The Sun S describes the circle A A A on the 21st December once in 24 hours; hence in that period day and night occur to every part of the Earth, except within the Arctic circle. The light of the Sun gradually diminishing from S, to the Arctic circle 1 2 3, where it becomes twilight, does so according to the well-known law of radiation, equally in all directions--hence, the circle 4 5 6 represents the whole extent of the Sun’s light at any given time. The arc 4 E is the advancing or morning twilight, and 6 E the receding or evening twilight; to every place underneath a line drawn across the circle through S to N it is noonday. It will now be easily understood that as the Sun S moves in the direction of the arrows or from right to left, and completes the circle A A A in 24 hours, it will produce in that period morning, noon, evening, and night to all parts of the Earth in succession. On referring to the diagram, it will be seen that to England, E, the length of the day at this time of the year is the _shortest_, the amount of light being represented by the arc E E E; and also that the northern centre N remains in darkness during the whole daily revolution of the Sun, the light of which terminates at the Arctic circle 1 2 3. Thus, morning, noon, evening, midnight, the _shortest_ days, or the Winter season, and the constant or six months’ darkness at the pole are all a part of one general phenomenon. As the Sun’s path begins now to diminish every day until in six months, or on the 15th of June, it describes the circle B B B, it is evident that the same extent of sunlight will reach over or beyond the pole N, as shown in the following diagram (Fig. 28), when morning, noon, evening, and night will again occur as before; but the amount of light passing over England, represented by the arc E E E, is now much larger than when the Sun was upon the circle A A A, and represents the _longest_ days, or the _Summer_ season, and the constant, or six months’ light at the pole. Thus, day and night, long and short days, Winter and Summer, the long periods of alternate light and darkness at the pole, arise simply from the Sun’s position in relation to the north pole. [Illustration: FIG. 28.]