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Midtown Manhattan from Weehawken September 2021 panorama 1.jpg
Midtown Manhattan from Weehawken September 2021 panorama 2.jpg
Midtown Manhattan from Weehawken September 2021 HDR panorama.jpg
A quarter-day cycle at Midtown Manhattan, from afternoon to dusk

A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.


A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years. A solar calendar organizes dates based on the Sun's annual cycle, giving consistent start dates for the four seasons from year to year. A lunar calendar organizes dates based on the Moon's lunar phase.

In common usage, a day starts at midnight, written as 00:00 or 12:00 am in 24- or 12-hour clocks, respectively. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time. Other conventions are sometimes used, for example the Jewish religious calendar counts days from sunset to sunset, so the Jewish Sabbath begins at sundown on Friday. In astronomy, a day begins at noon so that observations throughout a single night are recorded as happening on the same day.

In specific applications, the definition of a day is slightly modified, such as in the SI day (exactly 86,400 seconds) used for computers and standards keeping, local mean time accounting of the Earth's natural fluctuation of a solar day, and stellar day and sidereal day (using the celestial sphere) used for astronomy. In most countries outside of the tropics, daylight saving time is practiced, and each year there will be one 23-hour civil day and one 25-hour civil day. Due to slight variations in the rotation of the Earth, there are rare times when a leap second will get inserted at the end of a UTC day, and so while almost all days have a duration of 86,400 seconds, there are these exceptional cases of a day with 86,401 seconds (in the half-century spanning 1972 through 2022, there have been a total of 27 leap seconds that have been inserted, so roughly once every other year).


The term comes from the Old English term dæġ (/dæj/), with its cognates such as dagur in Icelandic, Tag in German, and dag in Norwegian, Danish, Swedish and Dutch – all stemming from a Proto-Germanic root *dagaz. [1] As of October 17,2015, day is the 205th most common word in American English, [2] and the 210th most common in English English. [2]


Apparent and mean solar day

Earth's rotation imaged by Deep Space Climate Observatory, showing axis tilt EpicEarth-Globespin-tilt-23.4.gif
Earth's rotation imaged by Deep Space Climate Observatory, showing axis tilt

Several definitions of this universal human concept are used according to context, need, and convenience. Besides the day of 24 hours (86,400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, the time it takes for the Sun to return to its culmination point (its highest point in the sky). Due to an orbit's eccentricity, the Sun resides in one of the orbit's foci instead of the middle. Consequently, due to Kepler's second law, the planet travels at different speeds at various positions in its orbit, and thus a solar day is not the same length of time throughout the orbital year. Because the Earth moves along an eccentric orbit around the Sun while the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. In recent decades, the average length of a solar day on Earth has been about 86,400.002 seconds [3] (24.000 000 6 hours). There are currently about 365.2421875 solar days in one mean tropical year.

Ancient custom has a new day starting at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, old style). [4] The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude and latitude, as well as altitude), and the time of year (as indicated by ancient hemispherical sundials).

A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).

In terms of Earth's rotation, the average day length is about 360.9856°. A day lasts for more than 360° of rotation because of the Earth's revolution around the Sun. With a full year being slightly more than 360 days, the Earth's daily orbit around the Sun is slightly less than 1°, so the day is slightly less than 361° of rotation.

Elsewhere in the Solar System or other parts of the universe, a day is a full rotation of other large astronomical objects with respect to its star. [5]

Civil day

For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at a central meridian. Such time zones began to be adopted about the middle of the 19th century when railroads with regularly occurring schedules came into use, with most major countries having adopted them by 1929. As of 2015, throughout the world, 40 such zones are now in use: the central zone, from which all others are defined as offsets, is known as UTC±00, which uses Coordinated Universal Time (UTC).

The most common convention starts the civil day at midnight: this is near the time of the lower culmination of the Sun on the central meridian of the time zone. Such a day may be called a calendar day.

A day is commonly divided into 24 hours, with each hour being made up of 60 minutes, and each minute composed of 60 seconds.

Sidereal day

Rotation of the dwarf planet Ceres PIA19547-Ceres-DwarfPlanet-Dawn-RC3-Animation-20150504.gif
Rotation of the dwarf planet Ceres

A sidereal day or stellar day is the span of time it takes for the Earth to make one entire rotation [6] with respect to the celestial background or a distant star (assumed to be fixed). [7] Measuring a day as such is used in astronomy. [7] A sidereal day is about 4 minutes less than a solar day of 24 hours (23 hours 56 minutes and 4.09 seconds), or 0.99726968 of a solar day of 24 hours. [8] There are about 366.2422 stellar days in one mean tropical year (one stellar day more than the number of solar days). [9]

Besides a stellar day on Earth, other bodies in the Solar System have day times, the durations of these being: [10] [11]

NameDaylength (hours)
Mercury 4222.6
Venus 2802
Earth's Moon 708.7
Mars 24.7
Ceres 9 [12] –9.1 [13]
Jupiter 9.9
Saturn 10.7
Uranus 17.2
Neptune 16.1
Pluto 153.3

In the International System of Units

In the International System of Units (SI), a day not an official unit, but is accepted for use with SI. [14] A day, with symbol d, is defined using SI units as 86,400 seconds; the second is the base unit of time in SI units. In 1967–68, during the 13th CGPM (Resolution 1), [15] the International Bureau of Weights and Measures (BIPM) redefined a second as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom". [16] This makes the SI-based day last exactly 794,243,384,928,000 of those periods.

In decimal and metric time

Decimal clock face, made in around the start of the 19th century Decimal Clock face by Pierre Daniel Destigny 1798-1805.jpg
Decimal clock face, made in around the start of the 19th century

Various decimal or metric time proposals have been made, but do not redefine the day, and use the day or sidereal day as a base unit. Metric time uses metric prefixes to keep time. It uses the day as the base unit, and smaller units being fractions of a day: a metric hour (deci) is 110 of a day; a metric minute (milli) is 11000 of a day; etc. [17] Similarly, in decimal time, the length of a day is static to normal time. A day is also split into 10 hours, and 10 days comprise a décade – the equivalent of a week. 3 décades make a month. [18] :35 Various decimal time proposals which do not redefine the day: Henri de Sarrauton's proposal kept days, and subdivided hours into 100 minutes; [18] :42 in Mendizábal y Tamborel's proposal, the sidereal day was the basic unit, with subdivisions made upon it; [18] :42–43 and Rey-Pailhade's proposal divided the day 100 cés. [18] :42

Other definitions

The word refers to various similarly defined ideas, such as:

Full day

Variations in length

Mainly due to tidal deceleration – the Moon's gravitational pull slowing down the Earth's rotation – the Earth's rotational period is slowing. [20] Because of the way the second is defined, the mean length of a solar day is now about 86,400.002 seconds, and is increasing by about 2 milliseconds per century. [21]

Since the rotation rate of the Earth is slowing, the length of a SI second fell out of sync with a second derived from the rotational period. [20] This arose the need for leap seconds, which insert extra seconds into Coordinated Universal Time (UTC). [20] Although typically 86,400 SI seconds in duration, a civil day can be either 86,401 or 86,399 SI seconds long on such a day. Other than the two-millisecond variation from tidal deceleration, other factors minutely affect the day's length, which creates an irregularity in the placement of leap seconds. [22] Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS), which measures the Earth's rotation and determines whether a leap second is necessary.

Geological day lengths

Discovered by paleontologist John W. Wells, the day lengths of geological periods have been estimated by measuring sedimentation rings in coral fossils, [23] [21] due to some biological systems being affected by the tide. [21] The length of a day at the Earth's formation is estimated at 6 hours. [21] Arbab I. Arbab plotted day lengths over time and found a curved line. [21] Arbab attributed this to the change of water volume present affecting Earth's rotation. [21]

DateGeological periodNumber of days per year [23] Duration of the day
PresentCurrent36524 hours
−100 million years Cretaceous 38023 hours and 20 minutes
−200 million years Triassic 39022 hours and 40 minutes
−300 million years Carboniferous 40022 hours
−400 million years Devonian 41021 hours and 20 minutes
−500 million years Cambrian 42520 hours and 40 minutes


Sun and Moon, Hartmann Schedel's Nuremberg Chronicle, 1493 Sun and Moon Nuremberg chronicle.jpg
Sun and Moon, Hartmann Schedel's Nuremberg Chronicle , 1493

For most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day's boundaries.

In the Hebrew Bible, Genesis 1:5 defines a day in terms of "evening" and "morning" before recounting the creation of the Sun to illuminate it: "And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day." The Jewish day begins at either sunset or nightfall (when three second-magnitude stars appear). Medieval Europe also followed this tradition, known as Florentine reckoning: In this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve, Halloween (“All Hallows’ Eve”), and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening.

The common convention among the ancient Romans, [24] ancient Chinese [25] and in modern times is for the civil day to begin at midnight, i.e. 00:00, and to last a full 24 hours until 24:00, i.e. 00:00 of the next day.

In ancient Egypt the day was reckoned from sunrise to sunrise.

Prior to 1926, Turkey had two time systems: Turkish, counting the hours from sunset, and French, counting the hours from midnight.


Humans have divided the day in rough periods, which can have cultural implications, and other effects on humans' biological processes. The parts of the day do not have set times; they can vary by lifestyle or hours of daylight in a given place. [26]


Daytime is the part of the day during which sunlight directly reaches the ground, assuming that there are no obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than night. The Sun is not a point but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. [27] Thus, daytime is on average around 7 minutes longer than 12 hours. [28]

Daytime is further divided into morning, afternoon, and evening. Morning occurs between sunrise and noon. [29] Afternoon occurs between noon and sunset, [30] or between noon and the start of evening. This period of time sees human's highest body temperature, [31] an increase of traffic collisions, [32] and a decrease of productivity. [33] Evening begins around 5 or 6 pm, or when the sun sets, and ends when one goes to bed. [34] [35] [36]


Twilight-dawn subcategories.svg

Twilight is the period before sunrise and after sunset in which there is natural light but no direct sunlight. [37] The morning twilight begins at dawn and ends at sunrise, while the evening twilight begins at sunset and ends at dusk. Both periods of twilight can be divided into civil twilight, nautical twilight, and astronomical twilight. Civil twilight is when the sun is up to 6 degrees below the horizon; nautical when it is up to 12 degrees below, and astronomical when it is up to 18 degrees below. [38]

Night in art Adolf Stademann Flusslandschaft im Mondschein.jpg
Night in art


Night is the period in which the sky is dark; [39] the period between dusk and dawn when no light from the sun is visible. [40] Light pollution during night can impact human and animal life, for example by disrupting sleep. [41] [42]

See also

Related Research Articles

Δ<i>T</i> (timekeeping) Measure of variation of solar time from atomic time

In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.

A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around 20 March and 23 September.

<span class="mw-page-title-main">Hour</span> Unit of time equal to 60 minutes

An hour is a unit of time historically reckoned as 124 of a day and defined contemporarily as exactly 3,600 seconds (SI). There are 60 minutes in an hour, and 24 hours in a day.

<span class="mw-page-title-main">Second</span> SI unit of time

The second is the unit of time in the International System of Units (SI), historically defined as 186400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. "Minute" comes from the Latin pars minuta prima, meaning "first small part", and "second" comes from the pars minuta secunda, "second small part".

<span class="mw-page-title-main">Year</span> Time of one planets orbit around a star

A year is the time taken for astronomical objects to complete one orbit. For example, a year on Earth is the time taken for Earth to revolve around the Sun. Generally, a year is taken to mean a calendar year, but the word is also used for periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. The term can also be used in reference to any long period or cycle, such as the Great Year.

A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

<span class="mw-page-title-main">Sidereal time</span> Timekeeping system on Earth relative to the celestial sphere

Sidereal time is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".

<span class="mw-page-title-main">Analemma</span> Diagrammatic representation of Suns position over a period of time

In astronomy, an analemma is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure eight. Globes of Earth often display an analemma as a two-dimensional figure of equation of time vs. declination of the Sun.

<span class="mw-page-title-main">Solar time</span> Calculation of elapsed time by the apparent position of the sun

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.

Sunrise is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon.

<span class="mw-page-title-main">Midnight sun</span> Natural phenomenon when daylight lasts for a whole day

Midnight sun, also known as polar day, is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When midnight sun is seen in the Arctic, the Sun appears to move from left to right. In Antarctica, the equivalent apparent motion is from right to left. This occurs at latitudes ranging from approximately 65°44' to exactly 90° north or south, and does not stop exactly at the Arctic Circle or the Antarctic Circle, due to refraction.

<span class="mw-page-title-main">Twilight</span> Atmospheric illumination by the Sun below the horizon

Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this illumination occurs.

A synodic day is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.

In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.

<span class="mw-page-title-main">Unit of time</span> Measurement unit for time

A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1."

<span class="mw-page-title-main">Earth's rotation</span> Rotation of Earth around its axis

Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Polaris, Earth turns counterclockwise.

<span class="mw-page-title-main">Daytime</span> Period of a day in which a location experiences natural illumination

Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime, but this article primarily discusses daytime on Earth.

A tropical year or solar year is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.

<span class="mw-page-title-main">Coordinated Universal Time</span> Primary time standard

Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communication, navigation, scientific research, and commerce.

<span class="mw-page-title-main">Lunar month</span> Time between successive new moons

In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.


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