500 LECTURE XLII. 



greatest apparent motions. Besides, the star Castor appears, Avhen viewed 

 with a telescope, to consist of two stars, of nearly equal magnitude; and 

 though they have both a considerable apparent motion, they have never been 

 found to change their distance a single 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 change 

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

 lono- 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 from his centre: 

 and since the centre of inertia of the whole system must ,be undisturbed by 

 any reciprocal actions or revolutions of the bodies composing it, the sun 

 must describe an irregular orbit round this centre, his greatest distance from 

 it being equal to his own diameter. Wc 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 presupposed know- 

 ledge 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 to- 

 wards the centre, our right hands will be eastwards, and our left westwards. 

 All the rotations of the diflferent bodies which compose the solar system, as 

 far as they have been ascertained, are in the same direction, and all their 

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