Motion of the Sun, and solar System. 237 
same motion, the stars must appear to an inhabitant of the 
earth to move in an opposite direction. In the triangle sp a, 
Plate VII. Fig. 1, let s p represent the parallactic motion of a 
star ; then, if this star is one that has no real motion, sp will 
also be its apparent motion ; but if the star in the same time, 
that by its parallactic motion it would have gone from s to p, 
should have a real motion which would have carried it from s 
to r, then will it be seen to move along the diagonal s a, of 
the parallelogram srp a; and p a, which is parallel and equal 
to s r, will represent its real motion. Therefore, in the above 
mentioned triangle spa, which I suppose to be formed in the 
concave part of the heavens by three arches of great circles, 
the eye of the observer being in the center, the three sides will 
represent, or stand for, the three motions I have named : sp the 
parallactic, p a the real, and sa the apparent motion of the star. 
The situation and length of these arches, in seconds of a degree, 
will express, or rather represent, not only the direction but 
also the quantity of each motion, such as it must appear to an 
eye in the above mentioned central situation. 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 which the 
sun is moving <p, the parallactic motion, when these are given, 
will be had by the expression ~~ = sp. This theorem, and its 
corollaries, of which frequent use will be made hereafter, it 
will not be necessary here to demonstrate. 
When I call the arch pa the real motion, it should be under- 
stood that I only mean its representative ; for it must be evi- 
dent that the absolute motion of a star in space, as well as its 
intrinsic velocity, will still remain unknown, because the incli- 
nation of that motion on which also its real velocity will 
