Earth, the name given to that member of the solar system on which we live. Travelling periodically round the sun it is classed among the planets. Its orbit is therefore an ellipse, in accordance with Kepler's laws of planetary motion, with the sun at one focus of the curve. The plane containing this ellipse is called the ecliptic, shown by a great circle on the celestial sphere. Its speed depends upon the solar distance, being greatest when this distance is a minimum, and least when we are farthest from the sun. The average distance as estimated by observations on transits of Venus is 92,800,000 miles. A difference of over 3,000,000 miles exists between maximum and minimum solar distances, the positions of which are called aphelia and perihelia respectively. The average rate of motion is over 65,000 miles an hour, the whole orbit being accomplished in one year. The elliptical orbit is subject to various perturbations (q.v.), on account of the attractions of the other heavenly bodies.
The earth has one satellite, the Moon (q.v.), which travels round the earth once a month in an elliptical path at an average distance of 238,000 miles. The exact length of the ordinary calendar year is 365 days 5h. 48m. 46s. This differs from the length of time taken for the orbit to be completely traversed, the difference being due to what is called the precession of the equinoxes (q.v.). This interval is known as the sidereal year, and is 365 days 6h. 9m. 9s. The earth's orbit is also gradually veering round towards the east, and if the time be taken from one perihelion to the next, the anomalistic year is obtained. This is about 365 days 6h. 13m. 48s.
Before giving the other motions of the earth, it is necessary to describe its shape. Like all other planets, it is a solid of revolution, being a sphere that has undergone a slight flattening at the opposite extremities or poles of the axis of revolution. More accurately, it is an oblate spheroid (q.v.), generated by the rotation of an ellipse about its minor axis. Such a figure would be assumed by a sphere of liquid rotating about a diameter, centrifugal force acting most vigorously at the equator, and tending to overcome the internal forces that keep the molecules together. The smallest diameter of the earth is that measured from pole to pole along the axis of rotation; this is 7,899.6 miles, or about 500,000,000 inches. The greatest diameters are those measured between opposite points on the equator; these are 7,926.6 miles, and, therefore, show that the eccentricity of the earth, or the extent of its departure from the perfect sphere, is very slight.
The circumference of the earth, measured along the equator, is 24,899 miles; the area is 197,000,000 square miles; and the volume is 260,000,000,000 cubic miles. Experiments on the comparative attraction of the earth [Cavendish Experiment] show that its density is about 5-1/2 times that of pure water at 4° C. Its mass is, therefore, approximately 6,000 trillion tons. The ordinary proofs of the sphericity of the earth are - (1) It can be circumnavigated; (2) the appearance of a vessel at sea always indicates a nearer convexity of the earth's surface; (3) the sea-horizon is always depressed equally in all directions when viewed from an elevation; (4) the elevation of the pole-star increases as we travel northwards from the equator; (5) the shadow of the earth on the moon during a lunar eclipse is spherical.
The earth rotates uniformly about its axis. The time taken to make a complete revolution of 360° is called a sidereal day, for it is the interval of time between consecutive transits of any distant star across any meridian of the earth. The time between consecutive transits of the sun across any meridian is called a solar day; the average of these throughout the whole year is called a mean solar day, and is the practical standard of time (q.v.) adopted by civilised nations. The ordinary proofs that the earth rotates are - (1) Bodies falling from a great height have an easterly deviation; (2) Foucault's pendulum experiment (q.v.); (3) a gyroscope delicately balanced so as to be free to change the direction of its axis in any way will, if rotated, exhibit an apparent deviation; (4) in northern hemispheres a projectile deviates to the right, in southern hemispheres to the left; (5) the trade winds (q.v.); (6) Dove's law of wind-change (q.v.).
The speed of a body on the equator, due to the diurnal rotation, is about 1,000 miles an hour. The centrifugal force due to this speed diminishes the weight of bodies; if the earth rotated in an hour, they would be thrown off from the surface at the equator.
The axis of the earth is not perpendicular to the ecliptic, but at angle of 66° 32' to it; the equator is, therefore, inclined to it at an angle of 23° 28'. This unsymmetrical placing of the bulging portions of the earth causes a slow wobbling, or precession (q.v.), of its axis, in the same sort of way as a spinning top will wobble when pushed over on one side. There is also a slight vibration or "nodding" motion of the earth's axis, known as nutation (q.v.). The period of each precession is about 21,000 years; if the earth's orbit occupied a constant position in its plane, the periods would be 26,000 years each. These motions have considerable influence on climate, the modern theories of the Ice Age being connected with the known facts of precessional motion.
The earth's temperature increases from the surface downwards at the rate of about 1° C. for every 100 feet of descent; this law does not hold for sea-depths, deep sea temperatures never exceeding 5° C. The phenomena of volcanoes, geysers, and earthquakes also point to a high internal temperature, and it is believed that the earth is gradually cooling down from a past condition of incandescence, more heat being lost from its surface every year by radiation than is received by the sun. It is probably solid throughout, the extreme internal pressure preventing liquefaction or vaporisation until restraint is removed and some outlet afforded. In fact, the rigidity of the earth must be comparable to that of steel in order to withstand with so little distortion the various gravitational and other stresses to which it is subjected.
Only the 2,000-millionth part of the sun's heat reaches the earth, and part is immediately reflected back into space. Seasons (q.v.) are due to variation in the obliquity of the sun's rays during the year, and to variation of its distance from the earth. Climate undoubtedly depends also on the lengths of summer and winter, using these terms to denote the two portions of the year into which it is divided by the equinoxes (q.v.). It must be understood that any hemisphere of the earth receives half the solar heat that arrives during a year. The summer in the northern hemisphere will, therefore, happen at the same time as the winter in the southern hemisphere, and conversely. It has been proved that so long as the obliquity of the ecliptic remains at its present value, of the total heat received from the sun in one year on a hemisphere, 63 per cent. is received during the summer and 37 per cent. in the winter. This is the case whatever be the shape of the earth's orbit or the position of the line of equinoxes. But this line alters the number of days in summer and winter, and its position will, therefore, affect the intensity of heat distribution during the summer or winter-days. The 37 per cent. winter-supply distributed over a long winter of 199 days is what obtains during the glacial epochs of the earth's history; whereas the 37 per cent. distributed over a short winter of 166 days is the case during the general epochs. At present the northern hemisphere is in an intermediate condition, its winter being of 179 days.