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If you watch a wheel turning round quickly, you will see that the middle part, which is called the axle, is quite still. When a top is spinning its fastest, sleeping, as boys say, the very middle of the top, right through, down to the point, is still. So, if you could spin round quickly on your heels, you might imagine a line through the middle of you, from your head to your heels, upon which you spin. That middle line would be still while all the rest of you was in motion; just as the knitting needle was still when you turned the orange round on it.

Everything which turns round or rotates in this way turns on a still middle line; not a real line; the stillness is real, but the line is only imaginary. Such a line is called an axis. If you could turn round upon your heels, you would turn upon an axis. The top spins upon its axis. The earth spins or rotates upon her axis once in twenty-four hours. You remember that the earth is a little flattened at the top and bottom; the axis runs between the two flattened parts.

The places where the axis would come out if it were a real, instead of an imaginary, line, are called poles. Your poles would be, one at the top of your head, the other at your heels. The earth's poles are at the two flattened parts. One of the poles always points to a particular star in the heavens called the pole star, and that is the north pole of the earth; the pole at the other flattened end is the south pole.

As the ends of the earth, where the poles are, are slightly flattened, the middle between the poles bulges out a little, as you may have seen an orange bulge in the middle. Bound this bulging middle, exactly between the two poles, there is another imaginary line called the equator, because it divides the earth into equal parts, and for another reason also. The equator helps us to know where places are, and you will find it marked upon all maps of the world. Sphere, as you know, is a name given to the earth because it is a round object; the word "hemi" means half; so half of the earth is a hemisphere.

The equator divides the earth into two hemispheres or half spheres, as you might divide an orange into two hemispheres by tying a string round the middle. The half between the equator and the north pole is the northern hemisphere : the other half, between the equator and the south pole, is the southern hemisphere.

Notebook Work:

1. Write the answers to the questions.

- a. What is the earth's axis?

- b. Where is the axis?

- c. What are the earth's poles?

- d. How long does it take the earth to rotate about its axis?

- e. When is it day where we are?

- f. When is it night where we are?

- g. What causes the change between day and night?

- h. What is the equator?

2. Draw a picture of the earth and label the north and south poles, the equator, and the northern and southern hemispheres.

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