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into the phenomenon called the harvest moon. It ismanifest also that all the circumstances of eclipses,solar as well as lunar, must be importantly modifiedby the remarkable variations which take place in themoon’s distance from the earth, and in her real andapparent motions. The eccentricity of the moon’sorbit also produces very interesting effects in relationto her librations. If the perigee and apogee alwaysheld a fixed position with respect to the nodes of thelunar orbit, the peculiarities thus arising would be lessremarkable ; but the continual shifting of the relativepositions ofthenodes andapses (astheperigee andapogeeare called) causes a continual variation, as we shall seehereafter, in the circumstances of the lunar librations.
Speaking generally it may be said that the lunar peri-gee advances at the mean rate mentioned in the pre-ceding chapter, that is in such a way as make a completecircuit in about 3232-575 days. Accordingly, applyingconsiderations resembling those applied to her motionwith respect to her nodes, we see that the period ofher motion from perigee to perigee must exceed asidereal month. Its actual length is found to beabout 27-555 days. This is the mean anomalisticmonth •* it exceeds the mean nodical month byrather more than the third part of a day; or moreexactly by O’342 of a day.
The actual motion of the perigee and apogee with
* The actual interval between the moon’s passages of her perigeevaries during the course of a year from about 25 days to about28^ days.