LESSON XVII.-PHASES OF THE MOON. ECLIPSES: HOW CAUSED. ECLIPSES OF THE MOON.

229. Let us now explain what are called the phases of the Moon,- that is, the different shapes the Moon puts on. We stated very early in this little book (Art. 12) that the Moon, like the Earth, got all her light from the Sun. Now, ja the first place, it is clear that the Sun can only light up that half of the Moon which is turned towards it; it is clear also that, if we could get on the same side of the Moon as the Sun is, we should see the lit-up half; if we' got on the other side, we should see just nothing at all of the Moon. But this is exactly what happens, and to explain this let us suppose the plane of the Moon's motion to be in the plane of the ecliptic. In Fig. 14, we suppose the Sun to lie to the right; the Earth and its orbit are shown, the half turned towards the Sun being of course lit up. We also represent the Moon's orbit. Let us first take the Moon at A. We represent it with the side turned to the Sun lit up. Now it is clear that, as we are on the side opposite to the Sun, we cannot see the lit-up portion-this is the position occupied by the New Moon —and practically we do not see the New Moon. Now let us take the Moon at B ; it is equally clear that at this point we face the lit-up portion and see all of it. Now this occurs at Full Moon, when the Moon arrives at such a position in her orbit that the Sun. Earth, and herself are in the same line, the Moon lying outside, and not in the middle as at New Moon.

230. At C and D our satellite is represented midway between these two positions. Again, it is evident that at C we shall see one-half of the lit-up Moon-that half

lying to the right, as seen from the Earth; at D we shall see the lit-up portion lying to the left, looking from the Earth. These positions are those occupied by the Moon at the First Quarter and Last Quarter respectively. When the Moon is at E and F we shall see but a smal

part of the lit-up portion, and we shall get a crescent Moon, the crescent in both cases being turned to the Sun. At G and H the Moon will be gibbous.

231. So that the history of the phases is as follows:

New Moon. The Moon is invisible to us, because the Sun is lighting up one side and we are on the other.

Crescent Moon. We just begin to see a little of the illuminated portion, but the Moon is still so

nearly in a line with the Sun, that we only catch, as it were, a glimpse of the side turned towards the Sun, and see the Moon herself for a short time after sunset.

First Quarter. As seen from the Moon, the Earth and Sun are at right angles to each other.

When the Sun sets in the west, the Moon is south. Hence, as the illumination is sideways, the right hand side of the Moon is lit up.

Gibbous Moon. The Moon is now more than half lighted up on the right-hand side.

Full Moon.

The Earth is now between the Sun and Moon, and therefore the entire half of the Moon which is illuminated is visible.

232. From Full Moon we return through the Second Quarter and other similar phases to the New Moon, when the cycle recommences. So that, from New Moon the illuminated portion of our satellite waxes, or increases in size, till Full Moon, and then wanes, or diminishes, to the next New Moon; the illuminated portion, except at Full Moon, being separated from the dark one by a semi-ellipse, called, as we have seen (Art. 222), the terminator.

233. In Fig. 14 we supposed that the Moon's motion was performed in the plane of the ecliptic. Our readers now know (Art. 215) that this is not the case: if it were so, every New Moon would put out the Sun; and as the Earth, and every body through which light cannot pass, both on the Earth and off it, casts a shadow, every Full Moon would be hidden in that shadow. These appearances are called eclipses, and they do happen sometimes.

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Let us see if we can show under what circumstances they do happen. One-half of the Moon's journey is performed above the plane of the ecliptic, one-half below it; hence at certain times-twice in each revolution-the Moon is in that plane, at those parts of it called the nodes. Now, if the Moon at that time happens to be new or full—that is, in line with the Earth and Sun

in one case we shall have an eclipse of the Sun, in the other we shall have an eclipse of the Moon.

This will be rendered clear by the accompanying figure. We have the Sun at bottom, the Earth at top, and the Moon in two positions marked A and B, the level of the page representing the level, or plane, of the ecliptic. We suppose therefore in both cases that the Moon is at a node,—that is, on that level, neither above nor below it.

234. At A, therefore, the Moon stops the Sun's light, its shadow falls on a part of the Earth; and the people, therefore, who live on that particular part of it where the shadow falls cannot see the Sun, because the Moon is in the way. Hence we shall have what is called an eclipse of the Sun.

235. At B the Moon is in the shadow of the Earth cast by the Sun ; therefore the Moon cannot receive any light from the Sun, because the Earth is in the way. Hence we shall have what is called an eclipse of the Moon.

236. It will easily be seen from the figure, that whereas the eclipse of the Moon by the shadow of the much larger Earth will be more or less visible to the whole side of the Earth turned away from the Sun, the shadow cast by the small Moon in a solar eclipse is, on the contrary, so limited, that the eclipse is only seen over a small

area.

237. In the figure two kinds of shadows are shown, one much darker than the other; the former is called the umbra, the latter the penumbra. If the Sun were a point of light merely, the shadow would be all umbra; but it is so large, that round the umbra, where no part of the Sun is visible, there is a belt where a portion of it can be seen; hence we get a partial shadow, which is the meaning of penumbra. This will be made quite clear if we get two candles to represent any two opposite edges of the Sun, place them rather near together, at equal distances from a wall, and observe the shadow they cast on the wall from any object; on either side the shadow thrown by both candles will be a shadow thrown only by one.

238. In a total eclipse of the Moon, as the Moon travels from west to east, we first see the eastern side of the Moon slightly dim as she enters the penumbra; this is the first contact with the penumbra, spoken of in almanacs. At length, when the real umbra is reached, the eastern edge becomes almost invisible; we have the first contact with the dark shadow; the circular shape of the Earth's shadow is distinctly seen, and at last she enters it entirely. When the Moon passes, however, into the shadow of the Earth, it is scarcely ever quite obscured; the Sun's light is bent by the Earth's atmosphere towards the Moon, and sometimes tinges it with a ruddy colour.

A total eclipse of the Moon may last about 12 hours. When the Moon again leaves the umbra we have the last