i?34 On the ^lotion of the Sun and Solar Si/stem. 



must appear to an eve in the above-mentioned central situa- 

 tion. And calling, the solar motion S, the distance of the 

 ■star from the sun d, and the sine of the star's distance from 

 the point towards vvliich the sun is moving ^, the parallac- 

 tic uiotlon, when these are £;iven, will be had by the ex- 

 pression -— = s /;. This theorem, and its corollaries, of 



r .tl 



which frequent use will be made hereafter, it will not be 

 necessary here to demonstrate. 



When I call the arch p a the real motion, it should be 

 understood that I onlv mean its representative ; for it must 

 be evident that the absolute motion of a star in space, as 

 well as its intrinsic velocity, vvill still remain unknown, be- 

 cause, the inclination of that motion on which also its real 

 velocity will depend, admits of the greatest variety of di- 

 rections. We arc only acquainted with the plane in which 

 llic motion must be performed, and with the length of tiie 

 arch in seconds bv which that motion n)ay be measured. 

 We may add that the chords of the arches representing the 

 three nrotions aie the smallest velocities of tliese motions 

 that can be admitted ; for in every other direction but at 

 right angles to the line of sight, the actual space over which 

 the star will move must be greater ihan the arch or chord 

 by which its motion is represented. 



Now, since a motion of the sun will occasion parallactic 

 motions of the stars, it follows that these again must indi- 

 cate a solar motion ; but in order to ascertain whether pa- 

 rallactic motions exist, we ought to examine tliosc stars 

 which are nio;U liable to be visibly affected by solar motion. 

 This requisite points out the brightest stars as the most pro- 

 per for our j)urpose ; for any star may have a great real mo- 

 tion, but in order to have a great parallactic one, it must 

 be in the neighbourhood of the sun. And as we can only 

 judge of the distance of the stars by their splendour, we 

 ought to choose the brightest, on account of a probability 

 that, being nearer than faint ones, they may be more within 

 ihc reach of parallax, and thus better qnalilied to show its 

 effects. 



We are also to look out for a criterion whereby paral- 

 lactic may be dislinguishc^l from real motions ; and this 

 we find in their directions. For, if a solar motion exists, 

 all parallactic motions will tend to a point in opposition to 

 the direction oi that motion ; whereas real motions will be 

 tllspersed indiscriminately to all parts of space. 



VVith tht;fc;.e distinctions in \ iew, we may examine the pro- 

 per 



