ページの画像
PDF
ePub

the causes affecting the direction and force of the wind, are very'nunerous, and too imperfectly known, to admit of any such attempt. We can calculate accurately the length of a particular day at any place, and the altitude of the sun at any hour of the day. In other words, we can state accurately the general causes which determine the temperature of such a day in such a place. But there are many modifying circumstances, as the direction of the wind, the clearness of the atmosphere, &c. which we cannot take into consideration; and therefore we can predict the temperature only in a general way, without being able to say precisely at what degree the thermometer will stand.

Fair weather and foul are the result of so many causes, some of which, as the chemical changes that are going on in the atmosphere, are so imperfectly understood, that we can make no pretensions whatever to foreknowledge in these particulars, as we do in regard to eclipses. There are, it is true, certain states of the air, and certain appearances in the heavens, that are generally followed by a change in the weather; and a long habit of observation may enable seamen and others deeply interested, to anticipate the weather for a few hours. But no one can lay claim to such knowledge a week beforehand, and still less in season to insert it in an Almanac. All such predictions, therefore, from whatever authority, are to be regarded as a species of quackery, the relics of astrology, of a system of fraud, which the selfish and designing are always ready to practise upon the credulous and unthinking part of society.

Some of our Almanacs continue to give the name of dog-days to a certain period extending from the latter part of July to the beginning of September; and many persons perhaps still believe, that the Dog-Star, or Sirius, has something to do with the warm and sultry weather which we usually have at this season. The fact is, that the sun was formerly in the neighbourhood of this star at this time of the year; and before other and better means were devised, the near approach of the sun to a star was used to denote the season. Two thousand years ago it so happened, that the sun passed this star in the warmest part of summer. But it does not pass it so soon now by a month, on account of the precession of the equi. noxes, and in about 8000 years, it will be later still by five months, and dog-days would happen in the middle of winter. Aware of this, Almanacmakers were in doubt for some time whether they should give the name of dog-days to the warmest part of summer, or to the time when the sun is near this star. But finding at length, that if they conformed to the original idea, they should gradually carry this period into autumn, they left the star, and returned to the season intended to be marked by it.

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

sun,

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 moon 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 293 days, it soon departs so far from the sun, that a narrow streak of light is discernible ; that is, the hemisphere presented to the and of course always illuminater, 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. The moon pursuing 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 de creases, becomes a crescent, and overtakes the sun again, at which time it disappears as before, and recommences the same changes.

As the moon completes a 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 os or 15° 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

30

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 favor. able 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. The next full moon, having, in some degree the same character, is sometimes called the Hunter's moon.

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

sun.

seen.

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

moon.

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 inperceptible 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

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 moơn, 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

« 前へ次へ »