Day

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Day

Day (/deɪ/; from Old English dæg) is a unit of time. In common usage, it is either an interval equal to 24 hours or daytime, the consecutive period of time during which the Sun is above the horizon.

Etymology

The term day originates from the Old English word dæg. The word is of Germanic origin and is related to the Old High German tag, the Old Norse dagr, and the Dutch dag.

Definition

In its most common usage, a day is a period of 24 hours. This is the time it takes for the Earth to rotate once on its axis. This period of time is divided into day time, when the Sun is above the horizon, and night time, when it is below. This usage is known as a solar day.

In a more general sense, a day can also refer to the period of light between dawn and dusk when the Sun is above the horizon. This is known as a daylight day.

Related Terms

  • Solar day: The period of time it takes for the Sun to return to the same position in the sky. This is approximately 24 hours, but varies slightly throughout the year.
  • Sidereal day: The period of time it takes for the Earth to make one complete rotation with respect to the stars. This is approximately 23 hours, 56 minutes, and 4 seconds.
  • Daylight day: The period of time between sunrise and sunset when the Sun is above the horizon.
  • Civil day: A day according to the civil calendar, which begins at midnight and ends at the following midnight.
  • Julian day: A continuous count of days since the beginning of the Julian Period, used primarily by astronomers.

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