Moon is: ferent 281. If the Earth had no annual motion, the Why the Sun would never appear to shift his place in the always ecliptic. And then every new Moon would fall in full in dif the samesign and degree of the ecliptic, and every full Moon in the opposite: for the Moon would go precisely round the ecliptic from change to change. So that if the Moon were once full in Pisces or Aries, she would always be full when she came round to the same sign and degree again. And as the full Moon rises at sun-set (because when any point of the ecliptic sets, the opposite point rises) she would constantly rise within two hours of sun-set, on the parallel of London, during the week in which she was full. But in the time that the Moon goes round the ecliptic from any conjunction or opposition, the Earth goes almost a sign forward: and therefore the Sun will seem to go as far forward in that time, namely, 27 degrees; so that the Moon must go 27 degrees more than round, and as much farther as the Sun advances in that interval, which is 2 degrees, before she can be in conjunction with, or opposite to the Sun again. Hence it is evident that there can be but one conjunction or opp sion of the Sun and Moon in a year in any Her periparticular part of the ecliptic. This may be fami- odical and liarly exemplified by the hour and minute-hands of synodical a watch, which are never in conjunction or oppo- exemplifi sition in that part of the dial-plate where they were ed. so last before. And indeed if we compare the twelve hours on the dial-plate to the twelve signs of the ecliptic, the hour-hand to the Sun, and the minute-hand to the Moon, we shall have a tolerable near resemblance in miniature to the motions of our great celestial luminaries. The only difference is, that while the Sun goes once round the ecliptic, the Moon makes 124 conjunctions with him: but, while the hour-hand goes round the dial-plate, the minutehand makes only 11 conjunctions with it; because the minute-hand moves slower in respect to the hourH h revolution The harvest and Moon. hand than the Moon does with regard to the Sun. 282. As the Moon can never be full but when she hunter's is opposite to the Sun, and the Sun is never in Virgo and Libra, but in our autumnal months, it is plain that the Moon is never full in the opposite signs, Pisces and Aries, but in these two months. And therefore we can have only two full Moons in the year, which rise so near the time of sun-set for a week together, as above-mentioned. The former of these is called the Harvest Moon, and the latter the Hunter's Moon. Why the 283. Here it will probably be asked, why we neregular ri- ver observe this remarkable rising of the Moon but sing is ne- in harvest, seeing she is in Pisces and Aries twelve ver per times in the year besides; and must then rise with in harvest. as little difference of time as in harvest? The answer ceived but is plain for in winter these signs rise at noon; and mer they rise about midnight, and the Sun being cuous. 284. At the equator, the north and south poles lie in the horizon, and therefore the ecliptic makes the same angle southward with the horizon, when Aries rises, as it does northward when Libra rises. Consequently as the Moon at all the fore-mentioned patches rises and sets nearly at cqual angles with the horizon all the year round, and about 50 minutes later every day or night than on the preceding, there can be no particular harvest-moon at the equator. 285. The farther that any place is from the equator, if it be not beyond the polar circle, the angle gradually diminishes which the ecliptic and horizon make when Pisces and Aries rise: and therefore when the Moon is in these signs she rises with a nearly proportionable difference later every day than on the former; and is for that reason the more remarkable about the full, until we come to the polar circles, or 66 degrees from the equator; in which latitude the ecliptic and horizon become coincident every day for a moment, at the same sidereal hour (or 3 minutes 56 seconds sooner every day than the former), and the very next moment one half of the ecliptic, containing Capricorn, Aquarius, Pisces, Aries, Taurus, and Gemini, rises, and the opposite half sets. Therefore, while the Moon is going from the beginning of Capricorn to the beginning of Cancer, which is almost 14 days, she rises at the same sidereal hour; and in autumn just at sun-set, because all the half of the ecliptic, in which the Sun is at that time, sets at the same sidereal hour, and the opposite half rises; that is, 3 minutes 56 seconds of mean solar time, sooner every day than on the day before. So while the Moon is going from Capricorn to Cancer, she rises earlier every day than on the preceding; contrary to what she does at all places between the polar circles. But during the above fourteen days, the Moon is 24 sidereal hours later in setting; for the six signs which rise all at once on the eastern side of the horizon are 24 hours in setting on the western side of it; as any one may see by making chalk-marks at the beginning of Capricorn and of Cancer, and then, having elevated the pole 66 degrees, turn the globe slowly round its axis, and observe the rising and setting of the ecliptic. As the beginning of Aries The harvest moons re is equally distant from the beginning of Cancer and of Capricorn, it is in the middle of that half of the ecliptic which rises all at once. And when the Sun is at the beginning of Libra, he is in the middle of the other half. Therefore, when the Sun is in Libra, and the Moon in Capricorn, the Moon is a quarter of a circle before the Sun; opposite to him, and consequently full in Aries, and a quarter of a circle hehind him, when in Cancer. But when Libra rises, Aries sets, and all that half of the ecliptic of which Aries is the middle, and therefore, at that time of the year, the Moon rises at sun-set from her first to her third quarter. 286. In northern latitudes, the autumnal full Moons are in Pisces and Aries; and the vernal full gular on Moons in Virgo and Libra: in southern latitudes, 'just the reverse, because the seasons are contrary. equator. But Virgo and Libra rise at as small angles with the both sides. of the horizon in southern latitudes, as Pisces and Aries do in the northern; and therefore the harvest-moons are just as regular on one side of the equator as on the other. 287. As these signs, which rise with the least angles, set with the greatest, the vernal full Moons differ as much in their times of rising every night, as the autumnal full Moons differ in their times of setting; and set with as little difference as the autumnal full Moons rise: the one being in all cases the reverse of the other. 288. Hitherto, for the sake of plainness, we have supposed the Moon to move in the ecliptic, from which the Sun never deviates. But the orbit in which the Moon really moves is different from the ecliptic: one half being elevated 5 degrees above it, and the other half as much depressed below it. The Moon's orbit therefore intersects the ecliptic in two points diametrically opposite to each other; and these intersections are called the Moon's nodes. So the Moon can never be in the ecliptic Moon's but when she is in either of her nodes, which is at The least twice in every course from change to change, nodes and sometimes thrice. For, as the Moon goes almost a whole sign more than round her orbit from change to change; if she passes by either node about the time of change, she will pass by the other in about fourteen days after, and come round to the former node two days again before the next change. That node from which the Moon begins to ascend northward, or above the ecliptic, in northern latitudes, is called the ascending node; and the other the descending node; because the Moon, when she passes by it, descends below the ecliptic southward. 289. The Moon's oblique motion with regard to the ecliptic causes some difference in the times of her rising and setting from what is already mentioned. For when she is northward of the ecliptic, she rises sooner and sets later than if she moved in the ecliptic; and when she is southward of the ecliptic, she rises later and sets sooner. This difference is variable even in the same signs, because the nodes shift backward about 193 degrees in the ecliptic every year; and so go round it contrary to the order of signs in 18 years 225 days. 290. When the ascending node is in Aries, the southern half of the Moon's orbit makes an angle of 5 degrees less with the horizon than the ccliptic does, when Aries rises in northern latitudes: for which reason the Moon rises with less difference of time while she is in Pisces and Aries, than she would do if she kept in the ecliptic. But in 9 years and 112 days afterward, the descending node comes to Aries; and then the Moon's orbit makes an angle 5 degrees greater with the horizon when Aries rises, than the ecliptic does at that time; which causes the Moon to rise with greater difference of time in Pisces and Aries than if she moved in the ecliptic. |