Twilight and Daylight Saving Time

What is Twilight?

Daylight saving time is possible because of the phenomenon of twilight. It is the manipulation of twilight, the period near and after sunset, that allows for the misleadingly named "daylight saving" to be instituted not by natural events but by man. As with time zones and the seasons it is the movement of the Earth relative to the Sun which causes twilight but with this effect many other factors are at play.

Twilight occurs, officially, when the Sun is not more than twelve degrees below the horizon. At low latitudes, in the tropical regions, the Sun sets more steeply than in the temperate zones. These are the areas between the tropics of Cancer and Capricorn and Arctic and Antarctic parallels of latitude respectively. In the tropics, with the Earth rotating at one degree every four minutes it takes only about forty-eight minutes for the Sun to dip more than twelve degrees below the horizon. Twilight is of very short duration in this region. Daylight saving time is never proposed by communities in tropical areas. At higher latitudes the sunset process takes much longer. Above the polar circle latitudes, in summer, the Sun does not get to twelve degrees below the horizon. Effectively the Sun does not set. Travelling from the tropics to the polar parallels the period of twilight increases. This phenomenon is emphasized in the summer months in both hemispheres because of the tilt of the earth's axis with respect to its plane of orbit round the Sun.

The other effects that happen to the Sun's rays as they impinge upon the Earth are that they are reflected and refracted by natural objects. This oceans, lakes, mountains, clouds and even the earth's atmosphere scatter the Sun's rays even though the Sun itself is below the horizon. In the higher latitudes it is possible to conduct many activities in a good light even though the Sun has set. The twilight period can extend for many hours.

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Daylight Saving Time is Twilight But Not of the Gods

It would be perfectly possible for humans to enjoy natural twilight periods. As is often the case man attempts to improve upon nature for his own benefit. By the simple process of re-setting clocks twice a year it is possible to make the normal working and business activities occur at times other than the natural Sun time. Thus the working day can begin and end earlier according to the clocks or closer to the natural dawn and before sunset respectively. A long period is then available for sport and recreational activities in good light and pleasant temperatures. This re-setting of the clocks is known as daylight saving time. There is, of course, no daylight saved in fact because this is a natural period the length of which cannot be changed. It is just made "administratively" possible for working hours to avoid the twilight period until the onset of winter when its length makes this less attractive and the start of the working day becomes too close to beginning in darkness before sunrise. Man-made clocks are then re-set for the winter season.

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After ...

Not every place observes "daylight saving time". This simply exacerbates the inconveniences of time zone differences when clocks are not re-set in common in different zones. This occurs most frequently in areas in or near the tropics where the twilight period is too short to make any adjustment of clocks necessary or worthwhile. Not all men agree about what is for the best but few would dispute that it is a very beautiful time of day.

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