CHAPTER XV. HARVEST MOON, AND POLAR APPEARANCES. FERGUSON has given so lucid an explanation of the Harvest Moon, on the common terrestrial globe, that I choose to use his language entirely. "It is generally believed that the Moon rises about 50 minutes later every day than on the preceding; but this is true only with regard to places on the Equator. In places of considerable Latitude, there is a remarkable difference, especially in the harvest time, with which farmers were better acquainted than astronomers till of late; and gratefully ascribed the early rising of the full Moon at that time of the year to the goodness of God, not doubting that He had ordered it so on purpose, to give them an immediate supply of moonlight after sunset, for their greater convenience in reaping the fruits of the Earth.' "In this instance of the harvest Moon, as in many other discoverable by Astronomy, the wisdom and beneficence of the Deity is conspicuous, who really ordered the course of 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 Who first observed the harvest Moon? To what was it attributed? Is it not justly ascribed to benevolent design? How doog the full Moon rise at the Equator? the former. At considerable distances from the Equator, where the weather and seasons are more uncertain, the autumnal full Moon rises very soon after sunset, for several evenings together. At the polar circles, where the mild season is of very short duration, the autumnal full Moon rises at sunset from the first to the third quarter. And at the poles, where the Sun is for half a year absent, the winter full Moons shine constantly without setting, from the first to the third quarter. It is soon said that all these phenomena are owing to the different angles made by the horizon and different parts of the Moon's orbit; and that the Moon can be full but once or twice in a year, in these parts of her orbit which rise with the least angles. But to explain this subject intelligibly, we must dwell longer upon it. The plane of the Equator is perpendicular to the Earth's axis: and therefore, as the Earth turns on its axis, all parts of the Equator make equal angles with the horizon both at rising and setting; so that equal portions of it always rise or set in equal times. Consequently, if the Moon's motion were equable, and in the Equator at the rate of 12° 11', it would rise and set about 50 minutes later every day than on the preceding; for 12° 11' of the Equator, rise or set in 50 minutes of time in all latitudes. But the Moon's motion is so nearly in the Ecliptic that we may consider her at present as moving in it. Now the different parts of the Ecliptic, on account of its obliquity to the Earth's axis, make very different angles with the horizon as they rise Those parts or signs which rise with the smallest angles set with the greatest and vice versa. How does the full Moon rise in the temperate zones?—at the poles? or set. In equal times, whenever this angle is least, a greater portion of the Ecliptic rises than when the angle is larger; as may be seen by elevating the pole of a globe to any considerable latitude, and then turning it round its axis in the horizon. Consequently, when the Moon is in those signs which rise or set with the smallest angles, it rises or sets with the least difference of time; and with the greatest difference, in those signs, which rise or set with the greatest angles. In northern Latitudes, the smallest angle made by the Ecliptic and horizon, is when Aries rises, at which time Libra sets; the greatest, when Libra rises, at which time Aries sets. From the rising of Aries to the rising of Libra, (which is twelve siderial hours,) the angle increases; and from the rising of Libra to the rising of Aries, it decreases in the same proportion. By this article and the preceding, it appears that the Ecliptic rises fastest about Aries, and slowest about Libra. All these things will be made plain by rectifying a globe to the Latitude of a given place, and then putting small patches on the Ecliptic of that globe, as far from one another, as the moon moves from any point of the celestial Ecliptic in 24 hours, which at a mean rate is 137 degrees; and then, in turning the globe round, observe the rising and setting of the patches in the horizon, as the index points out the different times in the hour circle." Those years, in which the harvest Moon will be most beneficial in the northern hemisphere for half a century, will be from 1835 to 1843, and from 1853 to 1861. How can this subject be illustrated on a globe? POLAR APPEARANCES. Could the enterprising adventurers now attempting it, reach the Arctic pole, they would see the Sun at the vernal equinox, begin to be barely visible above the horizon; and in 24 hours it would appear to pass quite through the whole points of the compass. The next 24 hours would present a similar appearance, but with the Sun a trifle more elevated above the horizon. This elevation would continue to increase till the 20th of June, the summer solstice. From that period the Sun would be more and more depressed till the 23d of September, when it would sink below the horizon. But the twilight would then remain at that point of the compass where the Sun was, as it appears to us a few minutes before, or after sunrise. As the Sun became vertical farther and farther south, this twilight would become more and more faint, till four fifths of its declination south of the Equator was reached; when its light would no longer be visible. While the declination of the Sun increased the other 50, and till its declination had decreased 5°, twilight would not be seen. From that time it would be visible and increase till the vernal equinox, and then the Sun would appear above the horizon again. This estimate is without allowing for the refractive powers of the atmosphere; a subject to be resumed in the next chapter. If the Moon revolved round the Earth exactly in the Ecliptic, it would, at the full, be as much above the horizon, at the north pole, as the Sun was south of the Equator. Of course, at the win What must be the annual appearances of the Sun at the north pole? What of the Moon? ter solstice, when no twilight was enjoyed at the Arctic pole, the Moon would appear above the horizon at its first quarter, and remain passing round and becoming more elevated till the full, then it would descend and diminish gradually till the last quarter, when it would disappear for two weeks. The Moon's variation from the Ecliptic does not essentially alter the appearances. Besides this amount of twilight and moonlight, there are frequent and very brilliant coruscations of the Aurora Borealis, supposed to be occasioned by the discharge of the electric fluid. Light at the north pole, adequate to all the purposes for which light is needed, is evidently enjoyed, and with much less inequality, when compared with equatorial regions, than is generally understood. The same remarks apply to the south pole in the opposite period of the year; and the intermediate nine years, out of the almost 19 years, which are required for a complete revolution of the Moon's nodes. He must be a dull scholar indeed, who does not in these circumstances of the harvest moon, and its polar appearances, discern legible inscriptions of the Creator's benevolent designs. CHAPTER XVI. ATMOSPHERE. DIVINE Goodness has surrounded the Earth with a fluid mass of matter, which like other matter gravitates, or tends towards the centre of the Earth, moves with the Earth in its daily and yearly revoutions, and is exceedingly elastic. This fluid is What other illumination there? |