162

THE MOON’S CHANGES

that its greatest range from the ecliptic is attainednearly at the points e and e, and is therefore appre-ciably equal to the sun’s range. The circumstancescf the moon’s motion must therefore resemble veryclosely those of the sun’s, the chief difference result-ing from the fact that the nodes of the moon’s orbitin the equator are some twelve or thirteen degreesin advance of the equinoctial points.

Lastly, similar considerations apply when the de-scending node of the moon’s orbit is near e, themoon’s path being in this case erne' m', and its nodeson the equator some twelve or thirteen degrees behindthe equinoctial points.

Now let it be noticed that the moon’s orbit passesthrough the complete cycle of changes (of which theabove four cases are the quarter changes) in about18‘6 years, the lunar node moving on the wholebackwards on the ecliptic. Thus, if such a cycle ofyears begin with the moon’s orbit in the positionWMEM' (fig. 53, Plate XIII.), then in about a fourthof the cycle (that is, in about 4‘65 years), the moon’sorbit is in or near the position e'm 'em, fig. 54, thenode having moved backwards from W to near orone quarter of a revolution. One fourth of the cyclelater,—that is, about 9‘3 years from the beginning ofthe cycle, the moon’s orbit is in or near the positionE»'ff m, fig. 53, the node having moved still back-wards from e' to near E. Yet another fourth of the

this kind need not detain us in a general explanation such as thatwe are now upon.