VOL. XLV.] PHILOSOPHICAL TBANSACTIONS. 43? 



the moon's nodes ; future astronomers may be enabled to settle the quantity of 

 the mean precession of the equinox, so far as it affects the declination of these 

 stars, with great certainty : and they may likewise discover, by means of the 

 stars near the solstitial colure, from what cause the apparent change in the 

 obliquity of the ecliptic really proceeds, if the mean obliquity be found to dimi- 

 nish gradually. 



The fore-mentioned points indeed can be settled only on the supposition, that 

 the angular distances of these stars do continue always the same, or that they 

 have no real motion in themselves; but are at rest in absolute space. A suppo- 

 sition which, though usually made by astronomers, yet seems to be founded on 

 too uncertain principles, to be admitted in all cases. For if a judgment may be 

 formed, with regard to this matter, from the result of the comparison of our best 

 modern observations, with such as were formerly made with any tolerable degree 

 of exactness ; there appears to have been a real change in the position of some 

 of the fixed stars, with respect to each other ; and such as seems independent of 

 any motion in our own system, and can only be referred to some motion in the 

 stars themselves. Arcturus affords a strong proof of this : for if its present de- 

 clination be compared with its place, as determined either by Tycho or Flam- 

 steed ; the difference will be found to be much greater than what can be sus- 

 pected to arise from the uncertainty of their observations. 



It is reasonable to expect that other instances of the like kind must also occur 

 among the great number of the visible stars : because their relative positions may 

 be altered by various means. For if our own solar system be conceived to change 

 its place, with respect to absolute space ; this might, in process of time, occa- 

 sion an apparent change in the angular distances of the fixed stars ; and in such 

 a case, the places of the nearest stars being more affected, than of those that are 

 very remote; their relative positions might seem to alter; though the stars them- 

 selves were really immoveable. And on the other hand, if our own system be 

 at rest, and any of the stars really in motion, this might likewise vary their ap- 

 parent positions ; and the more so, the nearer they are to us, or the swifter 

 their motions are. Or the more proper the direction of the motion is, to be ren- 

 dered perceptible by us. Since then the relative places of the stars may be 

 changed from such a variety of causes, considering that amazing distance at 

 which it is certain some of them are placed, it may require the observations of 

 many ages to determine the laws of the apparent changes, even of a single star : 

 much more difficult therefore must it be to settle the laws relating to all the 

 most remarkable stars. 



When the causes which affect the places of all the stars in general are known; 

 such as the precession, aberration, and nutation ; it may be of singular use to 

 examine nicely the relative situations of particular stars ; and especially of those 



