stars through which the sun successively passes form the zodiac.
The earth's daily motion about its own axis takes place in twenty-three hours, fifty-six minutes, and four seconds of mean time. This diurnal revolution is the occasion of the alternation of day and night. As the axis on which the earth performs its diurnal rotation is inclined towards the plane of its path about the sun at an angle of 66½°, and the angle between the plane of the ecliptic and the plane of the earth's equator is therefore 23½°, the sun ascends in the heavens, as seen from our northern latitudes, from 21st March to 21st June (the summer solstice), to about 23½° above the celestial equator, and descends again towards the equator from 21st June to 23rd Sept.; it then sinks till 22nd Dec. (the winter solstice), when it is about 23½° below the equator, and returns again to the equator by 21st March. This arrangement is the cause of the seasons, and the inequality of day and night attending them. For all places removed from the equator, day and night are equal only twice in the year (at the equinoxes). At the summer solstice in the northern hemisphere the north pole of the earth is turned towards the sun, and the south pole away from it, and for places within 23½° of the former there is a period of longer or shorter duration during which the sun is continually above the horizon throughout the twenty-four hours of each day, while round the latter there is an equal extent of surface within which the sun for similar periods is below the horizon. (See Day.) The reverse state of matters occurs at the winter solstice. The circles bounding these regions are called respectively the arctic and the antarctic circle, and the regions themselves the polar or frigid zones. Throughout a region extending to 23½° on each side of the equator the sun is directly overhead at any place twice in the year. The circles which bound this region are called the tropics, that in the northern hemisphere being the tropic of Cancer, that in the southern the tropic of Capricorn, while the region between is the torrid zone. The regions between the tropics and the polar circles are respectively the north and south temperate zones.
From the evidence furnished by volcanoes, hot springs, sinking of mines, &c., it is known that the earth has a high internal temperature. Taking the average of the various observed rates of increase this temperature seems to increase 1° F. for every 60 feet of descent. Assuming this to continue, the rocks at a depth of 2 miles would be as hot as boiling water, and at a depth of 50 miles the heat would be such as at the surface would melt every known solid. This being so, various theories as to the internal condition of the earth have been proposed: (1) that a thin envelope or crust surrounds a molten interior. It can be shown, however, that as tides must be produced in such a molten mass the cool outer crust would be unable to withstand the enormous force of these unless it were about 2000 miles thick. (2) That the interior is solid, with spaces here and there filled with liquid or gaseous material. This theory assumes that there are within the earth enormous cavities filled with molten rock, which escapes, when local pressure is removed, in the form of volcanic outbursts. (3) That the earth consists of a thin
crust, a large solid nucleus, and a liquid film between the nucleus and the crust. (4) That the earth is solid to the centre, but any part may become liquid if local pressure is removed. On this theory it is supposed that if water should percolate to liquefied rocks, it would be converted into steam, and produce the various volcanic phenomena.
The question of the constitution of the earth's interior has in recent years been much investigated by means of seismographic records. These appear to indicate that there are three distinct divisions. The outer crust has a thickness of from 20 to 40 miles. It possesses a high power of resistance to all kinds of stress. Beneath it is a large shell possessing a density and elasticity resembling fine steel. This shell has a high rigidity against changing forces of shorter duration, like tidal action, but in its outer parts at least yields in time to unvarying long-continued stress. The third or innermost division of the earth is probably molten, as it can transmit compressional waves, but yielding immediately to distortional or twisting forces, is unable to transmit distortional waves. This innermost portion appears to be a sphere of radius approximately one-half that of the earth as a whole. The transition between the crust and intermediate shell is abrupt, but that between the latter and the central portion is more gradual.
The earth (like the other planets) is believed to have condensed and solidified from a gaseous or nebular condition, and to have once had a far higher temperature than now. If such were the case, the outer surface, losing heat by radiation, would be the first part to cool quickly; while the interior, losing its heat by conduction, would not cool so rapidly, and, therefore, would naturally have a higher temperature than the portion at the surface. This is what all observations indicate the condition of the earth to be, and the shape of the earth also indicates that it must once have been in a fluid state. Calculations have been made of the time which has elapsed since solidification commenced, the estimates being in general of the order of hundreds of millions of years. See Nebular Hypothesis.
Another feature that the earth as a whole presents is its magnetism. When a magnetic needle is balanced on a point, it remains at rest in one position only, pointing then nearly due north and south. This can be explained only on the supposition that the earth acts as a great magnet. It has, in fact, two poles—a north and a south magnetic pole—which are not very far from the geographical poles. The magnetic equator, where the vertical force is zero and the dipping needle takes a horizontal position, does not diverge greatly from the geographical equator. The earth acts upon all magnets as they act upon each other, and it is for this reason that they point north and south.
The surface of the earth contains over 196,000,000 sq. miles, of which about two-sevenths is dry land, the remaining five-sevenths being water. The land is arranged into masses of irregular shape and size, the greatest connected mass being in the eastern hemisphere. The chief masses receive the name of continents, detached masses of smaller size being islands. The surface of the land is variously diversified, exhibiting mountains, valleys, plains, plateaus, deserts, &c. The water area of the earth is divided into oceans, seas, bays, gulfs, &c., while rivers and lakes may be regarded as features of the land surface. The great phenomena of the oceans are currents and tides. The population of the whole earth is estimated at from 1600 to 1700 millions. The earth is attended by the moon as a subordinate or secondary planet. See also such articles as Climate, Currents, Ocean, Earthquake, and Seasons.—Bibliography: A. von Humboldt, Cosmos; E. Reclus, The Earth and its Inhabitants; T. G. Bonney, The Story of our Planet; T. M. Reade, The Evolution of Earth Structure; Theory of Geomorphic Changes; A. T. Swaine, The Earth: its Genesis and Evolution, considered in the Light of the most Recent Scientific Research.
Earthenware, a name applied to the commoner sorts of pottery-ware. The older kinds of earthenware, such as Majolica, Delft-ware, Faïence, and Palissy-ware, are not only glazed, but are besides elaborately coloured and enamelled and ornamented with raised figures of various kinds. See Pottery.