daylengths

Daylengths, or photoperiods, denote the length of time between sunrise and sunset at a given location and date. They are determined by the tilt of the Earth's axis and its orbit around the Sun, and are affected by atmospheric refraction and the Sun’s apparent path across the sky.

Throughout the year, daylength varies with latitude. Near the equator it remains close to 12 hours, while higher latitudes experience longer summer days and shorter winter days. At latitudes within the Arctic Circle and Antarctic Circle, there are periods of continuous daylight or continuous darkness.

The longest and shortest days occur at the summer and winter solstices, when the Sun reaches its extreme northernmost or southernmost declination. The equinoxes, occurring in spring and autumn, are the days when day and night are most nearly equal.

In practice, daylength can be estimated from a location's latitude and the solar declination using astronomical formulas; sunrise and sunset times—adjusted for the Sun's disc and atmospheric refraction—provide observed daylength values.

Daylength is a major environmental cue that influences plant phenology, animal behavior and circadian rhythms, and it affects agricultural planning, energy demand, and human activities.