A day is the
time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...
period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
of a full
rotation of the Earth
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 Pola ...
with respect to the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. On average, this is 24
hours, 1440
minutes, or 86,400
seconds. In everyday life, the word "day" often refers to a
solar day
A synodic day (or synodic rotation period or solar 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.
The synodic day is distinguished from the sidereal day, which is ...
, which is the length between two
solar noon
Noon (or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time).
Sola ...
s or times the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
reaches the highest point. The word "day" may also refer to ''
daytime'', a time period when the location receives
direct and indirect sunlight. On Earth, as a location passes through its day, it experiences
morning
Morning is the period from sunrise to noon. There are no exact times for when morning begins (also true of evening and night) because it can vary according to one's lifestyle and the hours of daylight at each time of year. However, morning s ...
,
noon
Noon (or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time).
Sola ...
,
afternoon
Afternoon is the time after solar noon. It is the time when the sun is descending from its peak in the sky to somewhat before its terminus at the horizon in the west. In human life, it occupies roughly the latter half of the standard work a ...
,
evening
Evening is the period of a day that starts at the end of the afternoon and overlaps with the beginning of night. The exact times when evening begins and ends depend on location, time of year, and culture, but it is generally regarded as beginn ...
, and
night
Night (also described as night time, unconventionally spelled as "nite") is the period of ambient darkness from sunset to sunrise during each 24-hour day, when the Sun is below the horizon. The exact time when night begins and ends depends ...
. The effect of a day is vital to many
life
Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...
processes, which is called the
circadian rhythm.
A collection of sequential days is organized into
calendars as
dates
Date or dates may refer to:
*Date (fruit), the fruit of the date palm (''Phoenix dactylifera'')
Social activity
*Dating, a form of courtship involving social activity, with the aim of assessing a potential partner
**Group dating
* Play date, a ...
, almost always into
week
A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are of ...
s,
months and
year
A year or annus is the orbital period of a planetary body, for example, 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 hou ...
s. Most calendars' arrangement of dates use either or both the Sun with its
four seasons (
solar calendar) or the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
's
phasing
A phaser is an electronic sound processor used to filter a signal, and it has a series of troughs in its frequency-attenutation graph. The position (in Hz) of the peaks and troughs are typically modulated by an internal low-frequency oscil ...
(
lunar calendar
A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. The most commonly used calendar, t ...
). The start of a day is commonly accepted as roughly the time of the middle of the night or
midnight, written as 00:00 or
12:00 am in
24- or
12-hour clock
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin , translating to "before midday") and p.m. (from Latin , translating to "after midday"). For different opinions on represen ...
s, respectively. Because the time of midnight varies between locations,
time zone
A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it ...
s are set up to facilitate the use of a uniform
standard time. Midnight is not the only convention used to determine the start of a new day. Other defining moments have been used throughout history, and some are used even today, such as with the
Jewish religious calendar, which counts days from sunset to sunset, so the
Jewish Sabbath
Shabbat (, , or ; he, שַׁבָּת, Šabbāṯ, , ) or the Sabbath (), also called Shabbos (, ) by Ashkenazim, is Judaism's day of rest on the seventh day of the week—i.e., Saturday. On this day, religious Jews remember the biblical storie ...
begins at sundown on
Friday
Friday is the day of the week between Thursday and Saturday. In countries that adopt the traditional "Sunday-first" convention, it is the sixth day of the week. In countries adopting the ISO-defined "Monday-first" convention, it is the fifth d ...
.
Astronomers
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either obse ...
also have a convention where their day begins at high noon. This way, all of their observations throughout a single night are recorded as happening on the same day. This method removes ambiguity of a particular observation happening on a calendar day, eliminating the need to further determine which night it happened on. Because when using midnight as the start of day, each calendar day is associated with two separate night periods.
In specific applications, the definition of a day is slightly modified, such as in the
ISQ 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
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 Pola ...
and
sidereal day
Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) 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 coor ...
(using the
celestial sphere) used for
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
. In most countries outside of the
tropics
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referr ...
,
daylight saving time
Daylight saving time (DST), also referred to as daylight savings time or simply daylight time (United States, Canada, and Australia), and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typicall ...
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
A leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observ ...
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).
Etymology
The term comes from the
Old English
Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlers in the mid-5th c ...
term ''dæġ'' (), with its cognates such as ''dagur'' in
Icelandic, ''Tag'' in
German
German(s) may refer to:
* Germany (of or related to)
** Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ge ...
, and ''dag'' in
Norwegian
Norwegian, Norwayan, or Norsk may refer to:
*Something of, from, or related to Norway, a country in northwestern Europe
* Norwegians, both a nation and an ethnic group native to Norway
* Demographics of Norway
*The Norwegian language, including ...
,
Danish
Danish may refer to:
* Something of, from, or related to the country of Denmark
People
* A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark
* Culture of Denmark
* Danish people or Danes, people with a Danish a ...
,
Swedish
Swedish or ' may refer to:
Anything from or related to Sweden, a country in Northern Europe. Or, specifically:
* Swedish language, a North Germanic language spoken primarily in Sweden and Finland
** Swedish alphabet, the official alphabet used by ...
and
Dutch
Dutch commonly refers to:
* Something of, from, or related to the Netherlands
* Dutch people ()
* Dutch language ()
Dutch may also refer to:
Places
* Dutch, West Virginia, a community in the United States
* Pennsylvania Dutch Country
People E ...
– all stemming from a
Proto-Germanic
Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed proto-language of the Germanic branch of the Indo-European languages.
Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic bran ...
root ''*dagaz''.
[Mallory, James P.; Adams, Douglas Q. (2006). ''The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World''. Oxford, UK: Oxford University Press. p. 124. .] , ''day'' is the 205th most common word in
US English
American English, sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. English is the most widely spoken language in the United States and in most circumstances ...
,
and the 210th most common in
UK English
British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, "English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in England, or, more broadly, ...
.
Definitions
Apparent and mean solar day
![EpicEarth-Globespin-tilt-23](https://upload.wikimedia.org/wikipedia/commons/a/a0/EpicEarth-Globespin-tilt-23.4.gif)
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 moves along an
eccentric orbit
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betw ...
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 (24.000 000 6 hours) and there are currently about 365.2421875 solar days in one mean
tropical year
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time f ...
.
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). The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (
longitude
Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...
and latitude, as well as altitude), and the time of
year
A year or annus is the orbital period of a planetary body, for example, 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 hou ...
(as indicated by ancient hemispherical
sundials).
A more constant day can be defined by the Sun passing through the local
meridian
Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to
Science
* Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
* ...
, which happens at local
noon
Noon (or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time).
Sola ...
(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 celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract proj ...
; 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
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
or other parts of the
universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...
, a day is a full rotation of other large
astronomical objects with respect to its star.
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
Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a prep ...
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