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the earth therefore, varies incessantly. At perigee it is 26,000 miles nearer than in apogee : the mean distance is about 238,000 miles. It would require a chain of thirty globes equal in size to the earth to reach the moon. An express-train would take about a year to accomplish the journey. The moon completes its revolution (sidereal) around the earth in about 27| days; but, as the earth is constantly pass
ing on in its own orbit around the sun, it requires over two days longer before it comes into the same position with respect to the sun and earth, thus completing its synodic revolution.
The real path of the moon is the result of its own proper motion and the onward movement of the earth. The two combined produce a wave-like curve that crosses the earth's path twice each month; this, owing to its small diameter compared with that of the ecliptic, i3 always concave toward the sun. As the moon constantly keeps the same side turned toward us, it follows that it must turn on its axis once each month.
Dimensions.—Its diameter is about 2,160 miles. It would require fifty globes the size of the moon to equal the earth. Its apparent size varies with its distance; the mean is, however, about one half a
THE BIZE OP MOON AT HORIZON AND ZENITH. _
degree, the same as that of the sun. It always appears larger than it really is, on account of its brightness. This is the effect of what is termed in optics Irradiation. To illustrate, this principle, cut two circular pieces of the same size, one of black and the other of white paper. The white circle, when held in a bright light, will appear much larger than the black one. For the same reason it is often noticed that the crescent moon seems to be a part of a larger circle than the rest of the moon. As we have already said, the moon appears larger on the horizon than when high up in the sky. By an examination of the cut, it is easily seen that it is 4,000 miles nearer when on the zenith than when at the horizon. Besides these general variations in size, the moon varies in apparent size to different observers. Much amusement may be had in a large party or class by a comparison of its apparent magnitude. The estimates will differ from a small saucer to a wash-tub.
Librations (librans, swinging).—While the moon presents the same hemisphere to us, there are three causes which enable us to see about 576 out of the 1,000 parts of its entire surface. (1.) The axis of the moon is inclined a little to its orbit, as also its orbit is inclined to the earth's orbit; so when its north pole leans alternately toward and from the earth, we see sometimes past its north, and sometimes past its south pole. This is called libration in latitude. (2.) The moon's rotation on its axis is always performed in the same time, while its movement along its orbit is variable; hence it happens that we occasionally see a little further around each limb (outer edge) than at other times. This is called libration in longitude. (3.) The size of the earth is so much greater than that of the moon, that an observer, by the rotation of the earth, or by going north or south, can see further around the limbs.
Light And Heat.—If the whole sky were covered with full moons, they would scarcely make daylight, since the brilliancy of the moon does not exceed airolinnrtnat 0I*tne sun- That portion of the moon's surface which is exposed to the sun is supposed to be highly heated, possibly to the degree of boiling water, yet its rays impart no heat to us; indeed Prof. Tyndall considers them rays of cold. This is probably caused by the fact that our dense atmosphere absorbs all the heat, which in the higher regions produces the effect of scattering the clouds. It is a well-known fact that the nights are oftenest clear at full moon. (Herschel.)
Centre Of Gravity.—It is thought that the centre of gravity of the moon is not exactly at its centre of magnitude, but nearly thirty-three miles beyond, and that the lighter half is toward us. If that be so, this side is equivalent to a mountain of that enormous height. We can easily see that if water and air exist upon the moon, they cannot remain on this hemisphere, but must be confined to the side which is forever hidden from our view.
Atmosphere Of The Moon.—The existence of an atmosphere upon our satellite is at present an open question. If there be any, it must be extremely rarefied, perhaps as much so as that which is found in the vacuum obtained in the receiver of our best air-pumps.
Appearance Of The Earth To Lunarians.—If theie be any lunar inhabitants on the side toward us, the earth must present to them all the phases which their world exhibits to us, only in a reverse order. When we have a new moon, they have a full earth, a bright full-orbed mg ^
moon fourteen times as large as ours. The lunar inhabitants upon the side opposite to us of course never see our earth, unless they take a journey to the regions from whence it is visible, to behold this wonderful spectacle. Those living near the limbs of the disk might, however, on account of the librations, get occasional glimpses of it near their horizon.
The Earth-shine.—For a few days before and after new moon, we may distinguish the outline of the unillumined portion of the moon. In England, it is popularly known as "the old moon in the new moon's arms." This reflection of the earth's rays must serve to keep the lunar nights quite light, even in new earth.
Phases Of The Moon.—The phases of the moon show conclusively that it is a dark body, which shines only bv reflecting the light it receives from