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The Harvest Moon.
The Moon moves in her orbit (13° 10' 35'in each solar day of 24 hours. But as the motion of the Earth in the ecliptic, at the same time, is 59'8"', the apparent motion of the Sun, the excess of her motion over that of the Earth, or the Sun's apparent motion, is 12° 11' 27". A meridian of the Earth, in its diurnal rotation, moves this in 48' 38'' of time. But, the Moon at the same time moving forward, the meridian will not overtake her till 50' 28". The Moon rises at the equator, therefore, about 50' 28' later on each succeeding day, than it did on the preceding, at all seasons of the year. But the case is very different in high latitudes. Farmers in those latitudes have long observed(the early rising of the autumnal full moon.)
“ In this instance, says Mr. Ferguson, “as in many others, discoverable by astronomy,
the wisdom and beneficence of the Deity are conspicuous, who ordered the Moon so as to bestow more or less light on all parts of the Earth, as their several circumstances and seasons render it more or less serviceable. About the equator, where there is no variety of seasons, and the weather changes seldom, and at stated times, moonlight is not necessary for gathering in the produce of the ground; and there the Moon rises about 50 minutes later every day or night than on the former, In considerable distances from the equator, where the weather and seasons are more uncertain, the autumnal full Moons rise very soon after sunset for several evenings together. At the polar circles, where the mild season is of short duration, the autumnal full Moon rises at sunset from the first to the third quarter." The horizon of any place forms angles with the plane
of the Moon's orbit, differing from each other at different parts of the day, and at different seasons of the year. From the fvaried position of these circles arise the phenomena of the harvest Moon. About the 23d of September, when the Sun enters Libra, the Earth enters Aries. At any place in north latitude, the angle between the horizon and the Moon's orbit is less, at that season of the year, about the time of sunset, than at any other time of the day. The full Moon, about the autumnal equinox, being in that part of her orbit opposite the Sun, must rise at this angle in latitudes below the Arctic circle. The angle decreases from the equator to this circle, where it vanishes. While the Moon remains in this part of her orbit, in all places where the angle is small, her diurnal motion will make but little variation in the time of day when she rises, on each succeeding evening.
To illustrate the phenomena of the harvest Moon, put small pieces of paper on the ecliptic of a terrestrial) globe, on each side of the first degree of Aries, at 120 11' 27" from each other, representing the Moon's diurnal motion from the Sun ; rectify the globe for the latitude of the place, suppose 450 With the number of papers or pieces of paper corresponding to the days of a week, bring the westernmost to the eastern horizon; set the index of the hour circle at the time of the Moon's rising, on the evening nearest to three and a half days before her arrival at the first degree of Aries. Turn the globe westward, which will be the same in effect as the rotation of the horizon eastward in respect to the Moon, till the second paper is brought to the horizon; the index will point to the time of the Moon's rising on the succeeding evening. Bring in succession the papers to the horizon; the index will show the time of the Moon's rising on the succeeding days of the week. The inclination of the Moon's orbit to the plane of the ecliptic will not materially affect this representation.
When the full happens at the equinox, the Moon ar
rives at the first of Aries at that time. But when the full nearest the equinox happens any number of days before or after the 23d of September, her distance from that point at the full may be obtained with sufficient nearness, by taking a degree for each day between the full and the equinox, rejecting odd minutes both of time and motion. Compute the time of her arrival by her motion from the Sun. It will be found that the Moon is rarely more than one day's motion from the first of Aries at the time of the equinoctial full. If the index be set at 12, when the first paper
is brought to the horizon, and the other papers be brought in succession to that circle, the difference of time, when the Moon rises on the several nights, may be seen on the hour circle.
A more natural representation of the harvest Moon may be made by an artificial globe taken from the frame. Let a candle be placed on a stand, to represent the Sun. On a level with the candle, and a little distance to the west of it, let the globe be holden, the north pole so elevated as to form an angle of 23° 28' with the horizon. Let a small taper be placed under the globe, to represent the Moon at her first quarter. Carried to the west of the globe, the taper may represent her at the full in Aries. Over the globe, it will show her situation at her last quarter. By turning the globe round, and observing when any place, as Washington, comes into the light of the taper in its different situations, the appearance of the Moon rising at that place may be represented. If, in its western position, the taper be moved slowly and circularly up, so that the arch moved may subtend an angle of 125at the globe, while the globe itself is turned once round on its axis, and continued in this manner for several rotations, nearly an exact resemblance of the harvest Moon presented.
When the Moon rises with the least angle, she sets with the greatest; and when she rises with the great
est, she sets with the least. In other words, when the time of day, in which she rises for successive evenings, differs the least, the time of day in which she sets differs the most. At the full, the time of rising on successive evenings, differs most about the vernal equinox.
In every revolution, the Moon passes through the same signs; but, except in autumn, her rising with the least angle or difference of time, always about the first of Aries, is seldom observed. She enters Aries, in winter, about the first quarter, and, rising about midday, attracts no particular notice; about the change in spring, when, from nearness to the Sun, she is not visible; in summer, about the last quarter, and rising at or near midnight, is seldom observed.
The statement of Mr. Ferguson, respecting the rising of the Moon at the polar circles, is not strictly true. From the first quarter, when she rises about sunset at those circles, to nearly the last quarter, the Moon rises 3m. 56 s. earlier in the day on each succeeding evening.
The Moon is seldom full exactly at the equinox. When it is not, the fulls immediately before and after that time, exhibit phenomena resembling the equinoctial full. The nearer any full is to the equinox, the more resemblance it bears to the harvest Moon; the nearest being generally so denominated, whether it happen before or after the equinox.
The same phenomena are exhibited by the Moon in south latitudes, but at opposite times of the year; the autumnal equinox of south latitude being coincident with the vernal of the north.
The circumstances of the harvest Moon are in some measure affected by the inclination of the Moon's orbit to the ecliptic. This inclination is about 5° 9'. Moving backward, her nodes perform a revolution in about 18 y. 224 d. The harvest Moon is most beneficial during half of this time ; least beneficial during the other half; most, when her ascending node is in the first