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inclination increases ; while if the body is at Q, oranywhere on the arc M N', the node advances as ton' and the inclination diminishes.
It will be observed that as the motion of the nodeis here referred to the direction of the body’s motion,the result applies equally whether N be the ascendingor the descending node.
We have, then, these general rules :—Force towardsthe plane of reference,—inclination diminishes whilethe body’s distance from plane of reference is increas-ing, and vice versa; but nodal line regredes through-out. Force from the plane of reference,—inclinationincreases while the body’s distance from the plane ofreference is increasing, and vice versa,; but nodal lineadvances throughout.
Now let us apply these results to the moon’s motionround the earth, the plane of reference in this casebeing the ecliptic.
Let us suppose, first, that the line of the moon’snodes is placed (and remains during a complete revo-lution of the moon) as is shown in fig. 38, Plate X., Xbeing the ascending node, and N' the descending node.*Then the line A'A, to points in which all the perturbingforces act, lies in the plane of the moon’s orbit, beingcoincident with N N' in direction and situation. F
* The small lines surmounted by arrow-heads in this and suc-ceeding figures are intended to indicate the amount of the distanceof the corresponding points of the lunar orbit, above or below theplane of the ecliptic,—this last plane being supposed to he repre-sented by the plane of the paper. They are drawn to scale.