Imagine now that you are far away in space and are looking down on the north poles of both the earth and the sun just as you look down on the page before you. The earth would seem to move around the circle in the direction opposite to the hands of a clock, or counter clockwise. With the axis still pointing to the North Star place the pin in such a position that the regionS surrounding the north pole will get as much light as possible. That will represent the true position about June 21. Now carry the pin a quarter around the circle counter-clockwise, remembering still to keep it pointing to the north star. Now it is in the proper position for September 22, and the axis is inclined neither toward nor away from the sun. Place the pin in the proper positions for December 21 and March 21. Fin ish the work by revolving the earth through its orbit for twelve months, stopping on your birthday, or on the day when you study this page.
Human Habits and the Length of Daylight.—The inclination of the earth's axis causes the sun to remain above the horizon far longer in some places and at certain seasons than at others. Hence daylight and night vary greatly in length. This influences a multitude of human habits, such as hours of rising and times of recreation. In places like Norway or Alaska, where the period of daylight is long in summer, some people become so tired and nervous from lack of sleep that they are of ten irritable and sometimes become insane. The long winter nights, on the other hand, bring with them a period of comparative idleness which has a bad effect on the character.
The relative length of daylight and night has also an important bearing on temperature, and thus on plants and agriculture. In high latitudes the earth and air become very cold during the long winter nights. If snow falls, practically none melts during the short days, and it may accumulate so that even the long days of summer cannot melt all of it, and hence no crops can be grown. On the other hand, where little of the summer's heat is used up in melting, the long days cause the air to become warm in spite of the low position of the sun. Hence in Siberia and Canada, grain and vegetables can be raised as far north as the Arctic Circle.
Effect of Length of Daylight on Production of Seeds.—Another remarkable effect of the length of daylight is seen in the production of seeds. For many species of plants, and probably for all, a certain definite duration of daylight is necessary if flowers and seeds are to be produced. Temperature, moisture, and the intensity of light all have a marked effect on the vegetative growth, that is, on the size, shape, and vigor of the stems and leaves, but not on the time of flowering. This depends almost wholly on the length of the period of
light. For example, a certain kind of tobacco called Maryland Mammoth was long known to be valuable because it grows to great size, sometimes 12 to 15 feet high. It was hard to raise, however, because no matter how early it was planted it would not produce seeds except when transplanted to a greenhouse during the winter. Then it was found that plants started in the autumn and only 1 of 2 feet high would produce seeds in the winter at the same time as the great stalks that had been growing since spring. Finally experiments showed that if the tobacco were covered so that no light reached it during part of each day in summer, it would produce seed without regard to its size. In other words a healthy plant begins to produce seeds when the length of the period of daylight is reduced for a few weeks to eight or nine hours.
Other plants like the radish, for example, usually blossom only when the period of light is long. For that reason many of the common vegetables of the temperate zone will not produce seeds in the tropics, for there the daylight never lasts more than twelve hours. On the other hand, when such plants are grown in a greenhouse during the short days of winter, they can be made to blossom by subjecting them to electric light during part of the night.
Many kinds of trees that blossom early in the spring are stimulated to form flower buds by the short days of the autumn. Cold weather comes on, however, and checks their growth, but as soon as the air is sufficiently warm they blossom during the short days of spring.
All this is important to the farmer. If he wants his crops to blossom quickly without making much growth of stem and leaf, he should plant them only a little before the time when the length of the days causes the flower buds to develop. If he wants much vegetative growth, however, he must plant long before the time when the length of the day leads to flowering.
How Daylight and Night Vary in Length.—The cause of variations in the length of daylight and night is illustrated in Fig. 15. This represents the distribution of sunlight in the northern hemisphere during each month of the year. The proportions of the earth, sun, and orbit are far from true, but this is necessary in order to make the earth large enough. In the figure the North Pole is toward us, and the earth is revolving around the sun in the direction shown by the dates. It rotates in the same direction, that is, counter clockwise.