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.

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.

Some interesting features are as follows:

• On the Equator, the length of day is not exactly 12 hours all the year round, but rather — due to atmospheric refraction and the size of the Sun — exceeds 12 hours by about 7 minutes each day;

• Because the sun is north of the equator for almost 4 days more than half the year, the length of the average day at a given latitude in the northern hemisphere exceeds the length of the average day at the same latitude in the southern hemisphere by a few minutes;

• During a few days around the equinoxes—about March 19-22 and September 21-24—both poles experience 24 hours of daytime, due mainly to atmospheric refraction.

• At the poles there is only one sunrise and one sunset in the course of a year. This occurs around the time of the equinoxes.

Alternate definition

More conveniently, atmospheric refraction is ignored and 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.

With this definition the following features can be observed:

• At a given latitude, the length of the longest day in the northern hemisphere (on or about June 21) is the same as the length of the longest day in the southern hemisphere (on or about December 21);

• Similarly, at a given latitude, the length of the shortest day in the northern hemisphere (on or about December 21) is the same as the length of the shortest day in the southern hemisphere (on or about June 21);

• 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 on or 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 on or about December 21);

See also

• Sunrise
• Sunset
• Sunrise equation
• Sunset equation
• Sun declination
• Julian day
• Solar day
• Postglacial rebound
• Photoperiodism (for effects on living organisms)

External links

• Formulae to calculate day length, by Herbert Glarner
• Downloadable Excel Worksheet based on Glarner's formulae, by Don Whiteside
• Calculate sun rise/set, by U.S. Naval Observatory