40'2 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



To return to the principal subject of this paper, which is the proper motion of 

 the sun and solar system : does it not seem very natural, that so many changes 

 among the stars, — many increasing their magnitude, while numbers seem gra- 

 dually to vanish ; — several of them strongly suspected to be new-comers, while 

 we are sure that others are lost out of our sight ; — the distance of many actually 

 changing, while many more are suspected to have a considerable motion : — does 

 it, not seem natural that these observations should cause a strong suspicion that 

 most probably every star in the heaven is more or less in motion ? And though we 

 have no reason to think, that the disappearance of some stars, or new appearance 

 of others, nor indeed the frequent changes in the magnitudes of so many of them, 

 are owing to their change of distance from us by proper motions, which could 

 not occasion these phenomena without being inconceivably quick ; yet we may 

 well suppose that motion is some way or other concerned in producing these 

 effects. A slow motion, for instance, in an orbit round some large opaque body, 

 where the star, which is lost or diminished in magnitude, might undergo occa- 

 sional occultations, would account for some of those changes, while others might 

 perhaps be owing to the periodical return of large spots on that side of the sur- 

 face which is alternately turned towards us by a rotatory motion of the star. The 

 idea also of a body much flattened by a quick rotation, and having a motion 

 similar to the moon's orbit by a change of the place of its nodes, by which more 

 of the luminous surface would at one time be exposed to us than another, tends 

 to the same end ; for we cannot help thinking with M. de la Lande (Mem. 1776), 

 that the same force which gave such rotations, would probably also occasion mo- 

 tions of a different kind by a translation of the centre. Now, if the proper mo- 

 tion of the stars in general be once admitted, who can refuse to allow that our 

 sun, with all its planets and comets, that is, the solar system, is no less liable to 

 such a general agitation as we find to obtain among all the rest of the celestial 

 bodies. 



Admitting this for granted, the greatest difficulty will be, how to discern the 

 proper motion of the sun among so many other, and variously compounded, mo- 

 tions of the stars. This is an arduous task indeed, which we must not hope to 

 see accomplished in a little time ; but we are not to be discouraged from the at- 

 tempt. Let us, at all events, says Mr. H. endeavour to lay a good foundation 

 for those who are to come after us. I shall therefore now point out the method 

 of detecting the direction and quantity of the supposed proper motion ot the sun 

 by a few geometrical deductions, and at the same time show by an application of 

 them to some known facts, that we have already some reasons to guess which 

 way the solar system is probably tending its course. 



Suppose the sun to be at s, tig. 8, pi. 6 ; the fixed stars to be dispersed in all 

 possible directions and distances around, at s, s, s, s, &c. Now, setting aside the 



