Earth
From Encyclopediak
Earth, the planet of the solar system third outward from the sun, the fifth in size. Its mean distance from the sun is 92,500,000 m. its polar diameter is 7899 m. and the equatorial diameter is 79251. m. its weight is about five and one-half times that of a globe of water of equal size. It moves in its orbit at the rate of 1102.8 m. per minute its day is 23 b. 56 min., 4.09 s. and its year is 365.20, or practically 365 j4, days. When seen in an eclipse the earth has the appearance of a round, flat disk, and this is the shape which the ancients believed it to have. The actual shape, however, is that of a sphere, flattened on two opposite sides at the poles. That the earth is round, appears from the fact that as one ascends higher, his range of vision is extended, that ships sighted at sea show their rigging first and finally their hulls, that since 1519 the earth has been continuously circumnavigated and that as an ellipse it always casts a circular shadow. There is no definite knowledge of the condition of the interior of the earth, and numerous theories have been advanced concerning it. The oldest of these is that the interior is a molten mass and that the solid part, called the crust, is a sort of shell only a few miles in thickness. More recent theories consider the interior to be in a solid state and molten matter to exist only in small and widely separated localities, and that volcanoes are around these localities. While considering the solidity of the interior, the advocates of this theory agree that this interior is very hot. Investigations carried on in mines and artesian wells show that the temperature increases about 1 F. for every 60 ft. of descent below the surface. At this rate life temperature 50 m. beneath the surface would be sufficiently high to melt any known substance. The earth has two motions, one on its axis, causing day and night, and one around the sun, causing the change of the seasons See SEASONS.
Geographers have divided the earth's surface by means of imaginary lines called circles, extending about it from north to south and from east to west. Those which divide the earth into equal parts are called great circles. Those dividing it into unequal parts are called small circles. The small circles because they are parallel to each other are known as parallels. They mark the latitude. The equator is a great circle passing around the earth midway between the poles. Its plane is perpendicular to the earth's axis. The ecliptic is a great circle coinciding with the plane of the earth's orbit and forming an angle of 23-2 with the equator. Meridian circles are great circles passing through the poles. They mark longitude. A meridian is the half of a meridian circle extending from pole to pole. For conditions of surface and kindred topics, see PHYSICAL GEOGRAPHY, GEOLOGY.
