[AS.] One of the planets of the solar system, coming next to Mercury and Venus in the order of distance from the sun. The principal motions of the earth are (1) its daily revolution on its axis; (2) its yearly revolution in its orbit round the sun. It was long believed that the earth stood still and the sun, planets, and stars revolved around it, but astronomers have proved that the earth moves round the sun, completing one revolution in about 365 days 6 hours. The orbit is an ellipse, with the sun in one focus, so that the earth is not at the same distance from the sun at all periods of the year; the mean distance is about 92,800,000 miles. The motion of the earth in its orbit explains the apparent motion of the sun in the heavens during the course of the year. Many general considerations suggest to us the globular form of the earth. When a ship is sailing away from the land, the hull will be seen to disappear while the masts are still visible, and by degrees the masts also sink below the horizon. If the sea were flat, the body of the vessel would be visible as long as the masts. Mariners also have sailed round the earth, always steering in the same general direction. Measurements made at different parts of the earth's surface show that the length of a degree increases towards the poles, and that consequently the earth is not a perfect sphere, but is flattened at the poles. Its mean diameter is 7,918 miles, and circumference 24,875 miles, while it moves around the sun at a speed of 15 miles per second. The mass of the earth is rather more than five times as great as that of a globe of water of the same size would be. It has been found that the temperature increases about 1° F. for every 64 feet of descent. If the temperature were to increase at this rate inwards, then at no great depth the heat would be sufficient to melt the ordinary materials of the crust known to us. Hot springs and volcanoes show that the interior of the earth is much warmer than the exterior. It is therefore supposed that, though the materials in the interior are at an exceedingly high temperature, yet owing to the great pressure under which they exist they are most probably in the solid state.

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