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Daytime (astronomy)
Encyclopedia
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or (especially) direct sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
.
Other planets that rotate in relation to a luminous primary, such as a local star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
, also experience daytime of a sort, but this article primarily discusses daytime on Earth.
Characteristics
Approximately half of the Earth is illuminated at any given time by sunlightSunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
. 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
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
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 exactly perpendicular to the plane of its orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
(which is parallel 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
Sunrise
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...
, when the Earth's rotation towards the east first causes the Sun's disc to appear above the horizon, to sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...
, 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 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
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...
. 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 variations with latitude and seasons
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
s on the planet's surface, depending on the observer's latitude. Areas experiencing summer are tilted toward the sun. Their tilt toward the sun leads to over half of the day being in daylight and warmer temperatures due to the increased directness of the sun's rays. While increased daylight can have some effect on the increased temperature in the summer, most of the increase in temperature is due to the directness of the sun, not the increased daylight. The high (near 90 degrees) angles
Effect of sun angle on climate
The amount of heat energy received at any location on the globe is a direct effect of sun angle on climate, as the angle at which sunlight strikes the Earth varies by location, time of day, and season due to the Earth's orbit around the sun and the Earth's rotation around its tilted axis...
of the sun is what causes the tropics to be warm while low (barely above the horizon) angles at the poles is what causes them to be cold. Hours of daylight having little effect on temperature can be seen with the poles still being cold in their respective summers despite 24 hours of daylight, while the equator is warm with only 12 hours of daylight.
Although the length of the daytime period is always twelve hours at the Equator, in all seasons, at all other latitudes the length varies with the season. During the winter, the daytime period is shorter than twelve hours; during the summer, it is longer than 12 hours. When it is winter north of the Equator, it is summer south of the Equator, and vice versa.
At the Equator
At the Equator, the daytime period is always almost twelve hours in length, no matter what the season. The sun always rises nearly perpendicular to the horizon. From the March Equinox to the September Equinox, it rises a bit north of east, and sets a bit north of west. From the September Equinox to the March Equinox, it rises a bit south of east and sets a bit south of west. The path of the Sun lies entirely in the northern half of the sky for the period from the March Equinox to the September Equinox and is entirely in the southern half of the sky from the September Equinox to the March Equinox, with the Sun passing directly overhead at noon on the equinoxEquinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
es.
The fact that the Sun is always so close to the vertical at noon on the Equator (never being more than 23.5 degrees north or south) explains why equatorial regions are the hottest regions on the planet overall. Additionally, sunrise and sunset always occur very quickly at the Equator, because the Sun's path across the sky is so nearly vertical with respect to the horizon; at the equinox, the Sun requires only two minutes to traverse the horizon at sunrise and sunset.
In the tropics
The tropics occupy a band of the Earth's surface between 23.5° north latitude and 23.5° south latitude. Within this band, the Sun will pass almost directly overhead on at least one day per year. The line of 23.5° north latitude is called the Tropic of CancerTropic of Cancer
The Tropic of Cancer, also referred to as the Northern tropic, is the circle of latitude on the Earth that marks the most northerly position at which the Sun may appear directly overhead at its zenith...
, because when it was named, the Sun passed overhead at this location at the time of year when it was within the zodiac sign of Cancer. The equivalent line of south latitude is called the Tropic of Capricorn
Tropic of Capricorn
The Tropic of Capricorn, or Southern tropic, marks the most southerly latitude on the Earth at which the Sun can be directly overhead. This event occurs at the December solstice, when the southern hemisphere is tilted towards the Sun to its maximum extent.Tropic of Capricorn is one of the five...
, for similar reasons. If they had been named today using the constellation in which the sun is currently in at the time it is directly overhead the tropic line, they would have been called the tropic of leo
LEO
LEO as an initialism may refer to:* Low Earth orbit, a satellite path* Law enforcement officer, an official* Louisville Eccentric Observer, a newspaper* LEO , an electronic device* LEO , a lunar mission...
and aquarius
Aquarius
Aquarius may refer to:Astrology* Aquarius , an astrological sign* The Age of Aquarius, a time period in the cycle of astrological ages.Astronomy* Aquarius , a defined area of the sky containing a group of stars....
respectively. The sun enters and leaves each sign of the zodiac slightly later each year at the rate of about 1 day every 72 years. For more information, see precession of the equinoxes
Precession of the equinoxes
In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined...
.
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.
Near the poles
Near the poles, which coincide with the rotational axis of the Earth as it passes through the surface, the seasonal variations in the length of daytime are extreme. In fact, within 23.5° latitude of the poles, there will be at least some days each year during which the sun never goes below the horizon. In that there will 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, even without accounting for twilight). 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 sunMidnight sun
The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...
that is famous in some northern and southern countries. To the north, the Arctic Circle
Arctic Circle
The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. For Epoch 2011, it is the parallel of latitude that runs north of the Equator....
marks this 23.5° boundary. To the south, the Antarctic Circle
Antarctic Circle
The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. For 2011, it is the parallel of latitude that runs south of the Equator.-Description:...
marks the boundary. These boundaries correspond to 66.5° north or south latitude, respectively. Because the Sun's disc itself is about half a degree in diameter and is very bright, truly dark days during which the sun never seems to rise are only seen beyond 72° north or south latitude.
At and near the poles, the sun never rises very far 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
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
, 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 23.5° 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 northernly in the north and more southernly 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.
Middle latitudes
In the middle latitudesMiddle latitudes
The middle latitudes are between 23°26'22" North and 66°33'39" North, and between 23°26'22" South and 66°33'39" South latitude, or, the Earth's temperate zones between the tropics and the Arctic and Antarctic. The prevailing winds in the middle latitudes are often very strong...
, far from both the Equator and the poles, variations in the length of daytime are moderate. In the higher middle latitudes where Montreal
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
, Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
and Ushuaia
Ushuaia
Ushuaia may refer to the following:*Ushuaia, a city in Argentina.**Ushuaia Department, an administrative division**Ushuaia River**Ushuaia International Airport**Colegio Nacional de Ushuaia, National School of 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
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
and South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
are located, the seasonal difference may be quite small and only slightly noticeable to the locals.
Also in the middle latitudes
Middle latitudes
The middle latitudes are between 23°26'22" North and 66°33'39" North, and between 23°26'22" South and 66°33'39" South latitude, or, the Earth's temperate zones between the tropics and the Arctic and Antarctic. The prevailing winds in the middle latitudes are often very strong...
, the seasonal climate variations produced by changes in the length of daytime are the most marked, with very distinct periods of cold and heat, and other secondary seasonal changes such as snow and ice in winter that disappear in summer and so on. At high latitudes, it is cold most of the time, with constant snow and ice, so the seasons are less obvious; and in the tropics, it is hot most of the time, with no snow or ice at all, so again the seasons are less obvious.
Variations in solar noon
The exact instant of solar noon, when the sun reaches its highest elevation in the sky, varies with the seasons everywhere except at the poles and on the Equator. This variation is called the equation of timeEquation of time
The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...
, and the magnitude of the variation is usually in the range of minutes over the course of a year, depending on the observer's latitude.
See also
- DaylightDaylightDaylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...
- Daylight saving timeDaylight saving timeDaylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...
- Solar eclipseSolar eclipseAs seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...