day

Day is the period during which a planet completes a rotation relative to its primary light source. For Earth, this is the time between successive noons, the moment when the Sun is highest in the sky. A related measure is the sidereal day, defined by a full rotation relative to distant stars, which lasts about 23 hours 56 minutes 4 seconds. The difference arises because the Earth also orbits the Sun; as it rotates, it must turn a little more to bring the Sun back to the local meridian.

On average, the civil day is 24 hours in length. This is a mean solar day, the average time between noons, and it underpins most timekeeping and calendars. The length of the solar day can vary slightly due to irregularities in Earth's rotation and orbital dynamics. To keep civil time aligned with the solar day, leap seconds are occasionally added to Coordinated Universal Time (UTC).

Days are subdivided into 24 hours, hours into 60 minutes, and minutes into 60 seconds in common civil timekeeping. Time zones further divide the day across regions. The day is also culturally significant, governing daily work, sleep, and religious observances; the alternation of day and night is produced by the tilt of Earth’s axis relative to its orbit around the Sun.

The word day comes from Old English dag, related to other Germanic languages, and ultimately to proto-Germanic roots meaning daylight or time of day. In astronomy and earth sciences, the concept of the day contrasts with the year and with longer cycles such as the lunar month.