daynight

Daynight refers to the regular cycle of daylight and darkness caused by the rotation of the Earth on its axis relative to the Sun. The boundary between day and night is the terminator, a moving line that divides the illuminated hemisphere from the dark hemisphere.

The length of day and night varies with latitude and season. The axial tilt of about 23.44 degrees causes the Sun’s apparent path to rise higher in the sky during local summer and lower during winter, producing longer days in warm seasons and longer nights in cold seasons. At extreme latitudes, this can lead to polar day (the midnight sun) or polar night, where the Sun does not rise or set for extended periods.

Two related measures describe time in the day-night cycle: the solar day and the sidereal day. The solar day, averaging 24 hours, is based on the Sun’s apparent daily motion across the sky. The sidereal day, about 23 hours 56 minutes, is based on the rotation relative to distant stars. The solar day is longer because the Earth must rotate a little more to compensate for its orbital movement around the Sun.

Twilight describes periods of partial light around sunrise and sunset. Civil twilight ends when the Sun is 6 degrees below the horizon; nautical twilight ends at 12 degrees; astronomical twilight ends at 18 degrees. These distinctions affect outdoor lighting, navigation, and astronomical observations.

The day-night cycle is a fundamental feature of rotating bodies with a Sun-like illumination and influences daily life, scheduling, and cultural practices related to daylight and darkness.