22. I SHALL first consider the changes which are made in the plane of the moon’s orbit. The moon not moving in the same plane with the earth, the sun is seldom in the plane of the moon’s orbit, viz. only when the line made by the common intersection of the two planes, if produced, will pass through the sun, as is represented in fig. 97. where S denotes the sun; T the earth; A T B the earth’s orbit described upon the plane of this scheme; C D E F the moon’s orbit, the part C D E being raised above, and the part C F E depressed under the plane of this scheme. Here the line C E, in which the plane of this scheme, that is, the plane of the earth’s orbit and the plane of the moon’s orbit intersect each other, being continued passes through the sun in S. When this happens, the action of the sun is directed in the plane of the moon’s orbit, and cannot draw the moon out of this plane, as will evidently appear to any one that shall consider the present scheme: for suppose the moon in G, and let a straight line be drawn from G to S, the sun draws the moon in the direction of this line from G toward S: but this line lies in the plane of the orbit; and if it be prolonged from S beyond G, the continuation of it will lie on the plane C D E; for the plane itself, if sufficiently extended, will pass through the sun. But in other cases the obliquity of the sun’s action to the plane of the orbit will cause this plane continually to change. 23. SUPPOSE in the first place, the line, in which the two planes intersect each other, to be perpendicular to the line which joins the earth and sun. Let T (in fig. 98, 99, 100, 101.) represent the earth; S the sun; the plane of this scheme the plane of the earth’s motion, in which both the sun and earth are placed. Let A C be perpendicular to S T, which joins the earth and sun; and let the line A C be that, in which the plane of the moon’s orbit intersects the plane of the earth’s motion. To the center T describe in the plane of the earth’s motion the circle A B C D. And in the plane of the moon’s orbit describe the circle A E C F, one half of which A E C will be elevated above the plane of this scheme, the other half A F C as much depressed below it.