Page:Popular Science Monthly Volume 8.djvu/726

706 in middle latitudes, in short, of all that makes the rivalry of nations, and civilization a necessity. To answer this question it will be necessary to turn again to astronomy, and to study for a few moments some of its more abstruse problems.

In addition to the rotation of the earth on its axis once every day, and its revolution about the sun once in a year, there is also a slow, rolling motion of the equator, caused by the attraction of the sun on the excess of matter in equatorial diameters over the polar. It is precisely as when one touches the rim of a top in rapid motion: there is set up at once a slow, gyrating or tilting roll, and the upper end of the stem describes a small circle. Just so the sun lays hold of the protuberant rim of the great terrestrial top, and immediately it begins to oscillate in the long secular period of 25,868 years; while the polar axis, extended to the heavens, describes in the same length of time a small circle of $23 1⁄2$° radius among the northern or southern stars. This is the motion which occasions what is called the precession of the equinoxes. The plane of the earth's equator crosses the plane of its orbit; and, when the earth is at the points of junction, the days and nights are equal the world over. These two points, therefore, are the equinoxes; and the earth passes through them about the 21st days of March and September. Owing to the rolling motion of the equator, above described, these points, always in the line of intersection of the two planes, pass successively through the twelve signs or constellations, making slowly the entire circuit of the heavens. The vernal equinox, which now points to, or is on a line between, the sun and the constellation of the Fish, after about 26,000 years will have traveled the great circle of the heavens and come back again to point to the same cluster of stars which is now overhead at midnight on the 21st of March.

But the time of this revolution, so far as it affects the climate of the earth, is modified by the following circumstance: The ellipse or oblong circle in which the earth revolves about the sun is itself all the time slowly revolving. The long diameter of it—the major axis—makes a complete revolution in the heavens once in 110,000 years. Now, as this revolution is forward, or in the same direction among the constellations that the sun appears to move, while that of the equinoxes is retrograde, it follows that the extremities of the major axis, which are the perigee and the apogee, advance to meet the equinoctial points; so that the revolutions, or rather the conjunctions, of the equinoxes, which have to do with terrestrial climate, are accomplished in the shorter period of 21,000 years.

Now, all this astronomy amounts simply to this: that in the year of our Lord 1248 the earth was at its nearest approach to the sun on the 21st day of December, our winter solstice; and that in 10,500 years from that time the same thing will happen on the 21st day of July, our summer solstice. In the period comprising the first case,