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VII. MOON'S PHASES, HARVEST MOON, ECLIPSES, &c.

THE moon does not shine by its own light. It is illuminated, like the objects about us, by the rays of the sun. Accordingly, only that part appears bright, which is turned toward the sun. As the moon revolves round the earth in nearly the same direction in which the sun revolves, it is sometimes between us and the sun, passing, however, for the most part, a little to the north or south, instead of coming into an exact line joining the earth and sun. At these times the illuminated part of the mocn is turned directly from us, and for a day or two we lose sight of the moon entirely. This is called the change or new moon. But as the moon moves among the stars so much faster than the sun as to go round and come up with the sun again in about 29 days, it soon departs so far from the sun, that a narrow streak of light is discernible; that is, the hemisphere presented to the sun, and of course always illuminated, is slightly turned toward the earth, exhibiting at first a semicircular thread of light, which soon enlarges to a crescent; and, at the end of seven days, becomes a semicircle. This is what we call the first quarter. At this time the line joining the earth and moon is at right angles to the line joining the earth and sun, and half the illuminated hemisphere is turned toward us. The moon now rises about noon, and passes the meridian about 6 P. M. its course farther and farther from the sun, the phase enlarges beyond a semicircle, and in about 7 days more it presents an entire circle, and is called the full moon. It is now opposite to the sun in the heavens, rising when the sun sets, and setting when the sun rises. From this time it begins to approach the sun on the other side, going through the same changes as before, only in an inverted order; so that in a little more than a week after the full, it becomes again a semicircle, rising near midnight, and setting near noon. This is called the third quarter. It thence decreases, becomes a crescent, and overtakes the sun again, at which time it disappears as before, and recommences the same changes.

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The moon pursuing

As the moon completes à revolution in about 29 or nearly 30 days, it must move at the rate of 3_6_0 or 12° a day nearly. Now the daily apparent motion of the heavens, or real motion of the earth on its axis, is at the rate of 60 or 15o in an hour. If, therefore, the sun and moon are on the meridian at the same time to-day, to-morrow, when the sun comes to the meridian, by the diurnal motion, the moon will be 12° to the east, and will of course arrive at the meridian nearly an hour (50m.) after the sun. This interval of 50 minutes will be doubled the second day, tripled the third, and so on. Similar intervals will take place also between the rising of the sun and moon; in other words, the moon will rise at a mean about 50 minutes later and later every night. We say at a mean, because the

moon's path is much more oblique to the horizon at some times than at others, on which account there is a much less difference in the time of rising and setting two successive nights. On the supposition that the moon's orbit coincides with the ecliptic (it is inclined only about 5°,) it would make the least angle with the horizon when in the 1st of Aries, and it would rise two successive nights with a difference much less than 50 minutes. But we take little note of the moon's rising, except when near the full, and there can be in a year only one full moon, or at most but two full moons, when the moon is near the 1st of Aries, or the vernal equinox, at which time the sun would be near the 1st of Libra; that is, this favorable circumstance in the moon's rising, will happen in the latter part of September or the beginning of October; and being at the busy season of harvest, when the light of this prolonged full moon facilitates the labors of the husbandman, it has obtained the name of the Harvest moon. next full moon, having in some degree the same character, is sometimes called the Hunter's moon.

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When the moon thus rises for several successive nights with the least difference of time, on account of the smallness of the angle which its path makes with the horizon, it sets with the greatest difference, since, at setting, the angle which its path makes with the horizon, is now the greatest. Each of these circumstances tends to prolong the time that the moon is above the horizon.

The conditions of the harvest moon are reversed at the opposite season, namely, in March, when the full moon takes place in Libra; and now it rises for several successive nights with the greatest difference, and sets with the least.

We have considered the moon's orbit or path as coinciding with the ecliptic, whereas it is inclined about 5°; and it will accordingly be sometimes inclined to the horizon 5° more and sometimes 5° less, than the ecliptic. On this account the difference in the time of rising of the harvest moon is continually varying from year to year, through a period of nearly 19 years, in which the series is completed.

Since the angle which the moon's path makes with the horizon becomes less and less as we increase our latitude, so the circumstance above noted, of the small difference in the rising of the harvest moon, is more conspicuous in high latitudes, and less so as we approach the equator. Thus those parts of the earth are most favored in this respect, which, on account of the shortening of the days, most need this benefit of the moon's presence.

We have already mentioned, that at new moon, or when the moon comes between us and the sun, it generally passes a little to the north or south of the sun, and hardly ever falls exactly in a line joining the earth and This is owing to the oblique position of the moon's orbit to the

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sun's or the ecliptic. But when new moon happens near the time the moon is passing its node or the point of intersection of its orbit with the ecliptic, then it will come into a line joining the sun and some part of the earth, and the sun will be more or less obscured. This is called an eclipse of the sun; and it will be seen, from what has been said, that this phenomenon can occur only at the time of new moon, and when the moon is near its node or the point of intersection of its orbit with the ecliptic. The moon passes between us and the sun just as a cloud does, which is driven along by the wind, and the eclipse will happen, in each case, only to those who are in the shadow cast by the opaque object. To those who are without the shadow, there will be no eclipse. It should be remarked, however, that there is, surrounding a shadow caused by the sun, an imperfect shade, called the penumbra, throughout which a part of the sun is This penumbra surrounds the proper shadow of all objects where the luminary is an extended surface, like the sun. Let a balloon be supposed to pass directly between us and the sun. If it is so near to us as to appear larger than the sun, it will completely hide it from us, and we shall then be in the proper or perfect shadow of the balloon. But let the balloon be raised so high as to appear smaller than the sun, or whether larger or smaller, let it pass directly between us and a part only of the sun's disc, and we shall fall into the penumbra, where the sun's light is only partially interrupted. Now, by far the greater part of the eclipses of the sun are of this description. Only a part of the moon is interposed between us and the sun, the sun's and moon's centre and the spectator not being in a straight line. Moreover, the moon is generally too small to cover the whole of the sun. Under the most favorable circumstances, that is, when the moon is in perigee, or approaches nearest to the earth, and the sun is at its greatest distance, the proper shadow would cover only a very small space, compared with the whole extent of the earth, and would pass by the spectator in eight minutes. This is the longest total eclipse of the sun that can ever take place; and it requires such a concurrence of circumstances as will scarcely ever be united.

When the moon's centre passes very nearly over the sun's centre, and it is at the same time too small to cover the sun, a ring of light appears all round the circumference. This is called an annular eclipse.

When the moon at the full comes very nearly into a line joining the earth and sun, it will cross the earth's shadow, and be obscured. This is an eclipse of the moon. It will be seen, that it can happen only at the full. The earth having a diameter nearly four times as great as that of the moon, will project a shadow nearly four times as far. It is worthy of remark, that the moon seems to have been so placed, by the hands of the Creator, that its shadow shall, for the most part, just fall short of the earth. But the earth's shadow, at the distance of the moon, has a diameter considerably

more than double that of the moon itself, so that a total eclipse of the moon may often happen, without the moon being exactly in the ecliptic, at the full. Total eclipses of the moon, therefore, are not unfrequent, and they may be of considerable duration. The eclipse, from beginning to end, under the most favorable circumstance, may last nearly four hours; and it may continue total for very nearly two hours.

An eclipse of the moon comes on so gradually, that it is difficult to find the beginning and end very accurately. This is on account of the imperceptible gradation of the penumbra. Eclipses of the moon, therefore, are of much less value in determining the longitude than those of the sun, the beginning and end of which can be noted with the greatest precision.

The earth presents a larger surface, in eclipses of the sun, for the moon's shadow to fall upon, than the section of the earth's shadow, at the distance of the moon, presents for the moon itself to pass over in eclipses of the moon. Hence there are, absolutely considered, more eclipses of the sun than of the moon. But what we call eclipses of the sun, being only eclipses of the spectator, they are, in fact, of very limited extent; while those of the moon, being real obscurations of the luminary itself, take place with respect to an entire hemisphere, or throughout the extent to which the luminary is visible at the time. Accordingly, although there are more eclipses of the sun put down in the Calendar, than of the moon, yet in any one place there are more actually seen of the latter than of the former.

The stars which are situated near the ecliptic, are liable, like the sun itself, to have their light intercepted by the moon. This phenomenon is called an occultation. As the moon moves among the stars about 12° a day, it moves about half a degree, that is, one diameter, in an hour; and consequently, a star may remain about an hour behind the moon, when it happens to take the direction of a diameter. The disappearance of the star, by the interposition of the moon, is denominated the immersion, and its reappearance, the emersion. Each of these phenomena is very sudden, and affords a favorable means, like the different phases of a solar eclipse, of determining the longitude of a place,

VIII. TIDES,

THE ebb and flow of the sea are evidently connected with the moon's motions. The level of the ocean is slightly disturbed by the attraction which is alternately exerted and withdrawn. The waters for a large space under the moon, being more attracted than the great body of the earth, are thus rendered lighter than those parts of the ocean which are at the same distance as the earth's centre; and being lighter, they are forced

upward a little by the surrounding mass, which is heavier; just as water and oil will stand at different heights in the two branches of a syphon tube; or just as ice, which is lighter than water, is made to rise a little higher, on that account, when placed in water.

If the earth rested immovably upon a fixed support, there would be a tide, or rising of the waters, only on the side toward the moon. But the great body of the earth is just as free to move as a single particle of the ocean, and if suffered to yield to the moon's attraction, would be carried just as fast. Hence, for the same reason that a particle of water on the side of the earth toward the moon is drawn away from the centre, or has its downward tendency diminished, so the solid earth itself is drawn away from the mass of waters, on the side of the earth farthest from the moon. It is the difference of attraction, in both cases, between the surface and the centre, which causes the lightness of the waters, and the consequent elevation. It will be seen, therefore, that, taking the whole earth into view, there are always two high tides diametrically opposite to each other and two low tides also, midway between the high ones. The high tides are two great waves or swells, of small height, but extending each way through half a right angle. These waves follow the moon in its monthly motion round the earth, while the earth, turning on its axis, causes any given place to pass through each of these swells and the intervening depressions, in a lunar day, or 24h. 50m.

What we have said with respect to the moon's influence in disturbing the level of the ocean, may be applied also to that of the sun; only, in the case of the sun, although its absolute action is about double that of the moon, yet, on account of its very great distance, the relative action upon the surface of the earth, compared with that at the centre, is about one third as great as that of the moon. At new and full moon, when the sun's and moon's actions conspire, the tides are highest, and are called spring tides. But at the first and last quarters of the moon, the action on one body tends to counteract that of the other, and the tides, both at ebb and flow, are smallest, and are called neap tides.

We have supposed the highest tides to happen at new and full moon, and the lowest at the quarters. But the waters do not yield instantly to the action exerted upon them; the greatest effect takes place some time after the attractive influence has passed its point of greatest power. Thus the spring and neap tides actually occur about a day and half after the times above indicated. So, also, for a similar reason, the real time of high water, in the daily tides, happens about three hours after the moon has passed the meridian.

It will be perceived, from what has been said, that the sun's and moon's influence will vary with a change of distance, being greatest when the attracting body is nearest, and vice versa,

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