NIGHT and DAY

Walter R. Steiger

George W. Bunton

Some people say they can tell time by the Sun,
but I can't see the numbers!
        .... Anonymoous 5th grader

T
he diurnal habits of mankind, like those of most other creatures of the Earth, have been derived in an environment influenced by the rotation of the Earth under the illumination of the Sun. Even in this modern day, when artificialities have greatly modified the natural environment, humans still regulate their lives largely by the rising and setting of the Sun. This is more true in rural than in urban communities where artificial daylight has been extended far into the night.

This dissertation attempts to describe and explain the various kinds of time used in astronomical as well as civil affairs, and to elucidate the relations among them. Practical applications include definitions and computations of twilight, sunrise, and sunset, among others, as well as their seasonal and geographical variations.

Almost all the flora and fauna of the Earth are light dependent, and many of the foraging animal species have developed visual ability under very feeble illumination . While the Sun is the the primary luminary of the Earth, there are other natural sources that provide enough light to accommodate some creatures. The full Moon is about one twenty-four thousandths as bright as the Sun, but the human eye can accommodate to that reduced light to permit one to read under a full Moon. When the Moon is not present, some light is provided by the stars, and even the upper atmosphere itself glows faintly at night. This airglow phenomenon has been observed and photographed by astronauts as a thin luminescence gracing the dark curve of the Earth where there was no solar illumination.

Many of the wonders of nature are beyond the appreciation of most of us, immersed as we are in the cocoon of civilization.

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