centre of revolution, then of course its whole circumference in 

 succession would be presented to the earth as well as to the sun ; 

 but in a regular orbital revolution, as that of the moon or the 

 earth, the presentation of the whole circumference to the orbit's 

 centre, without the revolving body's turning round on its axis, is 

 perfectly impossible. Were a carriage wheel to slide along a 

 rail with the same point of its circumference continually in con- 

 tact with the rail, would any one ever imagine it was all the time 

 turning round on its axle ? With just as little propriety and 

 show of reason do astronomers affirm that, when the moon slides 

 along the rail of her orbit with the same face continually look- 

 ing to the earth, she not the less turns round on her axis. For, 

 whether the progressive movement be made along a straight line 

 or a curve, the line of motion described by the progressing axis 

 is always and invariably parallel to, or concentric with, the tracks 

 or lines of motion described by the diametrically opposite points 

 of its circumference, each at the angular distance of 90"* from 

 those points of circumference intersected by the orbit of the axis, 

 and a line drawn through the axis, directly across, and perpen- 

 dicular to these three lines of motion, and through those diame- 

 trically opposite points, will always measure and indicate the 

 body's rotation. Such a line is the radius-vector of the moon or 

 other body revolving in an orbit ; and it is utterly beyond the 

 possibility of any reasonable question, that a revolving body, 

 which keeps the same point of its circumference continually at- 

 tached to the radius-vector, in precisely the same relation to the 

 axis and all these other points, must revolve without rotating or 

 turning round on its axis. It is, indeed, nothing less than a po- 

 sitive contradiction in terms to say that the moon rotates, when, 

 as admitted, she really does not turn round about on her track like 

 a wheel; for, without turning round about on her track like a 

 wheel, she cannot turn round about on her axis. The one is the 

 indispensable condition and accompaniment of the other. 



Dr Johnson defines " rotation" to be " the act of whirling 

 round like a wheel ; the state of being so whirled round ; whirl ; 

 vicissitude of succession." Well ; how do the phenomena of 

 the moon's motion correspond with these definitions ? She pre- 

 sents continually the same face to the centre of her orbit ; and 

 astronomers allege that phenomenon to be the effect of her rota- 

 tion^ or whirling round like a wheel on her axis once during the 

 period of her orbital revolution ; but, strange to say, this alleged 

 rotation, or whirling round, so far from producing a " vicissitude 

 of succession" in respect of the fixed centre of her orbit, leavesj 

 the same point of circumference constantly in the same relative] 

 place towards that centre, just as if there were no rotation orj 

 whirling at all. The moon maintains precisely the same rela- 



