"I seem to remember something of the sort from my school days." "This angle of tilt may be assumed to be constant, for I won't bother with the precessions, nutations and other minor movements considered in accurate computations. As the earth moves around the sun, this tilt gives rise to what we call the sun's declination. You can readily see that at one time in the year, the north pole will be at its nearest point to the sun, speaking in terms of tilt and not in miles, while at another point on the elliptic, it will be farthest from the sun and the south pole nearest. There are two midway points when the two poles are practically equidistant." "Then the days and nights should be of equal length." "They are. These are the periods of the equinoxes. The point at which the sun is nearest to the south pole we call the winter solstice, and the opposite point, the summer solstice. The summer solstice is on June twenty-first. At that time the declination of the sun is 23-1/2 deg. north of the equatorial line. It starts to decrease until, six months later, it reaches a minus declination of 23-1/2 deg. and is that far south of the line. The longest day in the northern hemisphere is naturally June twenty-first." "And the shortest day when the sun has the greatest minus declination." * * * * * "Precisely, at the winter solstice. Now to explain what is happening. The year went normally until June twenty-first. That day was of the correct length, about fourteen hours and fifty minutes long. The twenty-second should have been shorter. Instead, it was longer than the twenty-first. Each day, instead of getting shorter as it should at this time of year, is getting longer. We have already gained some thirty-two minutes of sunlight at this latitude. The explanation is that the angle between the equator and the elliptic is no longer 23-1/2 deg. as it has been from time immemorial, but it is greater. If the continuing tilt keeps up long enough, the obliquity will be 90 deg. When that happens, there will be perpetual midday at the north pole and perpetual night at the south pole. The whole northern hemisphere will be bathed in a continuous flood of sunlight while the southern hemisphere will be a region of cold and dark. The condition of the earth will resemble that of Mercury where the same face of the planet is continually facing the sun." "I understand that all right, but I am still in the dark as to what is causing this increase of tilt."