The earth is represented at four successive points in its orbit about the sun. Since the axis of the earth is virtually unchangeable in its direction in space (leaving out of account the slow effects of the precession of the equinoxes), it results that at one time of the year, the north pole is inclined toward the sun and at the opposite time of the year away from it. It attains its greatest inclination sunward at the summer solstice, then the line between day and night lies 23½° beyond the north pole, so that the whole area within the arctic circle is in perpetual daylight. The days are longer than the nights throughout the northern hemisphere, but the day becomes longer in proportion to the night as the arctic circle is approached, and beyond that the sun is continually above the horizon. In the southern hemisphere exactly the reverse occurs. When the earth has advanced to the autumn equinox, the axis is inclined neither toward nor away from the sun. The latter is then perpendicular over the equator and day and night are of equal length all over the earth. When the earth reaches the winter solstice the north pole is inclined away from the sun, and now it is summer in the southern hemisphere. At the vernal equinox again there is no inclination of the axis either toward or away from the sun, and once more day and night are everywhere equal. A little study of this diagram will show why on the equator day and night are always of equal length. ] Now, with the lamp thus elevated above the equator, set the globe in rotation about its axis. You will perceive that all points in the upper hemisphere are longer in light than in darkness, because the plane dividing the illuminated and the unilluminated halves of the globe is inclined to the globe's axis in such a way that it lies beyond the upper pole as seen from the direction of the lamp. Consequently, the upper half of the globe above the equator, as it goes round, has more of its surface illuminated than unilluminated, and, as it turns on its axis, any point in that upper half, moving round parallel to the equator, is longer in light than in darkness. You will also observe that the ratio of length of the light to the darkness is greater the nearer the point lies to the pole, and that when it is within a certain distance of the pole, corresponding with the elevation of the lamp above the equator, it lies in continual light—in other words, within that distance from the pole night vanishes and daylight is unceasing. At the same time you will perceive that round the lower pole there is a similar space within which day has vanished and night is unceasing, and that in the whole of the lower hemisphere night is longer than day. Exactly on the equator, day and night are always of equal length.