Day (time unit)

A unit of time equal to 24 hours, conventionally 86,400 seconds. Its length is fixed by convention, unlike the varying duration of daylight or a solar day.

Second: The SI base unit of time, defined by the frequency of radiation from cesium-133 atoms. A conventional day contains exactly 86,400 seconds.
Solar day: The interval between successive passages of the Sun across a local meridian. Its duration varies, while the conventional day remains 24 hours.
Atomic clock: A clock that measures time using the stable frequencies of atomic transitions. Atomic clocks realize seconds precisely without relying on Earth's rotation.
Ancient Egyptian timekeeping: A system of time measurement in ancient Egypt that divided daylight and nighttime into twelve hours each. Its twelve-part day-and-night scheme helped establish the 24-hour division.
Hour: A unit of time equal to 60 minutes or 3,600 seconds. Twenty-four hours make up the conventional day.
Sidereal day: The time Earth takes to rotate once relative to distant stars, about 23 hours 56 minutes. It is shorter than the conventional day because Earth also orbits the Sun.
Coordinated Universal Time: The international time standard based on atomic seconds and adjusted to remain near Earth rotation time. It organizes civil time into days while accommodating the mismatch with Earth's rotation.
Babylonian astronomy: The systematic observation and recording of celestial motions in ancient Mesopotamia. Sexagesimal arithmetic associated with Mesopotamian scholarship shaped later divisions of time.
Minute: A unit of time equal to 60 seconds. The day is commonly divided into 1,440 minutes.
Mean solar day: The average interval between successive solar noons, smoothing the seasonal variation in apparent solar motion. It approximates 24 hours but is an astronomical average, not the unit's definition.

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Astronomical system of unitsBroader topic: The historical system used the day as its time unit.