harvest moon

the moon at and about the period of fullness that is nearest to the autumnal equinox.

[1700-10]

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▪ full moon

      the full moon nearest the autumnal equinox (about September 23). Near the time of the autumnal equinox, the angle of the moon's orbit relative to the Earth's horizon is at its minimum, causing the full moon to rise above the horizon much faster than usual. Since the difference of the moon's rising time on successive nights barely varies, the moon appears to rise at nearly the same hour for several nights in succession. Because the harvest moon, like any full moon, must rise near the hour of sunset, harvest workers in the Northern Hemisphere may be aided by bright moonlight after sunset on several successive evenings. A similar effect is observed in corresponding southern latitudes around March 21.

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