Fig. 7.
In summer the sun is higher, and nearer to the pole N, say at a point s. It rises at a point a in the ocean more to the north than E, the eastern point, and sets at a point b, also more north than W, the western point, and traverses the path a s b. But to traverse this path it takes longer than twelve hours, for a s b is more than half the circle a s b. Hence then it rises say at 4.30 a.m. and sets at 7.30 p.m. The night, during which the sun moves round the path from b to a, is correspondingly short, being only nine hours in length, from 7.30 p.m. till 4.30 a.m. So you have a long summer day and a short summer night. But in winter, when the sun gets nearer to the south pole of the heavens, it rises at a point C in the ocean at 7.30 a.m., and traverses the arc c t d, and sets at the point d at 4.30 p.m. So that the winter day is only nine hours long. But the winter night lasts from 4.30 p.m. till 7.30 a.m., and is therefore fifteen hours long, the sun going round the path d r c in the interval. It is therefore the obliquity of the poles N S, coupled with the fact that the sun’s position is nearer to one pole, N, in summer, and nearer to the other pole, S, in winter, that produces the inequality of days and nights in our latitudes. Suppose our island were on the equator. The north pole and the south pole would appear to be on the horizon, and then whether the sun moved in the circle a s b in the summer, or E S W at the vernal or autumnal equinoxes, or c t d in the winter, in each of these cases, though the places of rising and setting in the ocean might vary in summer from a and b to c and d in winter, yet in each of these cases the path from a to b, A to B, and c to d would still always be a half-circle and occupy twelve hours. Hence at the equator the days and nights never vary in length, but the sun always rises at six and sets at six. And, besides, it always rises straight up from the ocean and plunges down vertically into it, so that there is but little twilight and dawn.
Fig. 8.
But now let us suppose we were living at the north pole. In this case the north pole would be directly overhead, the south pole directly under our feet. At the vernal and autumnal equinoxes the sun would appear with half its disc above the ocean, and go round the ocean horizon, always appearing with half its disc above the sea. In summer it would appear at a point s nearer to the pole N. It would go round in the heavens, always appearing above the horizon, and would never set at all. As the summer waned the sun would become lower and lower, still, however, going round and round without setting till at the autumn equinox it reached the horizon. So that for six months it would never have set. But when it did set, there would then be six months without a sun at all.
Fig. 9.
Thus then all over the world the period of darkness and light is equivalent. At the tropics the days and nights are always equal. At the poles light for six months is followed by darkness for six months. In the intermediate temperate regions nights of varying lengths follow days of varying lengths, a short night following a long day and vice versâ.
