Day

A day is a unit of time based on Earth's rotation or a calendar convention. Its length depends on whether it is measured against the Sun, distant stars, or a civil timekeeping system.

Earth's rotation: Earth's spinning motion around its axis. A rotating Earth creates the recurring cycle underlying solar and sidereal days.
Solar day: The interval between successive passages of the Sun across a local meridian. This is the rotation-based day most closely tied to ordinary daylight and clocks.
Gregorian calendar: The calendar introduced in 1582 that is now the most widely used civil calendar. Its date changes at civil midnight, establishing the ordinary calendar day.
Circadian rhythm: A roughly 24-hour biological cycle regulating processes such as sleep and hormone release. Many organisms synchronize internal timing to the light-dark cycle of the day.
Hour: A unit of time equal to one twenty-fourth of a conventional day. Civil time divides the conventional day into 24 hours.
Sidereal day: The time Earth takes to rotate once relative to distant stars, about 23 hours 56 minutes. It isolates Earth's rotation from its simultaneous motion around the Sun.
International Date Line: A conventional boundary where the calendar date changes by one day. It coordinates civil dates across a rotating globe.
Diurnal cycle: A pattern that repeats over the course of a day, often driven by sunlight. Daily heating and illumination produce recurring environmental changes.
Mean solar time: Time measured using a fictitious Sun moving uniformly along the celestial equator. It smooths the uneven apparent motion of the real Sun to define average solar days.
Equation of time: The difference between apparent solar time and mean solar time. It explains why sundials and uniformly ticking clocks disagree by varying amounts.

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