Sunshine
Aiton's Encyclopedia (1910)
in meteorology, light falling from the sun upon the earth's surface, without hindrance of haze, mist, fog, cloud, or dust storm. As a matter of fact the sun is shining always. Some germs grow in darkness, but, speaking in a general way, light is one of the essentials of plant and animal growth. For the farmer, gardener, and stock raiser, no artificial light equals natural sunlight. Some crops are favored by a share of cloudy weather, but, like air, warmth, and water, clear sunshine is one of the great promoters of life--one of the essentials of prosperity. As half of the hours the world over belong to the night, a cloudless region may have not to exceed 365 times 12, or 4,380 hours of sunshine yearly. Sunshine is in one respect the opposite of rainfall. As a rule, the less rainfall, the less cloudy weather. Perpetual sunshine, without rain, is characteristic of an arid or desert region. North America , which is the greatest food-producing region in the world, is particularly fortunate in the matter of sunshine. An area in Arizona, Colorado, New Mexico , and Mexico has an average of 3,250 hours of sunshine yearly. Surrounding this sunshine center is a zone having 3,000 hours of clear sunlight. A third encircling zone reaches the Pacific Coast, embracing in its sweep the fruit-producing valleys of California, the uplands of Kansas, the tobacco fields of Kentucky, and the cotton fields of Texas. Northern California, southern Louisiana, West Virginia , and the famous corn belt of Ohio, Indiana, Illinois, and Iowa enjoy 2,500 hours of annual sunshine. A belt crossing the continent from Massachusetts, via Minnesota, to southern Oregon, has 2,250 hours. Nova Scotia , Manitoba, Washington, and "sunny" Alberta, have 1,750 hours or about fifty-five per cent as much sunshine as Arizona. May and June are the sunniest months in the southern part of the United States . July is the sunniest month farther north.
What Has Changed in the Last Century
Aiton's 1910 entry treats sunshine almost entirely as an agricultural asset, measured in hours per year with the burning-glass recorders of the day and mapped in bands from Arizona's 3,250 hours down to Alberta's 1,750. Modern instruments measure not hours but energy: pyranometers and satellite radiometers record watts per square metre, which is why the same Southwest "sunshine center" the entry praised for its crops is now covered with photovoltaic and solar-thermal plants instead. Two medical discoveries arrived just after this was written and cut both ways: vitamin D was identified in the early 1920s, confirming sunlight's role in preventing rickets, while later work tied ultraviolet exposure to skin cancer, leading to sunscreens and the public UV Index introduced in the 1990s. Meteorologists have also found that sunshine reaching the ground is not fixed, tracking a mid-century "global dimming" from air pollution and a partial brightening since about 1990.
Written for this edition, 2026.