On the Motion of the Sun and Solar System. 935 



which are known to change iheir magnitudes periodically. 

 For, as we have great reason to ascribe these regular changes 

 o a rotatory motion of the stars*, a real motion in space 

 nay be expected to attend it; and the number of these 

 tars is so considerable, that their concurring testinjony 

 .vould he verv desirable. 



Perhaps Algol, which according to these ideas must have 

 a very quick rotatory motion, may be found to have also a 

 considerable progressive one , and if that should be ascer- 

 tained, the position of the axis of the rotation of this star 

 will be in a great measure thereby discovered. 



An aro;unient from the real motion to a rotatory one is 

 nearly of equal validity, and therefore all the stars that have 

 a motion in space may be surmised to have also a rotation 

 on their axes. 



Symptoms of parallactic Motions. 



But, setting aside theoretical arguments, T shall now pro- 

 ceed to such as may be drawn from observation ; and as all 

 parallactic motions are evident indications that the observer 

 of them is not at rest, it will be necessary to explain three 

 sorts of motions, of which the parallactic is one ; they will 

 often engage our attention in the following discussion. 



Let th"e sun be supposed to move towards a certain part 

 of the heavens; and since the whole solar system will have 

 the same motion, the stars must appear to an inhabitant ot 

 the earth to move in an opposite direction. In the triangle 

 spa, (Plate VIII.) Fig. 1, hi s p represent the parallactic 

 motion of a star ; then, if this star is one that has no real 

 motion, s p v/i'il also be its apparent m-otion ; 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 v/ill it be seen to 

 move along the diagonal s a, of the parallelogram s r p o ; 

 and p a, which is parallel and equal to s r, will represent its 

 real motion. Therefore, in the above-meniioned triangle 

 s p 0, 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 centre, the three sides will represent, 

 or stand for, the three motions I have named : s p the pa- 

 rallactic, p a the real, and s a the apparent motion of the 

 star. The situation and length of these arches, in secondu 

 of a degree, will express, or rather represent, not only the 

 direction but also the quantity of each motion, such as it 



• See Phil. Trans, for the year 175)5, p 68. 



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