Earth is the name given to the third planet in order from the sun. Like other members of the solar system, it revolves in an elliptical orbit, in one focus of which is the sun. The average distance of the earth from the sun is 92,800,000 miles. The earth has one moon, at a mean distance of 238,800 miles. It was anciently believed that the earth was a flat disk of land surrounded by water. It is now known that it has approximately the form of a sphere. On a wide, smooth surface, such as the sea, the upper part of a distant receding object, as a ship, remains in sight after the lower part has disappeared. This could be true only if the earth were round. The position of the stars shows the same thing. If one travels south, new stars, which before could not be seen, rise into view. Another very convincing proof is the fact that vessels steering always in the same genera] direction have gone round the earth, coming back to the point from which they started. The earth is not a perfect sphere, but is flattened slightly at the poles. If it were a perfect sphere, the arc of the surface, corresponding to a definite angle at the earth’s center, would be equal in every part of the circumference. But it is found that an angle of one degree has a longer arc toward the north and toward the south than near the equator, thus showing that the polar regions are flattened and the equatorial regions bulge out. Delicate experiments also show that the force of gravity is greater near the poles than at the equator; but this can be true only if the center of the earth is closer to the other attracting body at the poles than at the equator; that is, the polar regions must be flattened so as to lie nearer the earth’s center than is the equator. The shorter diameter of the earth is 7,899.6 miles, while the equatorial diameter is 7,926.6 miles. The character of the earth’s interior can be inferred from certain tidal phenomena which have led Lord Kelvin to the conclusion that the rigidity of the earth is greater than that of glass. Estimates regarding the age of the earth vary enormously. Kelvin places it somewhere between 20 million and 400 million years.
The earth has four principal motions: that of rotation on its own axis; that of revolution around the sun; that of precession; and that of nutation. As it rotates on its axis half is always exposed to the sunlight and half is always in darkness, one rotation being made every 24 hours. The time taken for the revolution of the earth around the sun is 365¼ days, and forms our year. If its axis were exactly perpendicular, the days would always be the same length. But in fact its axis is inclined at an angle of 23½°; and thus, when the northern end of the axis is directed away from the sun, the rays of light and heat do not reach it, but, on the other hand, the sun’s rays continually shine upon the southern end. This period is the winter of the northern hemisphere. When the earth has moved a quarter of the way around its orbit, the sun’s rays reach both the north and south poles. At the end of the second quarter of its revolution—or the summer solstice—the northern end of the axis is inclined toward the sun, and thus is continually lighted; while at the third quarter of the revolution once more the light reaches both poles. In the first position, the further north we go the shorter is the time during which the sun’s rays touch any one point, and thus the days are short and the nights long; and, as the days are short and the sun shines only for a short time, the weather is cold. This is the season of winter. Now, as the earth moves through the first quarter of its orbit, the days become longer on the northern hemisphere and shorter in the southern, till they become just equal about the 21st of March. This is called the spring-equinox. At the end of the second quarter the days are long in the north, the nights short and the season therefore warm; and at the third quarter once more the length of days and nights becomes equal, about September 23. At the summer-solstice of the northern hemisphere, when the north pole is inclined toward the sun, sunlight falls 23½° beyond the pole, and, as the earth rotates, all this region remains in daylight the whole 24 hours. At this time the south pole is turned away from the sun to the same extent. The circles bounding these regions of continuous daylight or darkness at the solstices are called the Arctic and Antarctic circles, and the spaces within them the north and south frigid zones. At the same time the sun is vertical at a distance of 23½° north of the equator. This is the highest northern latitude at which the vertical sun is experienced, and is called the tropic of Cancer, from the constellation in which the sun appears at that time. At the winter-solstice of the northern hemisphere the sun is vertical at a distance of 23½° south of the equator, or on the tropic of Capricorn. As the sun appears overhead in all places between these tropics twice in the year, and thus exerts its greatest heating power, this broad belt of the earth is called the torrid zone. The belts between the tropics and the polar circles are called the northern and southern temperate zones.
The sun’s heat is constantly at work breaking down the higher rocks and spreading the broken matter as soil over the lower ground. The circulating of water is the great instrument for this work: vapor raised from the oceans and carried by winds is condensed as rain on the highlands, and, returning to the sea in the forms of springs and streams, has a chief share in wearing down the surface of the land. This process would finally reduce the land to a common low level, were it not counteracted by the continual gentle elevations and depressions of the surface, due to internal changes. Animal and vegetable life is spread all over the globe and has had a large share in producing the condition and aspect of many parts of the earth, as is witnessed by the great coal-fields of the earth, the chalk, limestone and marble found in many regions and the coral reefs and islands of tropical seas. Man, too, has helped to change the appearance of the face of the earth.
The average density of the earth is about five and one half times as great as that of water. Since the density of the earth’s crust is very much less than this, it is not unlikely that the interior of the earth has a density as high as 7 or 8 and that it is composed largely of metals.
Some of the articles which will be useful in this connection are Africa, America, Australia, Earthquake, Europe, Geology, Glaciers, Gravitation, Pacific, Planets, Sun, Tides, Volcano.