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A day is approximately the period of time during which the Earth completes one rotation around its axis. [1] A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. [2]

Earths rotation rotation of the solid Earth around its own axis

Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion. As viewed from the north pole star Polaris, Earth turns counter clockwise.

Solar time

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. Two types of solar time are apparent solar time and mean solar time.


In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. [3] A civil day is usually 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time. [4] [5]

Second SI unit of time

The second is the base unit of time in the International System of Units (SI), commonly understood and 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. Mechanical and electric clocks and watches usually have a face with 60 tickmarks representing seconds and minutes, traversed by a second hand and minute hand. Digital clocks and watches often have a two-digit counter that cycles through seconds. The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.

SI base unit one of the seven units of measurement that define the Metric System

The International System of Units defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the metre for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.

Time dimension in which events can be ordered from the past through the present into the future

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Time is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. Time is often referred to as a fourth dimension, along with three spatial dimensions.

Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. [6] However its use depends on its context, for example when people say 'day and night', 'day' will have a different meaning. It will mean the interval of light between two successive nights; the time between sunrise and sunset, [7] in this instance 'day' will mean time of light between one night and the next. [8] However, in order to be clear when using 'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period; [9] this is since daytime usually always means 'the time of the day between sunrise and sunset. [10] The word day may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns (circadian rhythms) of humans and many other species are related to Earth's solar day and the day-night cycle.

Midnight 12 oclock at night; transition time period from one day to the next

Midnight is the transition time from one day to the next – the moment when the date changes. In ancient Roman timekeeping, midnight was halfway between sunset and sunrise, varying according to the seasons. By clock time, midnight is the opposite of noon, differing from it by 12 hours.

Night part of the day when the Sun is not aloft

Night or nighttime is the period from sunset to sunrise in each twenty-four hours, when the Sun is below the horizon. However it can be defined differently and is subjective. Night can be defined as the time between bedtime and morning. There is no exact time for when night begins and ends. The start of night begins when evening ends, which is subjective, but is typically believed to end at astronomical sunset, which is when night may begin. There can be no precise definition in terms of clock time, but it is usually considered to start around 9 pm and to last to about 5 am. Since sunset and sunrise vary throughout the year there can be no precise definition in terms of clock time. Night and morning overlap when one considers morning to start past 12 am, which can be described as 'morning-night duality'.

Sunrise instant at which the upper edge of the Sun appears over the eastern horizon in the morning

Sunrise is the moment when the upper limb 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 and its accompanying atmospheric effects.

Daytime image of the bay of Naples Daytime image of the bay of Naples.jpg
Daytime image of the bay of Naples


Dagr, the Norse god of the day, rides his horse in this 19th-century painting by Peter Nicolai Arbo. Dagr by Arbo.jpg
Dagr, the Norse god of the day, rides his horse in this 19th-century painting by Peter Nicolai Arbo.

Apparent and mean solar day

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, defined as the time it takes for the Sun to return to its culmination point (its highest point in the sky). Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. Because the Earth orbits the Sun elliptically as 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 [11] (24.000 000 6 hours) and there are about 365.2422 solar days in one mean tropical year.

A tropical year is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. Because of the precession of the equinoxes, the seasonal cycle does not remain exactly synchronized with the position of the Earth in its orbit around the Sun. As a consequence, the tropical year is about 20 minutes shorter than the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, oldstyle). [12] The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude as well as latitude), and the time of year (as indicated by ancient hemispherical sundials).

Longitude A geographic coordinate that specifies the east-west position of a point on the Earths surface

Longitude, is a geographic coordinate that specifies the east–west position of a point on the Earth's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane through the Prime Meridian and a plane through both poles and the location in question.

Year Orbital period of the Earth around the Sun

A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility. The current year is 2019.

Sundial device that tells the time of day by the apparent position of the Sun in the sky

A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate and a gnomon, which casts a shadow onto the dial. As the Sun appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.

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.

Meridian (geography) line between the poles with the same longitude

A (geographic) meridian is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. Each meridian is perpendicular to all circles of latitude. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km.

Noon 12 oclock in the daytime

Noon is 12 o'clock in the daytime, as opposed to midnight. The term 12 p.m. is sometimes used for noon.

In observational astronomy, culmination is the instant of time of the transit of a celestial object across the observer's local meridian. During each day, every celestial object appears to move along a circular path on the celestial sphere due to the Earth's rotation creating two moments when it crosses the meridian. Except at the geographic poles, any celestial object passing through the meridian has an upper culmination, when it reaches its highest point above the horizon, and nearly twelve hours later, is followed by a lower culmination, when it reaches its lowest point. The time of culmination is often used to mean upper culmination.

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).

Stellar day

A day, understood as the span of time it takes for the Earth to make one entire rotation [13] with respect to the celestial background or a distant star (assumed to be fixed), is called a stellar day . This period of rotation is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.1 seconds) and there are about 366.2422 stellar days in one mean tropical year (one stellar day more than the number of solar days). Other planets and moons have stellar and solar days of different lengths from Earth's.


A day, in the sense of daytime that is distinguished from night time, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. 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. [14] Thus, daytime is on average around 7 minutes longer than 12 hours. [15]


The term comes from the Old English dæg, with its cognates such as dagur in Icelandic, Tag in German, and dag in Norwegian, Danish, Swedish and Dutch. All of them from the Indo-European root dyau which explains the similarity with Latin dies though the word is known to come from the Germanic branch. As of October 17,2015, day is the 205th most common word in US English, [16] and the 210th most common in UK English. [16]

International System of Units (SI)

A day, symbol d, defined as 86 400 seconds, is not an SI unit, but is accepted for use with SI. [17] The Second is the base unit of time in SI units.

In 1967–68, during the 13th CGPM (Resolution 1), [18] 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. [19]

This makes the SI-based day last exactly 794 243 384 928 000 of those periods.

Leap seconds

Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in minor variations for both solar days and stellar "days". The Earth's day has increased in length over time due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86 400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2 700 years). The length of a day circa 620 million years ago has been estimated from rhythmites (alternating layers in sandstone) as having been about 21.9 hours.

In order to keep the civil day aligned with the apparent movement of the Sun, a day according to Coordinated Universal Time (UTC) can include a negative or positive leap second. Therefore, 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.

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.

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 referred to as a calendar day.

A day is commonly divided into 24 hours of 60 minutes, with each minute composed of 60 seconds.

Decimal and metric time

In the 19th century, an idea circulated to make a decimal fraction (110 000 or 1100 000) of an astronomical day the base unit of time. This was an afterglow of the short-lived movement toward a decimalisation of timekeeping and the calendar, which had been given up already due to its difficulty in transitioning from traditional, more familiar units. The most successful alternative is the centiday , equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple kilosecond (1 000 seconds) and equal to the traditional Chinese unit, .


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

Full day


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. Common convention among the ancient Romans, [21] ancient Chinese [22] and in modern times is for the civil day to begin at midnight, i.e. 00:00, and 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. 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, and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening. Prior to 1926, Turkey had two time systems: Turkish (counting the hours from sunset) and French (counting the hours from midnight).

Validity of tickets, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g., public transport) operates from for example, 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day. For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) there is a risk of ambiguity. For example, a day ticket on the Nederlandse Spoorwegen (Dutch Railways) is valid for 28 hours, from 0:00 to 28:00 (that is, 4:00 the next day); the validity of a pass on Transport for London (TfL) services is until the end of the "transport day" – that is to say, until 4:30 am on the day after the "expiry" date stamped on the pass.

Midnight sun

In places which experience the midnight sun (polar day), daytime may extend beyond one 24 hour period and could even extend to months

Extraterrestrial bodies

Besides a stellar day on Earth, determined to be 23 hours 56 minutes and 4.1 seconds, there are related such days for bodies in the Solar System other than the Earth. [23] For example:

See also

Related Research Articles

Astronomical unit mean distance between Earth and the Sun, common length reference in astronomy

The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149597870700 metres or about 150 million kilometres. The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

Equinox astronomical event where the Sun is directly above the Earths equator

An equinox is commonly regarded as the instant of time when the plane of Earth's equator passes through the center of the Sun. This occurs twice each year: around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator.

Hour unit of time

An hour is a unit of time conventionally reckoned as ​124 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

Leap second extra second inserted to keep civil time in sync with the Earths rotation

A leap second is a one-second adjustment that is occasionally applied to civil time Coordinated Universal Time (UTC) to keep it close to the mean solar time at Greenwich, in spite of the Earth's rotation slowdown and irregularities. UTC was introduced on January 1, 1972, initially with a 10 second lag behind International Atomic Time (TAI). Since that date, 27 leap seconds have been inserted, the most recent on December 31, 2016 at 23:59:60 UTC, so in 2018, UTC lags behind TAI by an offset of 37 seconds.

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.

Sidereal time time standard

Sidereal time is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".

Twilight illumination of the Earths lower atmosphere when the Sun itself is not directly visible because it is below the horizon

Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.

Rotation period (of a celestial object) time that it takes to complete one revolution around its axis of rotation relative to the background stars

In astronomy, the rotation period of a celestial object is the time that it takes to complete one revolution around its axis of rotation relative to the background stars. It differs from the planet's solar day, which includes an extra fractional rotation needed to accommodate the portion of the planet's orbital period during one day.

Decimal time representation of the time of day using units which are decimally related

Decimal time is the representation of the time of day using units which are decimally related. This term is often used specifically to refer to the time system used in France for a few years beginning in 1792 during the French Revolution, which divided the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal seconds, as opposed to the more familiar UTC time standard, which divides the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds.

Unit of time measurement unit for time

A unit of time or midst unit 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 definition, from the National Institute of Standards and Technology is:

Daytime period on any given point of the planets surface during which it experiences natural illumination from sunlight

On Earth, daytime is roughly the period of the day during which any given point in the world experiences natural illumination from especially direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. During daytime, an observer sees indirect sunlight while in the shade, which includes cloud cover. 'Day' is sometimes used instead of 'daytime', in this case 'day' will mean 'the period of light between dawn and nightfall; the interval from sunrise to sunset', which is synonymous with daytime. However, in this context, in order to be clear "daytime" should be used distinguish it from "day" which typically refers to a 24-hour period.

December solstice astronomical phenomenon; solstice that occurs each December, typically between the 20th and the 22nd day of the month according to the Gregorian calendar

The December solstice, is the solstice that occurs each December – typically on Dec 21, and can vary ± 1 day according to the Gregorian calendar. In the Northern Hemisphere, the December solstice is the winter solstice, whilst in the Southern Hemisphere it is the summer solstice. It is also known as the southern solstice.

Coordinated Universal Time Primary time standard by which the world regulates clocks and time

Coordinated Universal Time is a time standard by which the world regulates clocks and time. It tracks Greenwich Mean Time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries mean solar time is official, but time signals for general use are based on UTC. Greenwich Mean Time (GMT) is not a synonym for UTC and predates UTC by nearly 300 years.


  1. Weisstein, Eric W. (2007). "Day" . Retrieved 2011-05-31.
  2. Weisstein, Eric W. (2007). "Solar Day" . Retrieved 2011-05-31.
  3. BIPM (2014) [2006]. "Unit of time (second)". SI Brochure (8th ed.).
  4. Weisstein, Eric W. (2007). "Solar Day" . Retrieved 2011-05-31.
  5. Weisstein, Eric W. (2007). "Day" . Retrieved 2011-05-31.
  6. "day – Definition of day in English by Oxford Dictionaries". Oxford Dictionaries – English.
  7. "day" via The Free Dictionary.
  8. "Definition of DAY".
  9. Online Dictionary Definitions of "day".
  10. Online Dictionary Definitions of "daytime"
  11. The average over the last 50 years is about 86 400.002. The yearly average over that period has ranged between about 86 400 and 86 400.003, while the length of individual days has varied between about 86 399.999 and 86 400.004 seconds. See this graph: Deviation of day length from SI day.svg (data from "Earth Orientation Parameters". International Earth Rotation and Reference Systems Service. Archived from the original on April 26, 2015.).
  12. L. Holford-Stevens, The History of Time (Oxford 2005) p. 6
  13. Certain authors caution against identifying "day" with rotation period. For example: Courtney Seligman. "Rotation Period and Day Length" . Retrieved 2011-06-03. A Cautionary Note: Because the rotation period of the Earth is almost the same as the length of its day, we sometimes get a bit sloppy in discussing the rotation of the sky, and say that the stars rotate around us once each day. In a similar way, it is not unusual for careless people to mix up the rotation period of a planet with the length of its day, or vice versa.
  14. 32′2 + 34′ = 50′
  15. 50°/60 ÷ 360° × 2(for sunrise and set) × 24 hours ≈ 7 min
  16. 1 2 "English Words". Oxford Dictionaries Online (ODO). Oxford University Press. Retrieved 2015-10-17.
  17. BIPM (2014) [2006]. "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI Brochure (8th ed.).
  18. "SI Unit of Time (Second)". Resolution 1 of the 13th CGPM (1967/68). Bureau International des Poids et Mesures (BIPM). Retrieved 2015-10-17.
  19. "Unit of Time (Second)". SI Brochure: The International System of Units (SI) (8 ed.). Bureau International des Poids et Mesures (BIPM). 2014 [2006]. Retrieved 2015-10-17.
  20. "Definition of NYCHTHEMERON". Retrieved 2017-02-01.
  21. See Plutarch, Quaestiones Romanae, 84.
  22. s:zh:清史稿/卷48: 起子正,盡夜子初
  23. Griggs, Mary Beth (18 January 2019). "Shaky rings help scientists measure Saturn's days – Speedy planet". The Verge . Retrieved 18 January 2019.
  24. McCartney, Gretchen; Wendel, JoAnna (17 January 2019). "Scientists Finally Know What Time It Is on Saturn". NASA. Retrieved 18 January 2019.
  25. Mankovich, Christopher; et al. (17 January 2019). "Cassini Ring Seismology as a Probe of Saturn's Interior. I. Rigid Rotation". The Astrophysical Journal . 871 (1): 1. arXiv: 1805.10286 . Bibcode:2019ApJ...871....1M. doi:10.3847/1538-4357/aaf798.