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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 moou 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 gooboo that of the sun. That portion of the moon's surface which is directly exposed to the sun is believed to be highly heated, possibly to the degree of boiling water, yet its rays impart very little, if any, heat to us. (See page 10.) This is perhaps 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 there 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

Fig. 42. 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 by reflecting the light it receives from

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the sun. Let as compare its various appearances with the positions indicated in the figure.

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We see it (1) as a delicate crescent in the west just after sunset, as it first emerges from the sun's

rays at conjunction. It soon sets below the horizon. Half of its surface is illumined, but only a slender edge with its horns turned from the sun is visible to us. Each night the crescent broadens, the moon recedes about 13° further from the sun, and sets correspondingly later, until at quadrature half of the enlightened hemisphere is turned toward us, and the moon is said to be in her first quarter. Continuing her eastern progress round the earth, the moon (2) becomes gibbous* in form, and, about the fifteenth day from new moon, reaches the point in the heavens directly opposite to that which the sun occupies. She is then in opposition, the whole of the illumined side is turned toward us, and we have a full moon. She is on the meridian at midnight, and so rises in the east as the sun sets in the west, and vice versa.

The moon (3) passing on in her orbit from opposition, presents phases reversed from those of the second quarter. The proportion of the illumined side visible to us gradually decreases; she becomes gibbous again ; rises nearly an hour later each evening, and in the morning lingers high in the western sky after sunrise. She now comes into quadrature, and is in her third quarter.

From the third quarter the moon (4) turns her enlightened side from us and decreases to the crescent form again; as, however, the bright hemisphore

* Gibbous means less than a half and more than a quarter circle.

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