CHAPTER V. PHENOMENA OF THE HARVEST MOON. "In this The moon's mean motion in her orbit in each solar day of 24 hours, is 13° 10′ 35′′. But as the earth moves, in the ecliptic at the same time 59′ 8′′, the apparent motion of the sun, the moon's motion from the sun is 12° 11' 27". Any meridian of the earth in its diurnal rotation, moves this distance in 48′ 38" of time. But as the moon is also moving, the meridian will not overtake her till 50 28". At the equator, therefore, the moon rises about 50′ 28′′ later, each succeeding day, at all seasons of the year. But in high latitudes it is very different. In those latitudes, farmers have long observed the early rising of the autumnal full moon. 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, moon light 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.” These phenomena are caused by the varied positions of the horizon and the moon's orbit. To avoid embarrassment in the explanation, the moon may be considered as moving in the ecliptic, and the obliquity of her orbit, as affecting the harvest moon, afterwards considered. PHENOMENA OF THE HARVEST MOON. 85 The plane of the ecliptic forms unequal angles with the horizon of any place on the earth at different parts of the day, and at different seasons of the year. When the sun enters Libra, about the 23d of September, the earth enters Aries. Then at any place in north latitude, the angle between the horizon and the ecliptic is less about sun setting, than at any other time of day. The full moon, which happens about the autumnal equinox, being in that part of the ecliptic opposite the sun, must rise at this angle, in latitudes below the Arctic circle. There the angle, decreasing with the increase of latitude from the equator, vanishes. The diurnal motion of the moon in its orbit 13° 10′ 35′′ will make but little variation in the time of its rising on each succeeding evening, while it remains in this part of its orbit in all places, when the angle is small. Because the less the angle, the sooner the horizon will overtake the moon. For illustration, put small patches on the ecliptic of a terrestrial globe, each side of the first degree of Aries at 12° 11' 27" from each other, that they may represent the moon's diurnal motion from the sun; rectify the globe for the latitude of the place, suppose 45°, and with the number of patches 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, found by calculation or a diary, on the evening nearest to three and a half days before her arrival at Aries, turn the globe westward, which will repreгергеsent the rotation of the horizon eastward, in relation to the moon, till the second patch comes to the horizon, and the index will point at the time of the moon's rising on the succeeding evening. Bring the patches in succession to the horizon, and the index will show the time when the moon will rise on each day for a week. The moon arrives at Aries, at the equinox, when the full happens at that time. But when the equinoctial full falls any number of days before or after the 23d of September, multiply 59' 8" by the intervening days, the product reduced to degrees gives the moon's distance from the first of Aries at the full. Compute the time by the moon's diurnal motion. The arrival is later than the full preceding the equinox; earlier, when the equinox precedes the full. These calcula tions may be much more easily made, by taking a degree for each day, and rejecting odd minutes, both of time and motion. The arrival at the equinox may thus be ascertained with sufficient exactness. The number of patches may be increased or diminished at pleasure, and the rising exhibited for a longer or a shorter time. If the index be set at 12, when the first patch is brought to the horizon, and the other patches be brought to that circle in succession, the difference of time between the moon's rising on the several nights may be seen on the hour circle. The harvest moon may be more naturally represented by an artificial globe taken from the frame. Let a candle be placed on a stand to represent the sun, and the globe holden at a little distance west of the candle and on a level with it but the north pole so elevated, as to form an angle of 23° 28′ with the horizon. A small taper placed under the globe may represent the moon at the first quarter. The taper carried to the west of the globe, may represent her at the full in Aries Placed over the globe it may show her situation in the last quarter. By turning the globe round, and observing when any place, as Washington, comes into the light of the taper in its different positions, you may see the appearance of the moon rising at that place. If the taper in its western position be moved slowly and circularly up, so as to make 12° at the globe, while the globe turns once round, and thus continued for several revolutions, nearly the exact appearance of the harvest moon may be represented. When the moon rises with the least angle, it sets with the greatest; and, when it rises with the greatest, it sets with the least. The time of rising at the full differs the most about the vernal equinox. The moon passes the same signs in every revolution ; but her rising with the least difference, always about the first of Aries, is seldom observed, except in autumn. In winter she enters Aries about the first quarter, and rising in the day time, is scarcely noticed; about the change in spring, and being with the sun, is not seen; in summer, about the last quarter and rising near midnight, is not often observed. In the quotation from Mr. Ferguson, at the commencement of this article, it was stated, that "at the polar circles, the autumnal full moon rises at sunset from the first to the third quarter." This is not strictly true. At those circles, the moon rises about sunset at the first quarter, and afterwards at the end of each sidereal day nearly to the third.So that during that time it rises 3′ 56′′ earlier on each succeeding day. When the harvest moon does not happen at the equinox, the fulls immediately before and after exhibit phenomena nearly resembling the equinoctial full; more nearly, as they are nearer the equinox, and as often in Pisces as Aries. The moon exhibits the same phenomena in south latitudes at opposite times in the year. There also, wisely ordered by a kind Providence, the full moon in autumn rises with less variation, than at any other season. This happens, when the moon is in Virgo and Libra, the autumnal equinox of south latitude corresponding to the vernal of the north. The inclination of the moon's orbit to the ecliptic, about 5° 9', varies in some measure the circumstances of the harvest moon. Her nodes move backward in the ecliptic, performing a revolution in about 18 years 224 days. Half of this time the harvest moon will be most, the other half, least beneficial; most, when her ascending node is in the first degree of Aries, least, when her descending node is in that sign. The full moon in summer runs much lower in some years than in others. She runs low, when in that part of her orbit, which is south of the ecliptic; lowest, when her latitude south is greatest, or she is 90° from her nodes. The winter full moon runs high, when she is north of the ecliptic; highest, when she is fartherest north, or 90° from her nodes. Though some of the full moons of summer are lower than others, all are low, and their time above the horizon short.In winter they are high and long above the horizon, when the dreary and protracted nights make moonlight most beneficial. At the poles, the full moon is not seen for nearly half the year, being below the horizon. But this is in summer, when, from the continued shining of the sun, moon light would be of no use. In winter, when the light of the moon is most beneficial, at the benighted polar regions, she shines continually, from about the first to the third quarter. Who can contemplate these things, and not admire and adore the wisdom and goodness of Him, who not only "gave the sun for a light by day," but "the moon and stars for a light by night !" The following table shows in what years the harvest moon is least, and in what most beneficial. L stands over the columns least of all beneficial; M over those most of all beneficial. In both the columns marked S, the harvest moon is fartherest south, or lowest in the orbit; in those marked N, it is fartherest north or highest in the orbit. Some years come so near a mean between the least and most beneficial, as scarcely to belong to either. Such is the year 1815. HARVEST MOON OF THE 19TH CENTURY, N. Years least beneficial. L. S. 1806 1807 1808 1809 1810|181118121813|1814|1815| 1844 1845 1846 1847 1848 1849 1850 1851|1852| |