The earth not only travels around the sun in a year, but the whole of that time it is itself turning around, or rotating. Just so, a top, while spinning quickly about its axis, might at the same time move along the floor. Turn around a few times on your heels and you will see how. It takes you a much longer time to spin around than the top requires, because you are so much larger than the top.

The earth is so huge that it cannot rotate, or spin around its axis, in less than twenty-four hours, a whole day and night. As there are 365 days in a year the earth turns quite around 365 times about its axis while it is moving around the sun, as you might turn around ten times while moving across the room.

Have you ever wondered why it is we have bright day to work and play in, and then dark night to rest in, and that these never fail to come, the one after the other?

Our Earth, without the sun, would be quite dark and cold. Every ray of light, every ray of heat, comes from the sun. And that is why the earth is made always to journey around the sun, and never to wander away; for what could she do out in the cold and the dark?

But the earth is round, the shape of an orange. Some evening, hold an orange close to a flashlight, and you will see exactly half the orange made bright with the light. The other part is in the shade, and there is a clear though faint line between the light part and the dark part. Do the same thing with a very large ball and the light and dark parts will show more clearly. Hold any round object before a light, and half the object will be lighted up; the other half will remain dark.

The earth is a round object; the sun is its light. Is one half of the world bright, beautiful, and warm; and the other half always dark and cold and dreary, without moving creature or growing plant? No! and the reason of this you can easily prove. Run a skewer, chopstick, or knitting needle through your orange to serve as its axis, and turn the orange very slowly around on its axis before the flashlight. Half is always in the light; half, in the shade. But it is not always the same half. One bit after another gets into the light; what was in the light goes into the shade. Thus every bit of the orange by turns is in the light half, and every bit has its turn in the dark.

Now you see what a beautiful, kind contrivance it is to keep the earth continually turning around its axis before the sun, while she revolves around him. By far the greatest part of the earth, all the way around, has its turn in the light and its turn in the dark in twenty-four hours. It is because it takes the earth that time to turn completely around that our day and night last twenty-four hours. The half turned to the sun has day; the half turned from the sun has night. When it is night with us, the people on the opposite half have day, and when we are about our work, they are in bed.

This movement of the earth is called its diurnal motion. Diurnal means daily; motion is movement.



Notebook Work:

1. Write the answers to the questions.

- a. How long does it take the earth to rotate around its axis?

- b. It takes the earth one year to revolve around the sun. How many days is that?

- c. The rotation of Earth about its axis is diurnal. Does 'diurnal' mean daily, weekly, or monthly?

2. Draw a picture showing the difference between the earth 1) rotating on its axis every 24 hours and 2) revolving around the sun every 365 days.

3. Draw a picture showing the sun some distance from the earth. Make the part of the earth closest toward the sun bright and the part of the earth farthest from the sun dark.