398 LECTURE XLII. 



second ; a circumstance which is easily understood if both their apparent 

 motions are supposed to arise from a real motion of the sun, but which 

 is much less probable on the supposition of two separate and independent 

 motions. 



Besides this progressive motion, the sun is subjected to some small ch&iige 

 of place, dependent on the situations of the planetary bodies, which was 

 long inferred from theory only, but which has been actually demonstrated 

 by modern observations. Supposing all the planets to be in conjunction, 

 or nearly in the same direction from the sun, the common centre of inertia 

 of the system is at the distance of about a diameter of the sun fron}3 his 

 centre : and since the centre of inertia of the whole system must be undis- 

 turbed by any reciprocal actions or revolutions of the bodies comper: Jg it, 

 the sun must describe an irregular orbit round this centre, his greatest 

 distance from it being equal to his own diameter. We may form an idea 

 of the magnitude of this orbit by a comparison with the orbit of the 

 moon : a body revolving round the sun, in contact with his surface, must 

 be nearly twice as remote from his centre as the moon is from the earth, 

 and the sun's revolution round the common centre of gravity of the system 

 must therefore be, where it is most remote, at four times the distance of 

 the moon from the earth. 



The sun revolves on his axis in 25 days 10 hours, with respect to the 

 fixed stars : this axis is directed towards a point about half way between 

 the pole star and Lyra, the plane of the rotation being inclined a little 

 more than 7 to that in which the earth revolves. The direction of this 

 motion is from west to east, terms which we can only define from our pre- 

 supposed knowledge of the stars, by saying that the motion is such, that a 

 point of the sun's surface at first opposite Aries, moves towards Taurus. 

 Nor have we any better mode of describing north and south, or right and 

 left : we can only say comparatively, that if we are placed with our heads 

 northwards, and looking towards the centre, our right hands will be east- 

 wards, and our left westwards. All the rotations of the different bodies 

 which compose the solar system, as far as they have been ascertained, are 

 in the same direction, and all their revolutions, excepting those of some of 

 the comets, of which the motions are retrograde, and those of some of the 

 satellites of the Georgian planet, which revolve in planes so distant from 

 those of the other planetary motions, that the directions of their revolutions 

 can scarcely be called either direct or retrograde. 



The time and direction of the sun's rotation is ascertained by the change 

 of the situation of the spots,* which are usually visible on his disc, and 

 which some astronomers suppose to be elevations, but others, apparently 

 on better foundations, to be excavations or deficiencies in the luminous 

 matter covering the sun's surface. These spots are frequently observed to 

 appear and disappear, and they are in the mean time liable to great varia- 

 tions, but they are generally found about the same points of the sun's 

 surface. Lalande t imagines that they are parts of the solid body of the 



* Discovered by Fabricius. See his treatise De Maculis in Sole observatis, Wit- 

 tenb. 1611. 



f Hist, et Mem. 1776. Brugnatelli, Bibliot. Fisic. i. 55. 



