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line of stars representing the ecliptic or concave circle of the hea* 

 vens ; the continuous ring, the orbit of the earth ; and the body in 

 the centre, the sun or other primary, as the case may be. When 

 the planet or satellite is in its place on the orbit at N, it is then 

 in line with both the star N and the sun, and were it to turn 

 round on its axis at that point, both the sun and the star would 

 equally indicate the completion of the rotation, because they are 

 both, as well as the straight line that connects them, at right 

 angles with or perpendicular to the orbit At the same time, 

 the straight line, forming in part the planet's radius-vector, con- 

 tinued across the orbit, points directly to the star S, at the an- 

 gular distance of 180° from the star N. Then move the planet 

 to the octant point NW. The diametric line of the orbit run- 

 ning through the planet's axis and the sun will then point to 

 two different stars, each at the angular distance of 45* from the 

 opposite ends of the line NS ; but the sun will remam just where 

 he was, in precisely the same place in the centre of the orbit, at 

 the same direct distance, and still subtending with his radius a 

 perfect right angle at the planet's axis on the orbit. The same 

 process repeated at each octant or at each degree of the circle 

 will shew precisely the same results. At the 1 80 degree of re- 

 volution, the planet will be in precisely the same relation to the 

 star S that it formerly was in to the star N ; which of them is to 

 measure the rotation now ? They both alike measure it exactly, 

 because they are once more in a rectangular position with re- 

 spect to the axis on the orbit ; and because they have fallen short 

 of the true measm*e of rotation to the extent of 1 80**, or half the 

 planet's circumference each ; so that, with respect to the planet, 

 they have virtually come into each other's places. Precisely the 

 same results would follow were the planet to stand still at N, 

 while the circle of the stars revolved, if that were possible, round 

 the centre of their circle, and then they would shew unquestion- 

 ably that it is only where the planet's protracted radius-vector 

 touches the circle of the heavens that the star which may be for 

 the time at or opposite to that extremity of the radius, can indi- 

 cate the completion of the planet's rotation. If, while the pla- 

 net stands at N, and rotates without revolving, will the star 

 NE be the true measure of rotation ? No, certainly. Then 

 just as little can the star N be the true measure of rotation 

 when the planet revolves to NW. Let the small circle which 

 has hitherto been considered as representing the earth's circum- 

 ference now be considered to represent the moon's orbit, with 

 the earth in its centre, and every thing that has been said will 

 apply to the moon's case, as exactly as if she were really, what 

 she is virtually, a point of the earth's circumference. Slie does 

 not lag behind the earth as the latter progresses; she is some- 



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