual variations that would have 



