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from falling. For the same reason a quoit retains its direction when whirled, and it will keep in the same plane at whatever angle it may be thrown. A man slating a roof wishes to throw a slate to the ground; he simply whirls it, and as it descends it will strike on the edge without breaking. As long as a top spins there is no danger of its falling, since its tendency to preserve parallel its axis of rotation is greater than the attraction of the earth. This wonderful law would lead us to think that the axis of the earth always points in the same direction, even if we did not know it from direct observation.
III. The rays of the sun strike the various portions of the earth, when in any position, at different angles.—Example. When the earth is in Libra, and also when in Aries, the rays strike vertically at the equator, and more and more obliquely in the northern and southern hemispheres, as the distance from the equator increases, until at the poles they strike almost horizontally. This variation in the direction of the rays produces a corresponding variation in the intensity of the sun's heat and light at different places, and accounts for the difference between the torrid and polar regions.
IV. As the earth changes its position the angle at which the rays strike any portion is varied.—Example. Take the earth as it enters Capricornus (yj) and the sun in Cancer (22) He is now overhead, 23J° north of the'equator. His rays strike less obliquely in the northern hemisphere than when the earth was in Libra. Let six months elapse: the earth is now in Cancer and the sun in Capricornus; and he is overhead, 23}2° south of the equator. His rays strike less obliquely in the southern hemisphere than before, but in the northern hemisphere more obliquely. These six months have changed the direction of the sun's rays on every part of the earth's surface. This accounts for the difference in temperature between summer and winter.
V. Tlie Equinoxes.—At the equinoxes one half of each hemisphere is illuminated: hence the name .Equinox (cequus, equal, and nox, night). At these points of the orbit the days and nights are equal over the entire earth,* each being twelve hours in length.
VL Northern and southern hemispheres unequally illuminated.—While one half of the earth is constantly illuminated, at all points in the orbit except the equinoxes the proportion of the northern or southern hemisphere which is in daylight or darkness varies. When more than half of a hemisphere is in the light, its days are longer than the nights, and vice versa.
VTI. The seasons and the comparative length oj days and nights in the South Temperate Zone, at any specified time, are the reverse of those in (lie North Temperate Zone, except at the Equinoxes, where the days and nights are of equal length.
* Except a small space at each pole.
VIII. The earth at the Summer Solstice.—When the earth is at the summer solstice, about the 21st of June, the sun is overhead 233° north of the equator, and if its vertical rays could leave a golden line on the surface of the earth as it revolves, they would mark the Tropic of Cancer. The sun is at its furthest northern declination, ascends the highest it is ever seen above our horizon, and rises and sets 23j2° north of the east and west points. It seems now to stand still in its northern and southern course, and hence the name Solstice (sol, the sun, sto, to stand). The days in the north temperate zone are longer than the nights. It is our summer, and the 21st of June is the longest day of the year. In the south temperate zone it is winter, and the shortest day of the year. The circle that separates day from night extends 23£° beyond the north pole, and if the sun's rays could in like manner leave a golden line on that day, they would trace on the earth the Arctic Circle. It is the noon of the long six months polar day. The reverse is true at the Antarctic Circle, and it- is there the midnight of the long six months polar night.
IX. The earth at the Autumnal Equinox.—The earth crosses the aphelion point the 1st of July, when it is at its furthest distance from the sun, which is then said to be in apogee. The sun each day rising and setting a trifle further toward the south, passes through a lower circuit in the heavens. We reach the autumnal equinox the 22d of September. The sun being now on the equinoctial, if its vertical raya could leave a line of golden light, they would mark on the earth the circle of the equator. It is autumn in the north temperate zone and spring in the south temperate zone. The days and nights are equal over the whole earth, the sun rising at 6 A. M. and setting at 6 p. M., exactly in the east and west where the equinoctial intersects the horizon.
X. The earth at the Winter Solstice.—Tfye sun after passing the equinoctial—" crossing the line," as it is called—sinks lower toward the southern horizon each day. We reach the winter solstice the 21st of December. The sun is now directly overhead 232° south of the equator, and if its rays could leave a line of golden light they would mark on the earth's surface the Tropic of Capricorn. It is at its furthest southern declination, and rises and sets 23^° south of the east and west points. It is our winter, and the 21st of December is the shortest day of the year. In the south temperate zone it is summer, and the longest day of the year. The circle that separates day from night extends 23^° beyond the south pole, and if the sun's rays in like manner could leave a line of golden light they would mark the Antarctic Circle. It is there the noon of the long six months polar day. At the Arctic Circle the reverse is true; the rays fall 23£° short of the north pole, and it is there the midnight of the long six months polar night. Here again the sun appears to us to stand still a day or two before retracing its course, and it is therefore called the Winter Solstice.
XI. The earth at tine Vernal Equinox.—The earth reaches its perihelion about the 31st of December. It is then nearest the sun, which is therefore said to be in perigee. The sun rises and sets each day further and further north, and climbs up higher in the heavens at midday. Our days gradually increase in length, and our nights shorten in the same proportion. On the 21st of March* the sun reaches the equinoctial, at the vernal equinox. He is overhead at the equator, and the days and nights are again equal. It is our spring, but in the south temperate zone it is autumn.
XII. The yearly path finished.—The earth moves on in its orbit through the spring and summer months. The sun continues its northerly course, ascending each day higher in the heavens, and its rays becoming less and less oblique. On the 21st of June it again reaches its furthest northern declination, and the earth is at the summer solstice. We have thus traced the yearly path, and noticed the course of the changing seasons, with the length of the days and nights. The same series has been repeated through all the ages of the past, and will be till time shall be no more.
XIII. Distance of the earth from the sun varies.—
* The precise time of the equinoxes and solstices varies each yeur, but within a small limit