Page:The Outline of History Vol 1.djvu/56

 to the sun in its midsummer position and the other most turned away at its midwinter position. And as the precession of the equinoxes goes on, a time will come when the summer-winter position will come not at aphelion and perihelion, but at the half-way points between them. When the summer of one hemisphere happens at perihelion and the winter at aphelion, it will be clear that the summer of the other hemisphere will happen at aphelion and its winter at perihelion. One hemisphere will have a short hot summer and a very cold winter, and the other a long cold summer and a briefer warmish winter. But when the summer-winter positions come at the half-way point of the orbit, and it is the spring of one hemisphere and the autumn of the other that is at aphelion or perihelion, there will not be the same wide difference between the climate of the two hemispheres.

Here are three wavering systems of change all going on independently of each other; the precession of the equinoxes, the change in the obliquity of the equator to the orbit, and the changes in the eccentricity of the orbit. Each system tends by itself to produce periods of equability and periods of greater climatic contrast. And all these systems of change interplay with each other. When it happens that at the same time the orbit is most nearly circular, the equator is at its least inclination from the plane of the earth's orbit, and the spring and autumn are at perihelion and aphelion, then all these causes will be conspiring to make climate warm and uniform; there will be least difference of summer and winter. When, on the other hand, the orbit is in its most eccentric stage of deformation, when also the equator is most tilted up and when further the summer and winter are at aphelion and perihelion, then climates will be at their extremest and winter at its bitterest. There will be great accumulations of ice and snow in winter; the heat of the brief hot summer will be partly reflected back into space by the white snow, and it will be unequal to the task of melting all the winter's ice before the earth spins away once more towards its chilly aphelion. The earth will accumulate cold so long as this conspiracy of extreme conditions continues.

So our earth's climate changes and wavers perpetually as these three systems of influence come together with a common tendency