HARVEST MOON. In our latitudes, at the time of full moon nearest the autumnal equinox, it happens that the moon rises for several days nearly at sunset, and about the same time by the clock, instead of rising, as it usually does, 52' later on one day than on the This phenomenon is owing to the fact, that at this time the moon is in Aries, when the part of the ecliptic below the horizon makes the least angle with it, as shown in the following figure, where AB represents a portion of the equator; 1 l' a portion of the horizon; Cl,, a portion of the ecliptic when C represents the equinoctial point of Aries; CI:, a portion of the same if C were the equinoctial point of Libra. Then the moon to move in the ecliptic, a supposition not far from the truth, and one which simplifies the explanation of this phenom enon), if the moon be at C (point of Aries) on one night, it will have retrograded to L hy the same time on the following night; and, by the revolution of the earth in the direction of NM', will appear on the hori zon at II, and the distance LH reduced to time will give the moon's retardation. If C be the equinoctial point of Libra, then L' will be the moon's position on the second night, and it will rise at II' after the earth has revolved so as to carry the whole of the line HI' above the horizon; this line, when reduced to time, gives the retardation. Hence, as the moon when at the full is in Aries at the sun's autumnal equinox,
and in Libra at the sun's spring equinox, the retardation is least in the first instance and greatest in the second, being respectively CN — CM, and CN CN + CM. In the latitude of Edinburgh (55° 58') the greatest retardation is 1 hour 8' 24% and the least 11' 44'; in lat. 04° 27', the least retardation is zero, or the moon rises at the same time on two successive evenings, while at the arctic circle (67° 30) it rises 4' earlier on the second evening. As this phenomenon occurs at a time (about Sept. 23) when the farmer is busy with his harvest, and very opportunely supplies hint for several evenings with light sufficient to continue his operations after sunset, the moon at this stage has received the name of " harvest moon. ' As the moon's orbit is inclined to the ecliptic. this irregularity will be increased or diminished according as the ascending node is between Capricorn and Cancer, or between Cancer and Capricorn. It is nothing at the equator, increasing as we proceed north. At our antipodes the greatest retardation occurs in Sept., and the least in March.