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The Moon. D The moon is the constant companion of the earth in its annual revolution round the sun, and next to that body, it s to us the most remarkable in our system; it supplies us with light during the absence of the sun, and furnishes us with a measure of time. The mean distance of the moon from the earth is 240,000 miles; its diameter is 2,160, being to that of the earth as 3 to 11. It turns roună the earth in 27 days, 7 hours, 43 minutes, and is carried round the sun with the earth in 1 year. Between one new moon and another are 29 days, 12 hours, 44 minutes; this is the foundation of the division of time into months. It turns round its axis in the same time, and hence it always presents the same face to us.
As the moon shines by borrowed light, and the enlightened part is not always turned towards the earth, it is only in one position that the moon appears round; this is, when it is in opposition to the sun, the whole of the enlightened side being then turned towards the earth : this appearance is called full moon. When it is in conjunction with the sun, the enlightened side is turned from us, and the moon is consequently invisible : this is called new moon. A few days after conjunction, it is seen in the shape of a crescent, and it gradually enlarges till the whole of the enlightened side appears. After full moon, it again loses its circular form, and the enlightened part decreases as before it increased.
As the moon affords light to the earth, so the earth, in return, affords light to the moon; but the surface of the earth being 13 times greater than that of the moon, it affords 13 times more light to the moon. The length of the day and night to the moon being nearly 30 of our days, the sun will be 15 days above the horizon, and the night will be of the same duration.
When the moon is only a few days old, the unenlightened part of it is, in favourable circumstances, partially visible. This phenomenon is caused by the light reflected from the earth - the moon's earthlight which is then great, as the enlightened side of the earth is turned towards the moon.
Numerous mountains and caverns render the surface of the moon very uneven.
This fact is proved by the following considerations :41. When the moon is horned, or gibbous, the boundary line of light and darkness is notched and broken, which is exactly the aspect that elevations and depressions would produce. 2. Close by the illuminated portion, yet within the dark part, there are small shining points which gradually join the luminous space and new ones appear. These are evidently the tops of mountains whose summits catch the illumination of the sun's rays before the plains below, just as Mont Blanc is enlightened while the Valley of Chamouni, at its foot, is in darkness. 3. Further evidence is afforded by the facts that the mountains project shadows in a direction from the sun, that the caverns are dark on the side nearest the sun and illuminated on the opposite side, and that the shadows shorten as the sun's rays became more direct, and lengthen as his beams fall more obliquely.
The form of the lunar mountains is various. There are many isolated peaks of a sugar-loaf form; one of these, Pico, is 7,000 ft. high. There are several mountain chains, but the most striking peculiarity in lunar mountains are ring fences, or circular ramparts, enclosing plains and hollows of various diameters, the most extensive having isolated peaks jutting from their bosom.
The moon is supposed to have neither atmosphere nor seas.
Beautiful as is the appearance of the moon as seen from the earth, the earth must be a still more striking object as seen from the moon.
If we could place ourselves in the middle of the lunar disc, we should enjoy a very singular spectacle ; we should see our earth placed in the zenith, like a motionless lamp, or only turning on its axis; and we should probably be able to distinguish the continents, islands, &c., as they would reflect more light than the oceans.
Supposing the moon inhabited, the inhabitants of that hemisphere next the earth will always see the earth in the same place in the heavens, while the sun will appear to perform his revolution in a month. The inhabitants of the opposite hemisphere, on the contrary, will never see the earth ; unless, prompted by curiosity, they make a journey to behold the extraordinary phenomenon.
Mars. Next to the earth is Mars. It may be known in the heavens by its dusky red appearance, which induced the ancients to give it the name of the God of War. Its diameter is little more than half that of the earth, being about 4,100 miles ; but the length of its day is nearly the same as ours, for it turns on its axis in 24 hours 39 minutes. Its distance from the sun is about 145 millions of miles ; the length of its year is equal to 687 days, and therefore it travels at the rate of 55,000 miles per hour. Mars has an atmosphere of considerable density. When viewed through a telescope, several spots are seen on its surface, some of which are permanent, others are not. Owing to its distance from the sun, the light and heat at Mars are only half of that which we enjoy. No moon has yet been discovered belonging to it. A cannon ball would take 13 years in passing from this planet to the sun.
Mars seems to shine with very different degrees of splendour. This is owing to the circumstance that in the course of its revolution it is situated at very different distances from the earth. When Mars is in opposition to the sun, it is only 50 millions of miles distant from us, but when it is in conjunction with the sun, it is 240 millions of miles distant from the earth. When nearest the earth, it presents a surface twenty-five times larger than when at its greatest distance.
Mars has a greater resemblance to the earth than any other planet in the system. Land and water diversify its surface. Owing to the inclination of its axis, it will have a change of seasons. White spots have been observed at its poles ; these have been conjectured to be snow, as they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of the polar winter of that planet.
THE ASTEROIDS. None of the four minor planets is sufficiently large to be visible to the naked eye. They are distinguished from the older planets by several peculiarities. They are all nearly at the same distance from the sun, and
complete a revolution round him in nearly the same time. They wander beyond the zodiac in consequence of their orbits being more inclined to the ecliptic than those of the other planets; their orbits are also more eccentric, and they cross one another.
Vesta () is about 225 millions of miles from the sun, and completes its revolution round it in 1,326 days. In size it resembles a star of the 5th magnitude.
JUNO ($) is situated at about 253 millions of miles from the sun, and completes its revolution in 1593 days
Ceres (2) is very nearly 263 millions of miles from the sun, and performs its revolution in 1,681 days. Ceres was the first of the asteroids that was discovered, and was first noticed by M. Piazzi, astronomer at Palermo, in Sicily, 1st Jan. 1801.
Pallas ( 4 ) is a little more than 263 millions of miles from the sun, and turns round the sun in 1686 days.
Before the discovery of the asteroids, the existence of a planet between Mars and Jupiter had been conjectured. It was observed that the interval between the orbit of each planet and that of the next goes on doubling as we proceed from the centre of the system ; but that the interval between Mars and Jupiter greatly exceeded the usual proportion. The attention of astronomers having been called to the circumstance, they were rewarded by the discovery of four small planets in the situation where they anticipated one large one to be. Some have supposed that the asteroids are the fragments of a larger orb which has been shattered by some internal convulsion.
Jupiter. 4 We come now to Jupiter, the largest of all the planets, and which the Greeks dignified with the name of their chief deity. It is easily known by its peculiar magnitude and brilliancy. Its diameter is about 87,000 miles; and hence it is 1,300 times larger than the earth. It turns on its own axis in 9 hours 55 minutes ; and revolves round the sun in 11 years 315 days, moving at the rate of about 30,000 miles per hour.
Being more than five times farther from the sun than the earth, viz., 494 millions of miles, the light and heat enjoyed by the inhabitants of Jupiter must be only the twenty-seventh part of that afforded by the sun to the earth. But this defect is partly supplied by 4 satellites, or moons, which constantly attend this planet, some of which will always be above the horizon. The density of Jupiter is little more than that of water. A cannon ball would take 47 years in passing from the sun to Jupiter.
The rotation of this planet on its axis is so rapid that the equatorial diameter exceeds the polar by 6,000 m. As the axis is very nearly perpendicular to the plane of its orbit, there will be no variety of seasons at Jupiter, and the days and nights will be constantly of equal length.
Jupiter, when viewed through a telescope, exhibits a series of dark zones or belts. They are variable, but are generally parallel to the equator of the planet. The dark belts are supposed to be the body of the planet, and the bright parts compact and undisturbed strata of clouds and vapour.
All the satellites of Jupiter, except the second, are rather larger than our moon; they were discovered by Galileo in January 1610. As they move in an orbit nearly parallel to the equator of the planet, they are always seen in a straight line coincident with the equator.
Eclipses of Jupiter's satellites happen very frequently. Sometimes they are seen passing before the planet and casting shadows on his disc, and sometimes disappearing behind the body or being hid in its shadow at a distance from it.
The eclipses of Jupiter's satellites were formerly much used in computing the longitude of places on earth. The time of an eclipse at Greenwich being given in the Nautical Almanac, and the time of the occurrence of the same eclipse at the place where longitude is required being ascertained by observation, the difference of time and hence of longitude can be deduced. This method is now little used, in consequence of the superior accuracy of that by lunar observations.
It was by the observation of the eclipses of Jupiter's satellites that the velocity with which light travels was ascertained. It was found that an eclipse occurred 8 m. sooner than the average time when the earth was nearest to Jupiter, and 8 m. later than the usual time when it was furthest from Jupiter. Hence it was inferred that light requires 16 m. to pass from the nearest to the furthest point of the earth's orbit, a distance of 95 millions of miles.