'be seen,, when only a part of it is enlightened by the sun Thus when the sky is clear, and the moon only two or three days old, it is not uncommon to see the brilliant new moon, with her horns enlightened by the sun, and at the same time, the old moon faintly illuminated by reflection from the earth. This phenomenon is sometimes called "the old moon in the new moon's arms.'

It was a disputed point among former astronomers whether the moon has an atmosphere; but the more recent discoveries have decided that she has an atmosphere, though there is reason to believe that it is much less dense than ours.

When the moon's surface is examined through a telescope, it is found to be wonderfully diversified, for besides the dark spots perceptible to the naked eye, there are seen extensive vallies, and long ridges of highly elevated mountains.

Some of these mountains, according to Dr. Herschel, are 4 miles high, while hollows more than 3 miles deep, and almost exactly circular, appear excavated on the plains. Astronomers have been at vast labor to enumerate, figure, and describe, the mountains and spots on the surface of the moon, so that the latitude and longitude of about 100 spots have been ascertain. ed, and their names, shapes, and relative positions given. A still greater number of mountains have been named, and their heights and the length of their bases detailed.

The deep caverns, and broken appearance of the moon's surface, long since induced astronomers, to believe that such effects were produced by volcanoes, and more recent discov eries have seemed to prove that this suggestion was not with. out foundation. Dr. Herschel saw with his telescope, what appeared to him three volcanoes in the moon, two of which were nearly extinct, but the third was in the actual state of eruption, throwing out fire, or other luminous matter, in vast quantities.

It was formerly believed that several large spots, which appeared to have plane surfaces, were seas, or lakes, and that a part of the moon's surface was covered with water, like that of our earth. But it has been found, on closely observing these spots, when they were in such a position as to reflect the sun's light to the earth, had they been water, that no such reflection took place. It has also been found, that when

What is said concerning the moon's atmosphere? How high are some of the mountains, and how deep the caverns of the moon? What is said concerning the volcanoes of the moon?

these spots were turned in a certain position, their surfaces appeared rough, and uneven; a certain indication that they are not water. These circumstances, together with the fact, that the moon's surface is never obscured by mist, or vapor, arising from the evaporation of water from her surface, have induced astronomers to believe, that the moon has neither seas, lakes, nor rivers, and indeed that no water exists there.

Eclipses.

Every planet and sattellite in the solar system is illuminated by the sun, and hence they cast shadows in the direction opposite to him, just as the shadow of a man reaches from the sun. A shadow is nothing more than the interception of the rays of light by an opaque body. The earth always makes a shadow, which reaches to an immense distance into open space, in the direction opposite to the sun. When the earth, turning on its axis, carries us out of the sphere of the sun's light, we say it is sun-set, and then we pass into the earth's shadow, and night comes on. When the earth turns half round from this point, and we again emerge out of the earth's shadow, we say, the sun rises, and then day begins.

Now an eclipse of the moon is nothing more than her falling into the shadow of the earth. The moon having no light of her own, is thus darkened, and we say she is eclipsed. The shadow of the moon also reaches to a great distance from her. We know that it reaches at least 240,000 miles, because it sometimes reaches the earth. An eclipse of the sun is occasioned whenever the earth falls into the shadow of the moon. Hence in eclipses, whether of the sun or moon, the two plan. ets and the sun, must be nearly in a straight line with respect to each other. In eclipses of the moon, the earth is between the sun and moon, and in eclipses of the sun, the moon is between the earth and sun.

If the moon went around the sun in the same plane with the earth, that is, were the moon's orbit on the plane of the

On

What is supposed concerning the lakes and seas of the moon? what grounds is it supposed that there is no water at the moon? What is a shadow ? When do we say it is sun-set, and when do we say it is sun-rise ? What occasions an eclipse of the moon? What causes eclipses of the sun? In eclipses of the moon, what planet is between the sun and moon? In eclipses of the sun, what planet is between the sun and earth? Why is there not an eclipse of the sun at every conjunction of the sun and moon?

ecliptic, there would happen an eclipse of the sun at every conjunction of the sun and moon, or at the time of every new moon. But at these conjunctions, the moon does not come exactly between the earth and sun, because the orbit of the moon is inclined to the ecliptic at an angle of 5 degrees. Did the planes of the orbits of the earth and moon coincide, there would be an eclipse of the moon at every full, for then the moon would pass exactly through the earth's shadow.

One half of the moon's orbit being elevated 5 degrees above the ecliptic, the other half is depressed as much below it, and thus the moon's orbit crosses that of the earth in two opposite points, called the moon's nodes.

As the nodes of the moon are the points where she crosses the ecliptic, it is obvious that she must be half the time above, and the other half below these points. The node in which she crosses the plane of the ecliptic upward, or towards the north, is called her ascending node. That in which she crosses the same plane downward, or toward the south, is called her descending node.

The moon's orbit like those of the other planets, is ellipti. cal, so that she is sometimes nearer the earth than at others. When she is in that part of her orbit, at the greatest distance from the earth, she is said to be in her apogee, and when at her least distance from the earth, she is in her perigee.

Eclipses can only happen at the time when the moon is at, or near, one of her nodes, for at no other time is she near the plane of the earth's orbit, and since the earth is always in this plane, the moon must be at, or near it also, in order to bring the two planets and the sun in the same right line, without which no eclipse can happen.

The reason why eclipses do not happen oftener, and at regular periods, is because a node of the moon is usually only twice, and never more than three times in the year, presented towards the sun. The average number of total eclipses of both luminaries, in a century, is about thirty, and the average number of total and partial, in a year, about four. There may be seven eclipses in a year, including those of both lu

How many degrees is the moon's orbit inclined to that of the earth? What are the nodes of the moon? What is meant by the ascending and descending nodes of the moon? What is the moon's apogee, and what her perigee? Why must the moon be at, or near one of her nodes, to occasion an eclipse? Why do not eclipses happen often, and at regular periods? What is the greatest, and what the least number of eclipses, that can happen in a year?

minaries, and there may be only two. two, they are both of the sun.

When there are only

When the moon is within 16 degrees of her node, at the time of her change, she is so near the ecliptic, that the sun may be more or less eclipsed, and when she is within 12 degrees of her node, at the time of her full, the moon will be more or less eclipsed.

But the moon is more frequently within 161 degrees of her node at the time of her change, than she is within 12 degrees at the time of her full, and consequently there will be a greater number of solar, than of lunar eclipses, in a course of years. Yet more lunar eclipses, will be visible, at any one place on the earth than solar, because the sun, being so much larger than the earth, or moon, the shadow of these bodies must terminate in a point, and this point of the moon's sha. dow never covers but a small portion of the earth's surface, while lunar eclipses are visible over a whole hemisphere, and as the earth turns on its axis, are therefore visible to more than half the earth: This will be obvious by figs. 206 and 207, where it will be observed that an eclipse of the moon may be seen wherever the moon is visible, while an eclipse of the sun will be total only to those who live within the space covered by the moon's dark shadow.

Lunar Eclipses. When the moon falls into the shadow of the earth, the rays of the sun are intercepted, or hid from her, and she then becomes eclipsed. When the earth's shadow covers only a part of her face, as seen by us, she suffers only a partial eclipse, one part of her disc being obscured, while the other part reflects the sun's light. But when her whole surface is obscured by the earth's shadow, she then suffers a total eclipse, and of a duration proportionate to the distance she passes through the earth's shadow.

Fig. 206 represents a total lunar eclipse; the moon being

Why will there be more solar than lunar eclipses, in the course of years? Why will more lunar than solar eclipses be visible at any one place?

in the midst of the earth's shadow. Now it will be apparent,

that in the situation of the sun, earth, and moon, as represented in the figure, this eclipse will be visible from all parts of that hemisphere of the earth which is next the moon, and that the moon's disc will be equally obscured, from whatever point it is seen. When the moon passes through only a part of the earth's shadow, then she suffers only a partial eclipse, but this is also visible from the whole hemisphere next the moon. It will be remembered that lunar eclipses happen only at full moon, the sun and moon being in opposition, and the earth between them.

Solar Eclipses. When the moon passes between the earth and sun, there happens an eclipse of the sun, because then the moon's shadow falls upon the earth. A total eclipse of the sun happens often, but when it occurs, the total obscurity is confined to a small part of the earth; since the dark portion of the moon's shadow never exceeds 200 miles in diameter on the earth. But the moon's partial shadow, or penumbra, may cover a space on the earth of more than 4,000 miles in diameter, within all which space the sun will be more or less eclipsed. When the penumbra first touches the earth, the eclipse begins at that place, and ends when the penumbra leaves it. But the eclipse will be total only where the dark shadow of the moon touches the earth.

Earth

Fig. 207.

Eclipse of the Sun

Sun

Why is the same eclipse total at one place, and only partial at another? Why is a total eclipse of the sun confined to so small a part of the earth?