Sun's changing declination is the variable length of days

The Sun travels in right ascension throughout the year, starting at 0h 0m 0s at the vernal equinox, or the first point of Aries, and returns to it the following year after having been all the way up to 24h. Right ascension is measured in hours as it is directly related to time. As the Sun's actual passage is along the ecliptic, and as right ascension is measured along the equator, the rate of advance varies during different seasons of the year.

This unequal change in declination, together with the variable speed of the Earth's travel around the Sun, makes the differences between sundial time and clock time. Because of them, we must adjust all solar observations by the "equation of time," which we mentioned in the chapter on Days.

The most obvious effect of the Sun's changing declination is the variable length of days and duration of twilight. Our own latitude is of great importance, too, so we shall take a journey both ways from the equator to the poles during various seasons of the year, and we shall also travel through one season at a single place.

For the start of our journey, let the season be mid-December and the place the equator. We shall then be situated where fewest seasonal changes occur. The Sun is to the south of us, but only by 23° 27'. It rises high each day and at nightfall it sinks rapidly below the horizon. Astronomers have generally set the end of twilight as the time when the Sun reaches the level of eighteen degrees below the horizon. Obviously, if the Sun is traveling in a vertical path, it will cover the distance more quickly than if its movement forms an oblique angle with the horizon. There is practically no twilight in the tropics, where the Sun is always nearly overhead at noon. The Sun "shoots up like thunder" and descends in the same precipitous fashion. Within half an hour after sunset the sky can appear as dark as midnight. On one occasion a photographer was trying to take a picture of a group who were facing west. He could not take it before sundown, because his subjects were posing themselves with their eyes shut against the glare. As soon as the Sun had set, he adjusted his camera, and he had just time to make two exposures before the light failed.

Nights near the tropics can be inky black with a blackness rarely seen from higher latitudes. Once in West Australia, I was going with a friend from our quarters at the Sons of Gwalia Mine (latitude 28° South) to the State Hotel about half a mile away. There was no danger of losing our way. The lights of the hotel shone clearly, and sheet lightning played in the sky; but there was no road--only a path which was indistinguishable in the darkness. We set off on a direct route for the hotel. Suddenly a flash of lightning illuminated a tree not five feet ahead of us. Without the lightning we could not have known that the tree was there until we bumped into its branches.

On return trips we had no direct lights to guide us. Our quarters were unlit, but there was an arc lamp outside the boiler house. Invariably we found ourselves near the boiler house first. From there we could turn and find our way home.

The same darkness persists until dawn. One morning I wanted to shoot some ducks on a pond about four miles away. To reach the place I had only to follow a railroad track. The watchman woke me at 4:00 A.M., and I made my way in complete darkness along the track. As I reached the pond I heard a weak "quack," but the bird was lost in obscurity. Dawn began to break, while I walked around the two-acre pond. Still there was no sign of a bird. Suddenly, almost without warning, up came the Sun. It had the effect of a blow. Instantly the temperature changed, and I felt a breath of really hot air. Not liking to go home without some trophy, I continued to walk around the pond. Before I realized that any time had passed, the Sun had risen to an elevation of twenty degrees in the sky. Eventually I saw one bird; I shot him, and, taking him home in triumph, found that I was not even late for breakfast.

Beginning now our journey south, we shall be approaching the summer, the days getting longer and warmer. There are longer twilights at both ends of the day. As we approached the Antarctic Circle, the altitude of the Sun ceases to fall more than eighteen degrees below the horizon line and at midnight the sky will not be dark.

As we continue south through the Antarctic Circle, we find that the Sun does not set at all at midsummer. From the poles, beginning at the spring equinox, the Sun appears to perform a continuous spiral, and it gradually reaches the altitude of 23° 27', at midsummer. Then it spirals down to the horizon at the other equinox and passing through the twilight zone, does not reappear again till the following spring. We know what will be happening during the time that it is below the horizon at one of the poles. It is above the horizon at the other and performing the same evolution.

If we go north from the equator in December, we shall have the opposite experience. The daylight will grow shorter and shorter. In London, latitude 51° 30', the soldiers can start their "midnight military maneuvers" as soon as they have finished their afternoon tea.

In Edinburgh, 56° North, the Sun just makes a pretense of showing itself. If there are no Scottish mists to interfere, it can actually be seen at the magnificent altitude of 101/2°, at noon on December 21.

Still farther north at the Arctic Circle, the Sun just fails to rise at all; and residents of North Cape have to be content with a feeble twilight during the winter months. North of them even twilight is lacking, and men have to do their outdoor work by the shining of the Moon.

Let us now return to our starting point in the tropics. Except for the rains there are no marked changes in the weather. The Sun may rise and set slightly to the north or south. It may appear to be nearly overhead.

"Nearly overhead?" Why should it not appear exactly overhead? We are in the tropics now, and presumably by the very definition of our residence, the Sun is vertically above us. That is quite true; but the Sun's declination is constantly changing. It may vary as much as one minute an hour. There is always the chance that the Sun will pass the overhead point of declination sometime during the night or early or late in the day. Then at high noon the Sun will not be straight above us, but already a little to the north or south. For that year at least we shall have missed the opportunity of standing directly under the Sun.

Even here in the tropics where the seasonal changes are so slight, there are curious results to be seen from them. In Ceylon, 2° North, an old resident can always pick out a tenderfoot, because the newcomer plants his trees to the south of his residence. He has been accustomed to plant them there in the northern latitudes, and here he is still north of the equator. "Ah yes," says the old resident, "but the hottest weather in Ceylon always comes when the Sun has a northern declination. We who know the country plant our shade trees to the north."

If we live on the equator we shall see the midsummer Sun set 23° 27' north of us, and as far south in winter. Daylight always lasts for about twelve hours. Here the old "unequal hours" become equal, and the old system has an advantage over modern clocks which show half an hour's variation in the rising and setting times of the Sun.