Tagesdauer

Tagesdauer, or day length, refers to the duration of daylight on a given location and date—the time interval between sunrise and sunset. It is determined by the geometry of the Earth’s tilt and orbit and is usually expressed in hours and minutes.

The precise definition of sunrise and sunset depends on convention. In practical terms, most common definitions consider sunrise the moment the Sun’s upper limb appears above the horizon and sunset the moment the upper limb disappears below it. This accounts for atmospheric refraction and the Sun’s disk size, and yields a standard value often around a solar altitude of about -0.833 degrees at the horizon. Under this convention, Tagesdauer is the time the Sun spends above the horizon.

Day length varies mainly with latitude and time of year. At the equator, day length remains close to 12 hours throughout the year. As one moves toward higher latitudes, the annual cycle becomes more pronounced: days are longest near the summer solstice and shortest near the winter solstice. Regions within the Arctic and Antarctic Circles experience polar day (continuous daylight) in summer or polar night (continuous darkness) in winter. The maximum and minimum day lengths depend on latitude, with extreme values occurring in high latitudes.

The seasonal pattern of Tagesdauer is driven by the tilt of the Earth's axis relative to its orbit around the Sun, causing the Sun’s apparent declination to change over the year. The length of daylight is also influenced by the equation of time, which affects the timing of solar noon but not the basic geometric length of the day.

Understanding day length has practical implications for agriculture, energy planning, ecology, and climate studies, and day-length data are routinely used in weather models and astronomical calculations.