It is necessary, however, to remark, that the Moon does not appear perfectly round when she is full in the highest or lowest part of her orbit; because we have not a full view of her enlightened side at that time. When full in the highest part of her orbit, a small deficiency appears on her lower edge; and the contrary, when full in the lowest part of her orbit. After the full she begins to decrease gradually as she moves through the other half of her orbit; and when the eastern half of her only is enlightened, she is said to be in her third quarter, and has the following appearance: thence she continues to decrease until she again disappears at the conjunction, as before.* *These various phases may be satisfactorily and pleasantly illustrated, by placing a lighted candle on a table to represent the sun, and a small ball at some distance from it to represent the earth, and then carrying a smaller ball round it, to represent the moon revolving round the earth. Between the third quarter and change, the Moon is frequently visible in the forenoon, even when the sun shines; and then she affords us an opportunity of seeing a very agreeable appearance, wherever we find a globular stone above the level of the eye, as suppose on the top of a gate. For, if the sun shines on the stone, and we place ourselves so as the upper part of the stone may just seem to touch the point of the Moon's lowermost horn, we shall then see the enlightened part of the stone exactly of the same shape with the Moon; horned as she is, and inclined the same way to the horizon. The reason is plain; for the sun enlightens the stone the same way as he does the Moon: and both being globes, when we put ourselves into the above situation, the Moon and stone have the same position to our eyes; and therefore we must see as much of the illuminated part of the one as of the other. The position of the Moon's cusps, or a right line touching the points of her horns, is very differently inclined to the horizon at different hours of the same days of her age. Sometimes she stands, as it were, upright on her lower horn, and then such a line is perpendicular to the horizon; when this happens, she is in what the astronomers call the Nonagesimal Degree; which is the highest point of the ecliptic above the horizon at that time, and is 90 degrees from both sides of the horizon where it is then cut by the ecliptic. But this never happens when the Moon is on the meridian, except when she is at the very beginning of Cancer or Capricorn. The inclination of that part of the ecliptic to the horizon in which the Moon is at any time when horned, may be known by the position of her horns; for a right line touching their points is perpendicular to the ecliptic. And as the angle which the Moon's orbit makes with the ecliptic can never raise her above, nor depress her below, the ecliptic, more than two minutes of a degree, as seen from the sun, it can have no sensible effect upon the position of her horns. Therefore, if a quadrant be held up, so as one of its edges may seem to touch the Moon's horns, the graduated side being kept toward the eye, and as far from the eye as it can be conveniently held, the arc between the plumb-line and that edge of the quadrant which seems to touch the Moon's horns, will shew the inclination of that part of the ecliptic to the horizon. And the arc between the other edge of the quadrant and plumb-line, will shew the inclination of a line, touching the Moon's horns, to the horizon. These various phases plainly demonstrate that the Moon does not shine by any light of her own; for if she did, being globular, she would always present a fully illuminated disc like the sun. That the Moon is an opaque body, is not only proved from her phases, but also by the occultation of stars, for her body often comes between the earth and a star, and while she is passing it, the star is hid from our view. MOTIONS OF THE MOON. The neighbouring moon her monthly round MILTON. It has already been remarked, that the motions of the Moon are very irregular. The only equable motion she has, is her revolution on her axis, which is completed in the space of a month, or the time in which she moves round the earth. This has been determined by the important and curious circumstance, that she always presents the same face to the earth, at least with very little variation. But as her motion in her orbit is alternately accellerated and retarded, while that on her axis is uniform, small segments on the east and west sides alternately appear and disappear. This occasions an apparent vibration of the Moon backwards and forwards, which is called her libration in longitude. A little more of her disc is also seen towards one pole, and sometimes towards the other, which occasions another wavering or vacillating kind of motion, called the libration in latitude. This shows that the axis of the Moon is not exactly, though nearly, perpendicular to the plane of her orbit; for if the axis of the Moon were exactly perpendicular to the plane of her orbit, or if her equator coincided with that plane, we should perceive no other libration than that in longitude. When the place of the Moon is observed every night, it is found * Increasing with horns towards the east ; decreasing with horns towards the west; and at the full. that the orbit in which she performs her revolutions round the earth, is inclined to the ecliptic at an angle of 5° 9′ at a mean rate; this angle is not only subject to some variation, but the very orbit itself is changeable, and does not always preserve the same form: for though it is elliptical, or nearly so, with the earth in one of the foci, yet its eccentricity is subject to some variation, being greater when the line of the apsides coincides with that of the syzygies, and least when these lines are at right angles to each other. But the eccentricity is always very considerable, and, therefore, the motion of the Moon is very unequal, for like all other planets, it is quickest in perigee and slowest in apogee. At a mean rate she advances, in her orbit, 13° 10′ per day, and comes to the meridian about 48 minutes later every day. As the Moon's axis is nearly perpendicu lar to the plane of the ecliptic, she can scarcely have any change of seasons. But what is still more remarkable, one half of the Moon has no darkness at all, while the other half has two weeks of light and darkness alternately. For the earth reflects the light of the Sun to the Moon, in the same manner as the Moon does to the earth; therefore, at the time of conjunction, or new Moon, one half of the Moon will be enlightened by the Sun, and the other half by the earth and at the time of opposition, or full Moon, one half of the Moon will be enlightened by the Sun, but the other half will be in darkness. The earth also exhibits similar phases to the Moon to what she does to the earth, but in a reverse order, for when the Moon is full, the earth is invisible to the Moon; and when the Moon is new, the earth will appear to be full to the Moon, and so on. has been already mentioned, that the Moon always presents the same face to the earth, from hence it is inferred, that one half of the Moon can never see the earth at all; whilst from the middle of the other half it is always seen overhead, turning round almost thirty times as fast as the Moon does. It From the circle which limits our view of the Moon, only one half of the earth's side next her is seen, the other half being hid below the horizon of all places on that circle. To the Moon, the earth seems to be the largest body in the universe, for it appears about thirteen times greater than the Moon does to the earth. OF THE HARVEST MOON. IT has long been known that the Moon when full, about the time of harvest, rises for several nights nearly at the time of Sun setting; but the cause of this remarkable phenomenon has not been so long known. This appearance was observed by the husbandman long before it was noticed by the Astronomer; and on account of its beneficial effects in affording a supply of light immediately after Sun-set, at this important season of the year, it is called the Harvest Moon. In order to conceive the reason of this phenomenon it must be recollected, that the Moon is always opposite the Sun when she is full, and of course in the opposite sign and degree of the zodiac. Now the Sun is in the signs Virgo and Libra in August and September, or the time of harvest; and therefore the Moon when full, in these months, is in the signs Pisces and Aries. But that part of the ecliptic in which Pisces and Aries is situated makes a much less angle with the horizon of places that have considerable northern latitude, than any other part of the ecliptic, and therefore a greater portion of it rises in any given time than an equal portion at any other part of it. Or, which is the same thing, any given portion of the ecliptic about Pisces and Aries rises in less space of time than an equal portion of it does at any other part. And as the Moon's daily motion in her orbit is about 13°, this portion of it will require less time to rise about those signs, than an equal portion at any other part of the ecliptic; consequently, there will be less difference between the times of the Moon's rising when in this part of her orbit than in any other.* At a mean rate the Moon rises 50 minutes later on any evening than she did the preceding evening; but when she is full about the beginning of September, or when she is in that part of her orbit which rises with the signs Pisces and Aries, she rises only about 16 or 17 minutes later than on the preceding evening; consequently, she will seem to rise for a few evenings at the same hour. Although this is the case every time that the Moon is in this part of her orbit; yet it is little attended to, except when she happens to be full at the time, which can only be in August or September. In some years this phenomenon is much more perceptible than in others, even although the Moon should be full on the same day, or in the same point of her orbit. This is owing to a variation in the angle which the Moon's orbit makes with the horizon of the place where the phenomenon is observed. If the Moon moved exactly in the ecliptic, this angle would always be the same at the same time of the year. But as the Moon's orbit crosses the ecliptic and makes an angle with it of 5° 9′, the angle formed by the Moon's orbit and the horizon of any place is not exactly the same as that made by the ecliptic and the horizon. Some years it is greater, and others less, even at the same time of the year; for it is subject to considerable variations, owing to the retrograde motion of the moon's nodes.† If the ascending node should happen to be in the first degree of Aries, it is evident, that this part of the Moon's orbit will rise with the least possible angle, and, of course, any given portion of it will require less time to rise than an equal portion in any other part of the orbit. The most favourable position of the nodes for producing the most beneficial harvest Moons is, therefore, when the ascending node *It would tend very much to make this phenomenon understood, if a terrestrial globe were at hand, and rectified for the latitude of London, when reading this description. + The nodes, or points where the moon's orbit crosses the ecliptic, move backward about 19° in a year, by which means they move round the ecliptic in 18 years 225 days. |