A fifth astronomical theory, and one which has of late years been received with great favour, is that so ably advocated by the late Dr. James Croll and by Professor James Geikie. Following the suggestions of the astronomer Adhémar, these writers have attempted to show that not only one Glacial epoch, but a succession of such epochs, has been produced in the world by the effect of the changes which are known to have taken place in the eccentricity of the earth's orbit when combined with the precession of the equinoxes--another calculable astronomical cause. [Illustration: Fig. 99.--Diagram showing effect of precession: _A._ condition of things now; _B._ as it will be 10,500 years hence. The eccentricity is of course greatly exaggerated.] It is well known that the earth's orbit is elliptical; that is, it is longer in one direction than in the other, so that the sun is one side of the centre. During the winter of the northern hemisphere the earth is now about three million miles nearer the sun than in the summer; but the summer makes up for this distance by being about seven days longer than the winter. Through the precession of the equinoxes this state of things will be reversed in ten thousand five hundred years; at which time we shall be nearer the sun during our northern summer, and farther away in winter, our winter then being also longer than our summer. Besides, through the unequal attraction of the planets the eccentricity of the earth's orbit periodically increases and diminishes, so that there have been periods when the earth was ten million five hundred thousand miles farther from the sun in winter than in summer; at which times, also, the winter was nearly twenty-eight days longer than the summer. Such an extreme elongation of the earth's orbit occurred about two hundred and fifty thousand years ago. It is easy to assume that such a change in astronomical conditions would produce great effects upon the earth's climate; and equally easy to connect with those effects the vast extension of ice during the Glacial period. Since, also, this period of extreme eccentricity terminated only eighty thousand years ago, the close of the Glacial period would, perhaps, upon Mr. Croll's theory, be comparatively a recent event; for if the secular summer of the earth's eccentricity lags relatively as far behind the secular movements as the annual summer does behind the vernal equinox, we should, as Professor Charles H. Hitchcock suggests, have to place the complete breaking up of the Ice period as late as forty thousand years ago.[DT] [Footnote DT: Geology of New Hampshire, vol. iii, p.327.]