daylength

Daylength, also called photoperiod in biological contexts, is the duration of daylight in a given location on a given date. It is measured as the time between sunrise and sunset and depends on the Sun's apparent path, the observer's latitude, and the tilt of the Earth's axis relative to its orbit.

Daylength varies with latitude and season. At the equator it stays close to 12 hours; toward higher latitudes the annual range grows. In temperate zones, days lengthen toward summer and shorten toward winter; at latitudes near 66.5° (the Arctic/Antarctic Circles) the Sun can stay above the horizon for 24 hours in summer or below it for 24 hours in winter.

Precise daylength is affected by atmospheric refraction and the convention used to define sunrise and sunset (the Sun's upper edge, not its center). Consequently observed daylength can differ by several minutes from simple geometric estimates.

Daylength can be estimated from the Sun's declination and the observer's latitude; in formula form, sunrise hour angle ω0 satisfies cos ω0 = -tan φ tan δ, and daylength = 2ω0 / 15° per hour. The concept is important in climate studies, ecology, agriculture, and biology, where photoperiod influences growth, reproduction, and behavior.