climate, and when the orbit is at its greatest eccentricity, there will be an age of cold with great extremes of seasonal temperature. These changes in the orbit of the earth are due to the varying pull of all the planets, and Sir Robert Ball declared himself unable to calculate any regular cycle of orbital change, but Professor G. H. Darwin maintained that it is possible to make out a kind of cycle between greatest and least eccentricity of about 200,000 years.
But this change in the shape of the orbit is only one cause of the change of the world's climate. There are many others that have to be considered with it. As most people know, the change in the seasons is due to the fact that the equator of the earth is inclined at an angle to the plane of its orbit. If the earth stood up straight in its orbit, so that its equator was in the plane of its orbit, there would be no change in the seasons at all. The sun would always be overhead at the equator, and the day and night would each be exactly twelve hours long throughout the year everywhere. It is this inclination which causes the difference in the seasons and the unequal length of the day in summer and winter. There is, according to Laplace, a possible variation of nearly three degrees (from 22° 6′ to 24° 50′) in this inclination of the equator to the orbit, and when this is at a maximum, the difference between summer and winter is at its greatest. Great importance has been attached to this variation in the inclination of the equator to the orbit by Dr. Croll in his book Climate and Time. At present the angle is 23° 27′. Manifestly when the angle is at its least, the world's climate, other things being equal, will be most equable.
And as a third important factor there is what is called the precession of the equinoxes. This is a slow wabble of the pole of the spinning earth that takes 25,000 odd years. Any one who watches a spinning top as it "sleeps," will see its axis making a slow circular movement, exactly after the fashion of this circling movement of the earth's axis. The north pole, therefore, does not always point to the same north point among the stars; its pointing traces out a circle in the heavens every 25,000 years.
Now, there will be times when the earth is at its extreme of aphelion or of perihelion, when one hemisphere will be most turned