Besides its annual motion round the sun the earth has a daily motion or rotation on its axis, which is performed from west to east and occu pies exactly 23 hours. 56 minutes, 4.090 seconds of ordinary mean solar time. On this motion (Wpmd the rising and setting of the sun. or the changes of day and night. The relative lengths of day and nielot depend upon the angle formed by the earth's axis with• the plane of its orbit. If the axis were perpendicular to the plane of the orbit. day and niebt would be equal during the whole year over all the earth, and there would be no change of seasons: hut lhe axis makes with the orbit au angle of 23.5°, and the consequence of this is all that variety of sea• sons and of climates that we find on the earth's surface. for it is only for a small strip (theoretie ally for a more line) lying under the equator that the days and nights are equal all the year; at all other places this equality (only ()Nears on the twch days in each year when the sun seems to pass through (he celestial i.e. about 'March 21-t and 2341. From March 21st the sun departs from the equator toward the north, till, about June 21st, he has reached a northern declination of 23.5', when he again approaches the equator. which he reaches about September 23d. Ile then advances southward, and about December 21st has reached a southern de•lina tion of 23.5'. when he turns once more toward the equator, at which he arrives March 21st. The •1-t of June is the day in the North ern Hemisphere, and the shortest in the South ern: with the 21st of December it is the reverse. The linear velocity of axial rotation at the earth's. surface evidently increases gradually from the poles to the equator, where it is about 24.540 miles a day, or about 1410 feet in a sec ond. A direct proof of the rotation of the earth is furnished by experiments with the pendulum, which. as we have already pointed out, show a decrease of the force of gravity from the poles toward the equator; and thmigh a part of this deerease is owing to Hie want of perfect spheric ity. a part also arises front the centrifugal force caused by the motion of rotation. Another di rect proof of rotation may lie drawn from the observation that bodies dropped from a consider able height deviate toward the east from the vertical line. The analogy of our earth to the other planets may be also adduced as an argu went, sinee their rotation. with the exception of the smallest and the most distant, is distinctly discernible. Finally, an additional proof of the earth's rotation was given in 1851 by Foucault's experiment with the pendulum. The principle of the experiment is this—that a pendulum once set in motion, and swinging freely, continues to swing in the same plane, while at any place on the earth the plane of the meridian continues to change its position relative to this fixed plane.
This experiment, being tried, verified the expect ed rotation most satisfactorily.
If the turning of the earth on its axis is once admitted to be the cause of the apparent daily motion of the heavens, it is an easy step to con sider the annual motion of the sun through the constellations of the zodiac as also apparent, and arising from that revolution of the earth about the sun which we have already mentioned. If we consider that the mass of the sun is about 332.000 times greater than that of the earth, and that by the laws of mechanics two bodies that revolve round each other must revolve about their common centre of gravity, the idea of the sun revolving about the earth is seen to be simply lin The common centre of grav ity of the two bodies, being distant from the centre of each inversely as their respective masses, is calculated to be far within the body of the sun, which has a diameter of 516.400 miles. But it is easy to see how the apparent motion of the sun on the ecliptic naturally arise: front a motion of the earth about the sun. The motion: of the planets, also, that ap pear so eomplieated and irregular as seen by us, can only he satisfactorily explained by assuming that they, too, revolve round the sun in the same direction as the earth. See the articles PnEcEssloN and Nr'r.tri0N for an account of a small periodic motion of the earth's axis and its effects.
THE Emmes TEMPFRATCRE. The phenomena of heat on the earth's surface are described under CLIMATE; MEryonor.ocv: and other heads. As we go below the surface, we reach a depth be yond which the interior of the earth seems to have no sympathy with the external causes of heat or cold, and its heat appears to be its own, and to increase according to a fixed law the deeper we descend. The average rate of observed increase is l' F. •for a descent of between 40 and 50 feet. The distribution of the land and water with their area, is discussed in the article GEmatArity (q.v.). For the physical features of the earth, see .EOLOGY.
II oci,kGRANi T. 1411'111t.iStur. GeSch ;eh te der Selli;pf ung (7th ed., Leipzig, 1872) ; Suess, Mrs tier Erde (Leipzig, 1883 et seq.) ; Kirch hoff, Unser Wisscii run (ler I:riti• Oh_ ltitiri et : Hann, llochstette•, and Pokorny. A 11 ye meinc 'noir (ih.. 1S96-99) ; Ratzel. fir unit this Li Iwo 1901 et ; `haler. „Is m:els of the Earth (New York, 1889) ; Hum boldt, Co.smos (new ed. London. 1889) : Ite•lus, The Earth end lls 1rchcbitorcls (trans, ed. by Keane and Havenstein. London, 1890 95); the Atlases of Physieal Geography of Alexa»der Keith .lohnston, Bartholomew. and Bergh:ins; Ilabenieht. I;rundris.s. t en Seliiiiifitoyage sehielite (Leipzig. 1896) ; Newcomb. of the .lotion of the Earth on Its Axis .kround the poper,, vi. ington. 18951; and Neumayr. Eriler.cch jell le (Leipzig, 1595).