« ПретходнаНастави »
for matter, it is found that it would take a million of our earths, to make a body equal to the sun.
272. Next to the sun, the moon is that of the heavenly bodies, which the most interests our curiosity. She is but 240,000 miles distant from the earth, only 2,000 miles in diameter, and 6,300 miles in circumference. She accompanies the earth in its annual orbit; and, during that period, goes herself nearly thirteen times round the earth in an orbit of her own
273. The moon goes round her orbit in 27 days 8 hours; but as the earth moves forward during the time, it is 29 days 12 hours before she returns again to a conjunction with the sun. The earth is sixty times larger than the moon: or it would require sixty moons to make up the bulk of the earth.
274. The mountains in the moon are, however, higher than those on the earth. For example, Mount Leibnitz in the moon, is five miles high,
which is a mile higher than Chimborazo, in Peru. The surface of the moon is, besides, covered with deep pits or vallies, some of them four miles deep. THE MOON, AS SEEN THROUGH A TELESCOPE.
275. The moon always keeps the same side towards the earth, so that she turns once on her axis as she moves round the earth ; and her day and night are, consequently, as long as the period from new moon to
But the earth acts also as a moon to her, being at the same time far more luminous : so when it is new moon to the earth, it is a full earth to the moon, and the contrary.
276. As the moon shines with no light besides that which she reflects from the sun ; it is evident, that the shape must depend on her position in regard to the sun and earth.
When the earth is exactly in the middle, the whole illumined side of the moon will be towards the earth, and it will be a full moon.
When the moon is in the middle, her dark side will be presented to the earth; and it will be new, or no
As she proceeds from new to full, more and more of her light side will appear, or it will increase; and on going from full to new, it will, of course decrease.
-The moon, refulgent lamp of night,
EXPLANATION. S is the sun; T the earth; the inner circle represents the moon in its orbit receiving its light from the sun. The outer circle is the portrait of the moon in each adjoining part of her orbit as seen at the earth. Thus at A it is full moon, or all light, as at a; and at E it is new, or all dark, as at e. At E it is in a position to produce an eclipse of the sun, or overshadow the earth ; and at A to be eclipsed itself, or receive the earth's shadow.
277. As both earth and moon cast long shadows, it is evident, if they moved on the same level, that every time the earth passes between the sun and moon, the earth's shadow would fall on it, and darken or eclipse the moon; and that every time the moon passes between the sun and earth, the moon's shadow would eclipse the sun.
278. The moon, however, ascends and descends five degrees in every revolution, so that in general the shadows pass under or over; but when the new or full moon takes place at the very time she is passing the plane of the earth's orbit in ascending or descending, ihen the striking phenomena of an eclipse takes place.
279. The shadow of the earth, as seen on the moon, demonstrates its rotundity; and the shadow of the moon on the earth, proves that it is nearer than the sun; so the passing of the moon over planets and stars, called Occultations, proves that they are more distant than the moon.
280. Occasionally, Venus and Meroury, the two planets nearer to the sun than the earth, pa
over the surface of the sun like black spots, called transits of Venus and Mercury.
This proves that those planets are nearer to the earth than the sun; and, by observing the progress of the transit at different parts of the earth, we can obtain the measure of an angle, by which we can determine the exact distance of the earth from the sun.
281. Having ascertained, by means of the observation of a transit, the distance of the earth from the sun, the distances of all the other planets are determined by that law of nature, which exactly proportions the cubes of the distances of the planets, to the squares of their respective periodical revolutions.
282. Besides affording us light, the moon effects the waters, and causes high tides; which obey her influence, as the seas pass beneath her. But as she moves forward in her orbit 12 or 13 degress every day, and consequently passes over every sea 50 minutes later one day than the day before, so the time of high water is always 50 minutes later each following day,
Obs. 1.--The tides, according to the theory of Newton, are caused by what he calls the atiraction of the moon and
But according to Phillips, they are phenomena of motion caused by the rotation or motion of the earth.
2. As such terms as attraction and repulsion ought not to be received as expressive of causes, we shall prefer the sys. tem which explains, or professes to explain, the causes, and omit that which, while it affects to describe the cause, in truth only defines the phenomena.
3. If, says Sir R. Phillips, the earth were composed of fixed matter only, there could be no tides; and if of fluid matter only, there could be no tides; because in neither case
would there be any variable transfer of motion to the several parts : but as it is composed of fixed and of fluid matter, intercepting each other (as the two great continents intercepting the two great seas), so a variable communication of motion arises between the fixed and fluid, and hence the tides of the seas.
4. The tides, he says, are but oscillations of the great ba. sins of the ocean, between the continents and islands and rocks. The waters serve, in truth, as a species of balancewheel, or pendulum, to the earth, each stroke being of six hours continuance, and its length from 30 to 50 miles, according to the velocity
5. The connexion of the moon and the sun with the tides, he ascribes to the operation of :he same uniform cause, (that is of transferred motion), on the whole, a uniform cause which operates on a system of bodies, producing necessarily simultaneous effects.
283. The san and moon concur in varying the motions: hence, we have high or spring tides, when their actions concur at the new and full moon; and low or neap tides, when the forces act in opposite directions, as at the quarters; or when the moon is half way between the conjunction and opposition.
Obs.-On small seas not readily communicating with others, and on which the rotary forces act generally, and produce but one wave, there is no appearance of tides.
284. All the terestrial phenomena, and all the problems on the globes and maps, may be reduced to one general principle; that the sun always illuminates one half of the earth, and that the other half is in darkness; and that, from every part of the earth, we always see one half of the heavens, the other half being invisible.
285. The circumference of the earth, the heavens, and of all circles of the earth and heavens, is supposed to contain 360 equal parts or degrees; consequently, half a circle, or half the heavens, is 180 degrees, and a fourth 90 degrees. On the surface of the earth, each degree is 69 miles; but the actual size of a degree, as carried out to the heavens, is indefinite, because space is without bounds.