In the following diagram ([Fig. 27]) A A A represent the Sun’s daily path on December 21st, and B B B the same on June 15th. N the North Pole, S the Sun, E Great Britain. The figures 1 2 3 the Arctic Circle, and 4 5 6 the extent of sunlight. The arrows show the direction of the Sun’s motion.
FIG. 27.
SECTION 6.
CAUSE OF DAY AND NIGHT, SEASONS, &c.
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.
FIG. 28.
If the Earth is a globe, it is evident that Winter and Summer, and long and short days, will be of the same character and duration in corresponding latitudes, in the southern as in the northern hemisphere. But we find that in many respects there is a marked difference; for instance, in New Zealand, where the latitude is about the same as in England, a remarkable difference exists in the length of day and night. In the Cook’s Strait Almanack, for 1848, it is stated, “At Wellington, New Zealand, December 21, Sun rises 4h. 31m., and sets at 7h. 29m., the day being 14 hours 58 minutes. June 21st, Sun rises at 7h. 29m., and sets at 4h. 31m., the day being 9 hours and 2 minutes. In England the longest day is 16h. 34m., and the shortest day is 7h. 45m. Thus the longest day in New Zealand is 1 hour and 36 minutes shorter than the longest day in England; and the shortest day in New Zealand is 1 hour and 17 minutes longer than the shortest day in England.”
In a recently published pamphlet, by W. Swainson, Esq., Attorney General, the following passage occurs:—“Compared with an English summer, that of Auckland is but little warmer, though much longer; but the nights in New Zealand are always cool and refreshing.... The days are one hour shorter in the summer, and one hour longer in the winter than in England! of twilight there is little or none.”
From a work, also recently published, on New Zealand, by Arthur S. Thompson, M.D., the following sentences are quoted:—“The summer mornings, even in the warmest parts of the colony are sufficiently fresh to exhilarate without chilling; and the seasons glide imperceptibly into each other. The days are an hour shorter at each end of the day in summer, and an hour longer in winter than in England.”