Day length

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Day length as a function of latitude and the day of the year (running from 1 to 365 in an ordinary year or 1 to 366 in a leap year).

Day length, or length of day, or length of daytime, refers to the temporal length of a day, or 24 hours, during which there is daylight.

Due to the diffusion and refraction of sunlight by the atmosphere, there is actually daylight even when the sun is slightly below the horizon.

Theoretically, the day length can be computed from the moment the upper limb of the sun's disk appears on the horizon during sunrise to the moment when the upper limb disappears at the horizon during sunset.

More conveniently, however, the center of the sun is often used in place of the upper limb for computing the day length. When sunrise and sunset do occur, the day length can be computed as 2ω_o/15°, where ω_o is the sunset hour angle in degrees (°) given by the sunset equation. When sunrise and sunset do not occur during the course of a day, the day length is either 0 or 24 hours.

To considerable accuracy, all the points at the same latitude on the same calendar date can be considered to have the same day length. The contour plot in the figure is computed using the sunset equation.

Some interesting features that can be easily recognised are as follows:

On the Equator, the length of day only varies slightly from 12 hours all the year round;

On the day of vernal equinox (about March 21) and autumnal equinox (about September 23), everywhere on the Earth has a 12-hour day and, consequently, a 12-hour night except the north pole and the south pole where the sun is exactly at the horizon;

On any given day, the length of day at a given latitude in the northern hemisphere is equal to the length of night at the same latitude in the southern hemisphere, and vice versa;

At any given point of the surface of the Earth, the average day length over the course of one year is 12 hours;

On any given day, the average day length over the entire surface of the Earth is 12 hours;

When the northern polar cap of certain range has length of daytime of 24 hours, the southern polar cap of the same range has length of nighttime of 24 hours, and vice versa.

In the northern hemisphere, the Arctic Circle is the southernmost latitude where 24-hour daylight can occur at least on one day in a year (northern summer solstice about June 21);

In the southern hemisphere, the Antarctic Circle is the northernmost latitude where 24-hour daylight can occur at least on one day in a year (southern summer solstice about December 21);