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XLIII. EARTH'S NEXT NEIGHBOR

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THE moon is the earth's

nearest companion. It is neither a planet nor a fixed star.

Its motion is round and round the earth. As the earth revolves about the

sun the moon goes along with it as its companion.

Some of the other planets also have moon attendants. The name for them is satellites. All the light of the moon is borrowed light. By means of telescopes, spectroscopes, and other very fine instruments people study the sky bodies. They agree that the moon has no appearance of having

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either air or water.

The most curious

thing about the moon is its different shapes; the pic

tures show some of them.

The moon cannot change

its form and a globe always looks round; it is plain,

EARTH'S NEXT NEIGHBOR

107

therefore, that except when we have the full moon, something gets more or less in the way of the light from the sun and casts a black shadow upon the part of it that we should

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otherwise see.

Another curious thing is that in its monthly journey the moon keeps the same side turned to the earth. We see the face but never the back.

The new moon is seen in the western sky. It looks

like a crescent with its horns pointed eastward. The reason for its small size is that only a little of the half that is turned toward the sun is also toward the earth. At full moon exactly the same

half is facing both the

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earth and the sun. It

rises in the east at about the time that the sun is

setting in the west.

Remember that about fifty moons, if well packed into one, would match the size of the earth.

When you see the new moon first it is about two days old. That is, it is two days since it passed between the earth and the sun. Just a week later it is a half-moon, and at sunset has made half its daily journey across our sky. In a week and a day we have the full moon. The other fifteen days of the moon's circuit it is going back to the place between the earth and the sun; for about a week of this time it is out of sight.

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XLIV. THE EIGHT SISTERS

THE earth and the moon are not the only sky bodies

that shine with borrowed light.

If you were to look closely on a clear night when no moon is shining, you could tell the difference between some of these and the real stars. They are planets, or wanderers, and you have heard the most of their names, -Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. Besides these there are smaller planets, or asteroids. The moon, not being a planet but an attendant on the earth, is called a satellite. Other planets have their satellites also.

Venus and Mars are like the

JUPITER

MERCURY

SATURN

MARS

VENUS

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EARTH

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NEPTUNE

URANUS

moon in this, that the face they hold to the earth is not always full. You can observe this of Venus for yourselves. The other planets are much farther

away, but wise watchers of the sky have brought them closer by the telescope and found out what before was their secret.

What makes all the planets seem like wanderers is that at different times they are nearer or farther from When they are nearest they seem much larger and brighter. The daily newspapers tell us when they are to be seen at their best.

us.

Sky studies have been going on ever since there were people to gaze into the bright sky. It would make a library if all the books about the sky were to be collected. When one astronomer became old, some

pupil of his would add to what he had learned.

The sun and the stars that revolve about it make what is called the Solar System. If we wish to describe the earth, we may say that it is a globe or ball in the great sky. It is never still, but moves with a steady motion, in a path like an ellipse, round and round, with the sun for its center. Two other planets are nearer than it to the sun. As you may see, there are four which are a great many times as large.

We cannot tell what might be written in a book called Mars and the Sky, or Saturn and the Sky.

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