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The Earth as a Planet 1

moon, sun, time, center, pull, satellites and bodies

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1. A huge body moving through space.— We have studied about the various parts of the earth's surface as the homes of men, but we have not yet studied about this earth as a whole. There are many interesting things about the earth as one of the many huge bodies of matter moving around through space. The earth is one of several planets revolving around the sun.

2. Revolving around the planets revolve around the sun very much as a weight on a string revolves around your hand when you hold the string and swing the weight around and around. The time re quired for a complete revolution of the earth around the sun we call a year, 365 days, 5 hours, 48 minutes and 46 seconds. But be cause of the part of a day over 365 days, every fourth year is leap year. But having leap year every four years is a little too often (can you tell why?), so leap year skips once every 400 years. • How long does it take the planet Neptune to revolve around the sun? It is nearly thirty times as far from the sun as the earth is. What is the average distance of the earth from the sun? The path, or orbit, of the earth around the 3. Satellites or moons.—Some of the plan ets have smaller bodies, called satellites, that revolve around them very much as the plan ets themselves revolve around the sun.

The earth has one satellite, the moon, which has about one-fourth (0.27) as great a diameter as the earth, and is about 240,000 miles from the earth.

Astronomers, who have a very interesting time finding out things about the earth and the heavenly bodies, tell us that the moon has no air, and for that reason is as cold as Greenland's ice cap.

4. The moon's phases—new moon and full moon.—The part of the moon that is turned toward the sun shines with sunlight which is reflected to us as is the light from any distant hill. That is the reason we see the moon. The part that is turned away from the sun is in the dark, as the dark side of the earth is at night. (Fig. B.) Look carefully at the new moon some time; then tell yourself where the sun is at that moment, and you can see how it is shining on one side of the moon and not on the other. Fig. B shows all this very well indeed. You can see that a person on the dark side of the earth (where it is night) would see more of the light side of the moon when it is at D than when it is at B. Where

should the moon be to show still more light? The moon revolves around the earth every 294 days. That is why we never see it in the same place two nights in succession.

5. Eclipses.—Fig. B also shows how we have eclipses of the sun and mode. The very black parts in the picture are the shadows of the earth and the moon as the sun shines upon them. Sometimes, as the moon goes 'round and 'round the earth, it gets in between us and the sun, as at A on the figure, and hides the sun completely from a small part of the earth's surface—that part under the shadow marked "Total." We then have an eclipse of the sun. Now look at the figure, and explain for yourself what an eclipse of the moon is.

6. The moons, or satellites, of other planets. —The table of solar system facts shows that satellites are quite the style in the solar sys tem, and that the earth is one of the poor brothers. Think what our night would be if we had as many reflectors as Jupiter! 7. Gravitation and tides.—The heavenly bodies are pulling each other all the time. Wicall this pull gravitation. It is the grav itation of the earth for the stone that gives the stone weight. The earth and the moon pull each other all the time, and the sun and the earth pull each other all the time. Now one of the Igps of gravitation is that its pull is harder on things that are near to each other than on things that are far apart. When it pulls the solid earth the earth moves as one piece, so the center of pull is at the center of the earth's weight, or center of gravity as it is called. That is somewhere near the center of the earth, about 4000 miles farther from the moon than the surface of the earth is. Because of this difference in distance, the moon pulls the sea water on the side of the earth nearest the moon farther than it pulls the earth. This makes the sea rise on the side toward the moon, and we call it high tide. But the water on the other side of the earth is 4000 miles farther away than the earth's center of gravity, so it is not pulled as much as the earth. That makes high tide on the other side of the world, too. Thus there are two high tides each day on our spinning world. One is on the part of the earth that is toward the moon, and one is on the part that is turned away from it.

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