CHAPTER XI

THE HEAVENLY BODIES

83. What is the Earth Like? We like to think of our earth as a great, solid floor on which we stand and on which we can depend. We speak of getting down to the firm earth. But if we wish to get a correct idea of what our earth is like, we must imagine ourselves as standing off in space and looking back at the earth. Then we shall see it as a great, round ball turning with dizzy speed from west to east and flying with still dizzier speed in its great path around the sun.

The ancient Greeks knew that the earth is round and that it turns or rotates. They had even worked out a way of calculating its size, but their work was afterwards forgotten. So it came about that when Columbus, in the latter part of the fifteenth century, said that our earth is a great ball, the people of his day made fun of him. As we know, the time the earth takes for one rotation is a day. The imaginary line, or axis, around which the earth spins, passes through the north and south poles.

With a tapeline measure carefully the distance around (the circumference of) a round plate, or hoop, or wheel; then measure the distance across it, through the center (the diameter). How many times as great as the diameter. is the circumference? Then measure the circumference

of a tennis ball, or croquet ball, or baseball, and calculate its diameter.

By very careful measurement men have found that the circumference of the earth at the equator is nearly 25,000 miles. What, then, is its diameter?

How rapidly must a spot at the equator be turning? Do you see that it must turn through about 25,000 miles in 24 hours, or at the rate of about 1000 miles an hour?

We may give the earth's rate of rotation in another way: We know that the distance around a circle may be measured in degrees, each complete circle having 360 degrees (written 360°). The distance around a sphere like the earth is measured on a circle that passes around it, and is also stated in degrees. Since the earth spins about on its axis once in every 24 hours, in one hour it goes through 360÷24, or 15 degrees.

84. What is the Sky? Have you ever looked along a straight picket fence and observed that the pickets all seem to be at about the same place? Yet our judgment tells us that they are one behind the other. Perhaps you have noticed how hard it is to tell which of a group of mountain peaks are near and which are farther away. All distant objects seem to be at about the same distance from us. So it is with the sky. The sky seems to be a hollow sphere and the heavenly bodies all seem to move across its surface. Yet the truth is that some of these bodies are enormously distant as compared with others. When we see the moon near a certain bright star, their nearness to each other is due to the fact that they are in the same direction from us, one beyond the other. Just in the same way the sun and moon at intervals of about

four weeks seem to be almost at the same place in the sky (the time of "new moon"), although the sun is much farther away than the moon.

While it is true that the sky is just a make-believe sphere, we must learn its parts if we want to study the heavenly bodies. The circle in which the sky and earth

The shortest? (Sug

FIG. 56. The path of the sun across the sky in the four seasons of the year. When is the sun above the horizon for the longest time? gested by Todd's Astronomy.)

seem to meet is the horizon (Fig. 56).

It is the line

beyond which we cannot look because the bulging surface of the earth is in the way. The horizon has its four points: north, east, south, and west. The point in the sky exactly above you is the zenith.

85. Why Do the Heavenly Bodies Rise and Set?As you look at the sky, you see that the moon and the stars, like the sun, rise in the east and set in the west.

Men used to suppose that all these bodies turned, or revolved, about the earth, but we now have many proofs that the earth is turning, or rotating, on its axis once every 24 hours, and that it is this rotation of the earth that brings us back each day to such a position that we can again begin to see the same heavenly bodies. This is what we mean by the rising of the heavenly bodies. If you were walking up a hill, behind which there is a church, the church might seem to rise when you could begin to see its steeple, although what is really happening is that you are moving toward the church and the church is standing still. When you move away, the church set."

seems to

Watch the path of the sun across the sky. Does the sun rise exactly at the east point of the horizon? It does so only twice a year: in March and in September. You know that in early summer the sun rises far north of the east point and sets far north of the west point; at this time it shines into the north windows of our houses (Fig. 56). In winter, on the other hand, the sun rises far south of the east point and sets far south of the west point.

86. What is the Path of a Star across the Sky?Are there any stars that do not rise and set, but are always above the horizon? If you look at the northern sky on a bright night, you will see the Big Dipper. This is a group, or constellation, of seven principal stars. The two bright stars, which make one side of the bowl of the dipper, point toward the North Star and are known as the "Pointers." If you live in the northern states

and watch the position of the Dipper at a given time say at 8 P.M. — every week or two during several months, you will find that it revolves about the North Star in a complete circle and is always above the horizon. If a star is somewhat farther away from the North Star than the Dipper is, it will be above the horizon part of the , time and below it part of the time; that is, it will rise. and set. The North Star is also called Polaris.

The stars that keep their positions with respect to one another, so that they can always be found in a given constellation, are called fixed stars.

If you were at the equator, all the stars would rise and set and their paths would all be perpendicular to the horizon. The North Star would be just at the horizon. Why is this? You must remember that the daily motions which the stars seem to have are due to the turning of the earth and that the north pole of the earth always points to the north pole of the sky. This makes all the stars seem to revolve around the north pole of the sky. The sky's north pole is near the North Star. 87. What are Some Star Groups? We have already learned of one star group, or constellation, called the Big Dipper. This is part of a larger group called the Great Bear. If you try to make out the shape of the Great Bear, you will need a great deal more imagination than to find the Dipper. Many of the constellations received their names centuries ago from the Egyptians, the Greeks, or the Arabs; others are more modern. The stars and constellations made out by the ancients were named after heroes, animals, or events.