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The Moon has a peculiar motion from east to west, which is known from observation; for if we attend to her any evening when she is situated near a given fixed star, we shall find her, in 24 hours, about 13 east of that star, and her distance continually increases, till at last, after a certain number of days, she returns again to the same star from the west, having performed a complete revolution in the heavens. From a long series of observations, it has been ascertained that the Moon makes a complete revolution in 24 d. 7 h. 43 m.; this is called the periodical month; but, if we refer to the time passed from new Moon to new Moon again, the month consists of 29 d. 12 h. 44 m., which is called the synodical month. This difference is occasioned by the earth's annual motion in its orbit. Thus, if the earth had no motion, the Moon would make a complete round in 27 d. 7 h. 4 m.; but while the Moon is describing her journey, the earth has passed through nearly a twelfth part of its orbit which the Moon must also describe before the two bodies come again into the same position that they before held with respect to the Sun: this takes up so much more time, as to make her synodical month equal to 29 d. 12 h. 44 m. The Moon's motion in her orbit is more unequal than the apparent motion of the Sun in one part of her orbit she moves faster, in another slower. By knowing the time of a complete revolution, we can easily calculate the mean motion for a day, or any given time, and the mean motion is called the mean anomaly. The true motion is called the true anomaly; the difference between the two is called the equation. Now the Moon's equation sometimes amounts to 6° 18' 32". Her apparent diameter varies with the velocity of her angular motion. When she moves the fastest, her diameter is largest; it is smallest when her angular motion is slowest. Hence it follows that the distance of the Moon from the earth varies.Kepler was the first person who ascertained that the

orbit of the Moon is an ellipse, having the earth in one of its foci. Her imaginary radius-vector describes equal areas in equal times, and her angular motion is inversely proportional to the squares of her dis

tances from the earth.

The point of the Moon's orbit which is nearest the earth is called the perigee; the opposite point is the apogee. The line which joins these opposite points, is called the line of the Moon's apsides. It moves slowly eastward, completing a sidereal revolution in about 9 years.

The Moon's orbit is inclined to the ecliptic at an angle of 5°, and the points where it intersects the ecliptic are called the nodes. Their position is not fixed in the heavens. They have a retrograde motion, that is to say, a motion contrary to that of the Sun; and they make a complete revolution in the heavens in a little less than 19 years. The ascending node is that in which the Moon rises above the ecliptic towards the north pole; the descending node, that in which she sinks below the equator towards the south pole.

The mean distance of the Moon from the earth is 240,000 miles, and, as their respective diameters are as about 11 to 3, the bulk of the earth is to that of the Moon as 113: 33; or as 1331: 27; or as 49:1. That is, the earth is 49 times as large as the Moon.

The different appearances or phases of the Moon constitute some of the most striking phenomena of the heavens. When she emerges from the rays of the Sun in an evening, she appears after sun-set as a small crescent just visible. The size of this crescent increases continually, as she separates to a greater distance from the Sun; and when she is exactly in opposition to that luminary, she appears under the form of a complete circle. This circle now declines, changing into a crescent as she approaches nearer that luminary, exactly in the same manner as it had increased, till at length she disappears alto

gether, plunging into the Sun's rays in the morning at sun-rise.

The Moon is an opaque globe, like the earth, and shines only by reflecting the light of the Sun; therefore, while that half of her which is towards the Sun is enlightened, the other half must be dark and invisible: hence she disappears when she comes between us and the Sun, because her dark side is then towards us. When she has gone a little way forward, we see a little of her enlightened side; which still increases to our view as she advances forward, until she comes to be opposite to the Sun, and then her whole enlightened side is towards the earth, and she appears with a round illuminated orb, which we call the full Moon, her dark side being then turned away from the earth. From the full she seems to decrease gradually, as she goes through the other half of her course, showing less and less of her enlightened side every day, till her next change or conjunction with the Sun, and then she disappears as before.

The axis of the Moon being nearly perpendicular to the ecliptic, it has scarcely any difference of seasons. One half of the Moon's surface has no darkness at any time, the earth constantly affording it a strong light in the absence of the Sun; while the other half has a fortnight's darkness and a fortnight's light by turns. Our earth, unquestionably, performs the office of a Moon to the Moon, waxing and waning regularly, but appearing thirteen times as large, and, of course, affording thirteen times as much light as she does to us. When she changes to us, the earth appears full to her; and when she is in her first quarter to us, the earth is in the third quarter to her, and vice versa. To the Moon the earth seems to be the largest body in the universe, and must indeed be a most magnificent sight.

We come now to speak of Eclipses, which formerly were subjects of dread and terror, but which

philosophers have converted to the purposes of utility and instruction. The Moon can only be eclipsed by the interposition of an opaque body, which intercepts from it the light of the Sun; and it is obvious that this opaque body is the earth, because the eclipses of the Moon never happen, except when the Moon is in opposition, and consequently when the earth is interposed between her and the Sun. The globe of the earth projects behind it a conical shadow, the axis of which is the straight line that joins the centres of the earth and Sun, and which terminates at the point when the apparent diameters of these two bodies become equal. The diameters of these two bodies, seen from the centre of the Moon in opposition, are nearly in the proportion of 3 for the Sun and 11 for the earth. Therefore the conical shadow of the earth is at least thrice as long as the distance between the earth and Moon, and its breadth at the point where it is traversed by the Moon more than double the diameter of that luminary. Moon, therefore, would be eclipsed every time it is in opposition, if the plane of the orbit coincided with the ecliptic. But in consequence of the mutual inclination of these two planes, the Moon, when in opposition, is often elevated above the earth's shadow, or depressed below it, and never can pass through that shadow unless when it is near the nodes. If the whole of the Moon's disk plunges into the shadow, the eclipse is said to be total; if only a part of the disk enter the shadow, the eclipse is said to be partial.

The

The Moon's diameter, as well as the Sun's, is supposed to be divided into 12 equal parts, called digits; and so many of these parts as are darkened by the earth's shadow, so many digits is the Moon said to be eclipsed. All that the Moon is eclipsed above 12 digits, shows how far the shadow of the earth is over the body of the Moon, on that edge to which she is nearest at the middle of the eclipse.

Eclipses of the Sun only take place during the conjunctions of the Sun and earth; they are occasioned by the Moon's body being interposed between the Sun and earth, or, in other words, by the earth's being plunged in the shadow of the Moon. The Moon, though much smaller than the Sun, is so much nearer to the earth, that its apparent diameter does not differ much from the diameter of that luminary; and, in consequence of the changes which take place in the apparent diameter of these bodies, it happens that, in some positions, the apparent diameter of the Moon is greater than that of the Sun. If we suppose the centres of the Sun and Moon in the same straight line with the eye of the spectator placed on the earth, he will see the Sun. eclipsed. If the apparent diameter of the Moon happen to surpass that of the Sun, the eclipse will be total; but if the Moon's diameter be the smallest, the observer, if properly situated, will see a luminous ring, formed by that of the Sun's disk, which exceeds that of the Moon's, and the eclipse in this case is called annular. If the centre of the Moon is not in the same straight line which joins the observer and the centre of the Sun, the eclipse can only be partial, as the Moon can only conceal a part of the Sun's disk: on these accounts, there must necessarily be a great variety in the appearances of solar eclipses. We may add also to these causes of variety, the elevation of the Moon above the horizon, which is the cause of considerable changes in the diameter; for it is a fact, well and generally known, that the Moon's diameter appears larger when she is near the horizon than when she is elevated above it: and as the Moon's height above the horizon varies according to the longitude of the observer, it follows that the solar eclipses will not have the same appearance to observers situated in different longitudes on the earth. One observer may see an eclipse which does not happen to another in

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