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Daytime as observed on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
is the period of the
day The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...

day
during which a given location experiences
natural illumination
natural illumination
from direct
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
. Daytime occurs when the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
appears above the local
horizon The horizon is the apparent line that separates the surface of a celestial body In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that st ...

horizon
, that is, anywhere on the globe's
hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western Hemisphere ** Land and water hemispheres * A half of the ...
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
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s and
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

natural satellite
s that rotate relative to a luminous
primary body A primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravity, gravitationally bound, multi-object system. This object constitutes most of that system's mass and will generally be located near ...
, such as a local
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
, also experience daytime, but this article primarily discusses daytime on Earth.


Characteristics

Approximately half of the Earth is illuminated at any time by the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
. The area subjected to direct illumination is almost exactly half the planet; but because of atmospheric and other effects that extend the reach of indirect illumination, the area of the planet covered by either direct or indirect illumination amounts to slightly more than half the surface. The
hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western Hemisphere ** Land and water hemispheres * A half of the ...

hemisphere
of the Earth experiencing daytime at any given instant changes continuously as the planet rotates on its own axis. The axis of the Earth's rotation is not perpendicular to the plane of its
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

orbit
around the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
(which is
parallel Parallel may refer to: Computing * Parallel algorithm In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their a ...
with the direction of sunlight), and so the length of the daytime period varies from one point on the planet to another. Additionally, since the axis of rotation is relatively fixed in comparison to the stars, it moves with respect to the Sun as the planet orbits the star. This creates seasonal variations in the length of the daytime period at most points on the planet's surface. The period of daytime from the standpoint of a surface observer is roughly defined as the period between
sunrise 250px, Skyline of Kaohsiung harbour, Taiwan at sunrise. Sunrise (or sunup) 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 a ...

sunrise
, when the Earth's rotation towards the east first causes the Sun's disc to appear above the horizon, to
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

sunset
, when the continuing rotation of the Earth causes the Sun's disc to disappear below the horizon to the west. Because the Sun is a luminous disc as seen from the Earth, rather than a point source of light, sunrise and sunset are not instantaneous and the exact definition of both can vary with context. Additionally, the Earth's atmosphere further bends and diffuses light from the Sun and lengthens the period of sunrise and sunset. For a certain period after sunset and before sunrise, indirect light from the Sun lightens the sky on Earth; this period is often referred to as ''
twilight Twilight is the of the lower when the Sun is not directly visible because it is below the . Twilight is produced by in the upper atmosphere, illuminating the lower atmosphere so that is neither completely lit nor completely dark. The w ...

twilight
.'' Certain groups, such as Earthly astronomers, do not consider daytime to be truly ended until the Sun's disc is actually well below the Earth's horizon, because of this indirect illumination.


Daytime length variations with latitude and seasons

Given that Earth's own axis of rotation is tilted 23.44° to the line perpendicular to its
orbital plane The orbital plane of a revolving body is the geometric plane in which its orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satel ...
, called the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

ecliptic
, the length of daytime varies with the
season A season is a division of the year based on changes in weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most ...

season
s on the planet's surface, depending on the observer's
latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

latitude
. Areas tilted toward the Sun are experiencing
summer Summer is the hottest of the four temperate In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenom ...

summer
. Their tilt toward the Sun leads to more than half of the day seeing
daylight Daylight is the combination of all direct and indirect sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is ...

daylight
and warmer temperatures, due to the higher directness of solar rays, the longer period of daytime itself, and less
absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
of
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
in the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...
. While increased daylight can have some effect on the higher temperatures in the summer, most of temperature rise results from the directness of the Sun, not the increased daylight. The high
angles The Angles ( ang, Ængle, ; la, Angli; german: Angeln) were one of the main Germanic peoples The Germanic peoples were a historical group of people living in Central Europe and Scandinavia. Since the 19th century, they have traditional ...
(around the
zenith The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "high ...

zenith
) of the Sun causes the
tropics The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% ...

tropics
to be warm, while low angles (barely above the horizon) causes the
polar regions The polar regions, also called the frigid zones, of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent ...
to be cold. The slight effect of daylight hours on average seasonal temperature can be seen with the poles and tropical regions. The poles are still cold during their respective summers, despite seeing 24 hours of daylight for six months, while the
Equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

Equator
remains warm throughout the year, with only 12 hours of daylight per day. Although the daytime length at the Equator remains 12 hours in all seasons, the duration at all other latitudes varies with the seasons. During the winter, daytime lasts shorter than 12 hours; during the summer, it lasts longer than 12 hours. Northern winter and southern summer concur, while northern summer and southern winter concur.


At the Equator

At the
Equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

Equator
, the daytime period always lasts about 12 hours, regardless of season. As viewed from the Equator, the Sun always rises and sets vertically, following an apparent path nearly
perpendicular In elementary geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relativ ...

perpendicular
to the
horizon The horizon is the apparent line that separates the surface of a celestial body In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that st ...

horizon
. Due to the
axial tilt In , axial tilt, also known as obliquity, is the angle between an object's and its al axis, or, equivalently, the angle between its ial plane and . It differs from . At an obliquity of 0 degrees, the two axes point in the same direction; i.e., ...
of Earth, Sun always lies within 23.44° north or south of the
celestial equator The celestial equator is the great circle A great circle, also known as an orthodrome, of a sphere of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a Geometry, geometrical object in solid geometry, three-dimensiona ...
, so the
subsolar point The subsolar point on a planet is the point at which its sun is perceived to be directly overhead (at the zenith); that is, where the sun's rays strike the planet exactly perpendicular In elementary geometry, the property of being perpendicu ...

subsolar point
always lies within the
tropics The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% ...

tropics
. From the
March equinox The March equinox or northward equinox is the equinox An equinox is the instant of time when the plane of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's s ...
to the
September equinox The September equinox (or southward equinox) is the moment when the Sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the September equinox may occur anytime from ...
, the Sun rises within
23.44° north
23.44° north
of due east, and sets within 23.44° north of due west. From the September
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

equinox
to the March equinox, the Sun rises within 23.44° south of due east and sets within 23.44° south of due west. The Sun's path lies entirely in the northern half of the
celestial sphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...

celestial sphere
from the March equinox to the September equinox, but lies entirely in the southern half of the celestial sphere from the September equinox to the March equinox. On the equinoxes, the equatorial Sun culminates at the
zenith The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "high ...

zenith
, passing directly overhead at
solar noon Noon (or midday) is 12 12-hour clock, o'clock in the daytime. It is written as 12 noon, 12:00 m. (for 12-hour clock, meridiem, literally 12:00 noon), 12 p.m. (for 12-hour clock, post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 2 ...
. The fact that the equatorial Sun is always so close to the zenith at solar noon explains why the
tropical zone The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several larg ...

tropical zone
contains the warmest regions on the planet overall. Additionally, the Equator sees the shortest
sunrise 250px, Skyline of Kaohsiung harbour, Taiwan at sunrise. Sunrise (or sunup) 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 a ...

sunrise
or
sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon The horizon is the apparent line that separates earth Earth is the third planet from the Sun and the only astronomical object known to harbor ...

sunset
because the Sun's path across the sky is so nearly perpendicular to the horizon. On the equinoxes, the solar disk takes only two minutes to traverse the horizon (from top to bottom at sunrise and from bottom to top at sunset).


In the tropics

The tropics occupy a zone of Earth's surface between 23.44° north and 23.44° south of the
Equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

Equator
. Within this zone, the Sun will pass almost
directly overhead
directly overhead
(or culminate) on at least one day per year. The line of 23.44° north latitude is called the
Tropic of Cancer The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted ...

Tropic of Cancer
, because when it was named, the Sun passed overhead at this location at the time of year when it was near the constellation of Cancer. The equivalent line of south latitude is called the
Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the that contains the at the December (or southern) . It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reaches 90 degrees below the horizon at solar ...
, for similar reasons. The sun enters and leaves each zodiacal constellation slightly later each year at the rate of about 1 day every 72 years. For more information, see
precession of the equinoxes In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's Rotation around a fixed axis, rotational axis. In particular, it can refer to the gradual shift in the orientation of E ...
. On the Tropical Circles, the Sun is directly overhead only once per year, on the corresponding solstice. At latitudes closer to the Equator and on the Equator itself, it will be overhead twice per year (on the equinoxes in the case of the Equator). Outside the tropics, the Sun never passes directly overhead.


Around the poles

Around the poles, which coincide with the rotational axis of Earth as it passes through the surface, the seasonal variations in the length of daytime are extreme. In fact, within 23.44° latitude of the poles, there will be at least some days each year during which the sun never goes below the horizon. There will also be days when the Sun never rises above the horizon. This number will be fewer, but close to the number of days in the summer where the sun doesn't set (for example the sunrise is usually a few days before the spring equinox and extends a few days past the fall equinox). This phenomenon of more daylight than night is not unique to the poles. In fact, at any given time slightly more than half of the earth is in daylight. The 24 hours of summer daylight is known as the
midnight sun upright=1.25, Midnight sun at the North Cape on the island of Magerøya in Norway The midnight sun is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germin ...

midnight sun
that is famous in some northern countries. To the north, the
Arctic Circle The Arctic Circle is one of the two s and the most northerly of the five major as shown on maps of . It marks the northernmost point at which the center of the sun is just visible on the and the southernmost point at which the center of the ...

Arctic Circle
marks this 23.44° boundary. To the south, the
Antarctic Circle 350px, ''Map of the Antarctic with the Antarctic Circle in blue.'' The Antarctic Circle is the most southerly of the five major circle of latitude, circles of latitude that mark maps of the Earth. The region south of this circle is known as the An ...

Antarctic Circle
marks the boundary. These boundaries correspond to 66.56° north or south latitude, respectively. Because the sky is still bright and stars can't be seen when the sun is less than 6 degrees under the horizon, 24-hour nights with stars visible all the time only happen beyond 73° north or south latitude. At and near the poles, the Sun never rises very high above the horizon, even in summer, which is one of reasons why these regions of the world are consistently cold in all seasons (others include the effect of
albedo Albedo (prounounced ; la, albedo, meaning 'whiteness') is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiat ...

albedo
, the relative increased reflection of solar radiation of snow and ice). Even at the summer solstice, when the Sun reaches its highest point above the horizon at noon, it is still only 23.44° above the horizon at the poles. Additionally, as one approaches the poles the apparent path of the Sun through the sky each day diverges increasingly from the vertical. As summer approaches, the Sun rises and sets become more northerly in the north and more southerly in the south. At the poles, the path of the Sun is indeed a circle, which is roughly equidistant above the horizon for the entire duration of the daytime period on any given day. The circle gradually sinks below the horizon as winter approaches, and gradually rises above it as summer approaches. At the poles, apparent sunrise and sunset may last for several days.


At middle latitudes

At
middle latitudes The middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) are a spatial region on Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29 ...
, far from both the Equator and the poles, variations in the length of daytime are moderate. In the higher middle latitudes where
Montreal Montreal ( ; officially Montréal, ) is the second-most populous city in Canada Canada is a country in the northern part of . Its extend from the to the and northward into the , covering , making it the world's . Its southern and w ...

Montreal
,
Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estimated population of 2,175,601 residents , in an area of more than . Since the 17th century, Paris ha ...

Paris
and
Ushuaia Ushuaia ( , ) is the capital city, capital of Tierra del Fuego Province, Argentina, Tierra del Fuego, Antártida e Islas del Atlántico Sur Province, Argentina, and the southernmost city of the country. Ushuaia claims the title of the southernmo ...

Ushuaia
are located, the difference in the length of the day from summer to winter can be very noticeable: the sky may still be lit at 10 pm in summer, but may be dark at 5 pm in winter. In the lower middle latitudes where southern
California California is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper i ...

California
,
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identi ...

Egypt
and
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is the world's 23rd-most populous nation and covers an area of . South Africa has three capital citie ...

South Africa
are located, the seasonal difference is smaller, but still results in approximately 4 hours difference in daylight between the winter and summer solstices. The difference becomes less pronounced the closer one gets to the equator. An approximation to the monthly change can be obtained from the
rule of twelfths The rule of twelfths is an approximation to a sine curve. It can be used as a rule of thumb The English phrase ''rule of thumb'' refers to a principle with broad application that is not intended to be strictly accurate or reliable for every situ ...

rule of twelfths
.


Variations in solar noon

The exact instant of
solar noon Noon (or midday) is 12 12-hour clock, o'clock in the daytime. It is written as 12 noon, 12:00 m. (for 12-hour clock, meridiem, literally 12:00 noon), 12 p.m. (for 12-hour clock, post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 2 ...
, when the Sun reaches its
highest point A list of highest points typically contains the name, elevation, and location of the highest point in each of a set of geographical regions. Such a list is important in the sport of highpointing Highpointing is the sport of ascending to the point ...
in the sky, varies with the seasons. This variation is called the
equation of time The equation of time describes the discrepancy between two kinds of solar time Solar time is a calculation of the passage of time Time is the continued of and that occurs in an apparently succession from the , through the , int ...

equation of time
; the magnitude of variation is about 30 minutes over the course of a year.


See also

*
Daylight saving time#REDIRECT Daylight saving time Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by ...

Daylight saving time
*
Solar eclipse A solar eclipse occurs when a portion of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The re ...

Solar eclipse
* Sunrise equation


References

{{Authority control Parts of a day Time in astronomy