TheInfoList

Sunlight is a portion of the
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

given off 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 ...

, in particular
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

,
visible Visibility is in meteorology, a measure of the distance at which an object or light can be seen. Visibility may also refer to: * Visual perception ** Naked-eye visibility * A measure of turbidity in water quality control * Interferometric visibili ...
, and
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

light. 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 ...

, sunlight is scattered and through
Earth's atmosphere The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...
, and is obvious as
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...

when the Sun is above 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 ...

. When direct
solar radiation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...
is not blocked by
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. Wate ...

s, it is experienced as sunshine, a combination of bright
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

and
radiant heat Thermal radiation in visible light can be seen on this hot metalwork. Its emission in the infrared is invisible to the human eye. Infrared cameras are capable of capturing this infrared emission (see Thermography). Thermal radiation is electro ...
. When blocked by clouds or , sunlight is
diffused Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present i ...
. Sources indicate an "Average over the entire earth" of "164 watts per square meter over a 24-hour day". The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for synthesis and a
mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of penta ...
. Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. A photon starting at the center of the Sun and changing direction every time it encounters a
charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spa ...
would take between 10,000 and 170,000 years to get to the surface. Sunlight is a key factor in
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

, the process used by plants and other
autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
ic organisms to convert
light energy Light or visible light is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ...

, normally from the Sun, into
chemical energy Chemical energy is the energy of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects th ...
that can be used to synthesize carbohydrates and to fuel the organisms' activities.

# Measurement

Researchers can measure the intensity of sunlight using a ,
pyranometer A pyranometer is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density (W/m2) from the hemisphere above within a wavelength range 0.3 μm to 3 μm. The name py ...

, or
pyrheliometer A pyrheliometer is an instrument for measurement of direct beam solar irradiance Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. ...

. To calculate the amount of sunlight reaching the ground, both the
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off- center, in geometry * Eccentricity (graph theory) of a ...
of Earth's
elliptic orbit In astrodynamics Orbital mechanics or astrodynamics is the application of ballistics Ballistics is the field of mechanics concerned with the launching, flight behavior and impact effects of projectiles, especially ranged weapon munitio ...

and the
attenuation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "P ...
by
Earth's atmosphere The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...

have to be taken into account. The extraterrestrial solar illuminance (), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by :$E_= E_ \cdot \left\left(1+0.033412 \cdot \cos\left\left(2\pi\frac\right\right)\right\right),$ where dn=1 on January 1; dn=32 on February 1; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dn–3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, therefore, the maximum occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338. The solar illuminance constant (), is equal to 128×103
lux The lux (symbol: lx) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...

. The direct normal illuminance (), corrected for the attenuating effects of the atmosphere is given by: :$E_=E_\,e^,$ where is the atmospheric extinction and is the relative optical
airmass In astronomy, air mass or airmass is a measure of the amount of air along the line of sight when observing a star or other celestial source from below Earth's atmosphere (#CITEREFGreen1992, Green 1992). It is formulated as the integral of air dens ...
. The atmospheric extinction brings the number of lux down to around 100 000 lux. The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1367 watts per square meter (the value when the Earth–Sun distance is 1
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metri ...

), then the direct sunlight at Earth's surface when the Sun is 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 ...

is about 1050 W/m2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W/m2. In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm). At the top of the atmosphere, sunlight is about 30% more intense, having about 8%
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

(UV), with most of the extra UV consisting of biologically damaging short-wave ultraviolet. has a
luminous efficacy Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux up Integrating sphere used for measuring the luminous flux of a light source. In photometry (optics), photometry, luminous flux or ...
lumens The lumen (symbol: lm) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...
per watt of
radiant flux In radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are ...
. Multiplying the figure of 1050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides an
illuminance In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ob ...

of approximately 98 000
lux The lux (symbol: lx) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...

(
lumens The lumen (symbol: lm) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...
per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the
South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the where intersects its surface. It is the southernmost point on Earth and lies on the of Earth from the . Situated on the ...
(see
insolation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...

). Dividing the
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

of 1050 W/m2 by the size of the Sun's disk in
steradian The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

s gives an average
radiance In radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are ...
of 15.4 MW per square metre per steradian. (However, the radiance at the center of the sun's disk is somewhat higher than the average over the whole disk due to
limb darkening Limb darkening is an optical effect seen in stars (including the Sun), where the central part of the disk appears brighter than the edge, or '' limb''. Its understanding offered early solar astronomers an opportunity to construct models with such g ...
.) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2.

# Composition and power

The
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...
of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 . The Sun emits EM radiation across most of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequency, frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energy, photon energies. The electromagnetic spectrum covers electromagnetic waves with f ...

. Although the Sun produces
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s as a result of the process, internal absorption and thermalization convert these super-high-energy
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

s to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, the Sun does not emit gamma rays from this process, but it does emit gamma rays from
solar flare A solar flare is an intense eruption of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Sp ...

s. The Sun also emits
X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''ph ...

,
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

,
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
,
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

, and even
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
s; the only direct signature of the nuclear process is the emission of
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

s. Although the
solar corona A corona (, in turn derived from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the f ...
is a source of
extreme ultraviolet 250px, 13.5 nm extreme ultraviolet light is used commercially for photolithography as part of the semiconductor fabrication process. This image shows an early, experimental tool. Extreme ultraviolet radiation (EUV or XUV) or high-energy ...
and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all solar
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

striking the
Earth's atmosphere The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...
spans a range of 100  nm to about 1  mm (1,000,000 nm). This band of significant radiation power can be divided into five regions in increasing order of
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

s: * Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ''ultraviolet'' refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the
human eye The human eye is a sense organ A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans several scales and are determined ...

). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation has germicidal properties, as used in
germicidal lamp A germicidal lamp is an electric light that produces ultraviolet C (UVC) light. This short-wave ultraviolet light disrupts DNA base pairing, causing formation of pyrimidine dimers, and leads to the inactivation of bacteria, viruses, and protoz ...
s. * Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the Earth's atmosphere, and along with UVC causes the
photochemical reactionOrganic photochemistry encompasses organic reactions that are induced by the action of light. The absorption of ultraviolet light by organic molecules often leads to reactions. In the earliest days, sunlight was employed, while in more modern times u ...
leading to the production of the . It directly damages DNA and causes
sunburn Sunburn is a form of radiation burn A radiation burn is a damage to the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. ...

. In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer, but is also required for
vitamin D Vitamin D is a group of fat-soluble secosteroid 250px, The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield D vitamins. A secosteroid () is a type of steroid , hypothetical a steroid with ...
synthesis in the skin of mammals. * Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

, and hence is used in cosmetic artificial
sun tanning Sun tanning or simply tanning is the process whereby Human skin color, skin color is darkened or tanned. It is most often a result of exposure to ultraviolet radiation, ultraviolet (UV) radiation from sunlight or from artificial sources, such ...

( tanning booths and
tanning bed Indoor tanning involves using a device that emits ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of ...
s) and
PUVA PUVA ( psoralen and ultraviolet A) is an ultraviolet light therapy treatment for skin diseases: eczema Dermatitis (also known as eczema) is inflammation Inflammation (from la, inflammatio) is part of the complex biological response of bo ...
therapy for
psoriasis Psoriasis is a long-lasting, noncontagious autoimmune disease An autoimmune disease is a condition arising from an abnormal immune response to a functioning body part. There are at least 80 types of autoimmune diseases. Nearly any body part c ...

. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of
free radicals In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...
and
reactive oxygen species Reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from O2. Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen (O2) produce ...
), and can cause cancer. *
Visible Visibility is in meteorology, a measure of the distance at which an object or light can be seen. Visibility may also refer to: * Visual perception ** Naked-eye visibility * A measure of turbidity in water quality control * Interferometric visibili ...
range or light spans 380 to 700 nm. As the name suggests, this range is visible to the naked eye. It is also the strongest output range of the Sun's total irradiance spectrum. *
Infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

range that spans 700 nm to 1,000,000 nm (1  mm). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength: ** Infrared-A: 700 nm to 1,400 nm ** Infrared-B: 1,400 nm to 3,000 nm ** Infrared-C: 3,000 nm to 1 mm.

## Published tables

Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.

# Intensity in the Solar System

Different bodies of the
Solar System The Solar SystemCapitalization 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 "Sola ...

receive light of an intensity inversely proportional to the square of their distance from Sun. A table comparing the amount of solar radiation received by each planet in the Solar System at the top of its atmosphere: The actual brightness of sunlight that would be observed at the surface also depends on the presence and composition of an
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 ...

. For example, reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds". Sunlight on Mars would be more or less like daylight on Earth during a slightly overcast day, and, as can be seen in the pictures taken by the rovers, there is enough
diffuse sky radiation upright=1.9, The visible spectrum, about 380 to about 740 nanometers (nm), shows the atmospheric water absorption band and the solar Fraunhofer lines. The blue sky spectrum presents across 450–485 nm, the wavelengths of the color blue. Diff ...
that shadows would not seem particularly dark. Thus, it would give perceptions and "feel" very much like Earth daylight. The spectrum on the surface is slightly redder than that on Earth, due to scattering by reddish dust in the Martian atmosphere. For comparison, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise (see
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...

for comparison table). Even on Pluto, the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full
moonlight Moonlight consists of mostly sunlight (with little earthlight (astronomy), earthlight) reflected from the parts of the Moon, Moon's surface where the Sun, Sun's light strikes. Illumination The intensity of moonlight varies greatly depending on ...

on Earth, a distance of about 500 AU (~69  light-hours) is needed; only a handful of objects in the Solar System have been discovered that are known to orbit farther than such a distance, among them
90377 Sedna 90377 Sedna, or simply Sedna, is a large planetoid A minor planet is an astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists i ...
and .

# Variations in solar irradiance

## Seasonal and orbital variation

On Earth, the solar radiation varies with the angle of the Sun above 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 ...

, with longer sunlight duration at high latitudes during summer, varying to no sunlight at all in winter near the pertinent pole. When the direct radiation is not blocked by clouds, it is experienced as ''sunshine''. The warming of the ground (and other objects) depends on the absorption of the electromagnetic radiation in the form of
heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these ...

. The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth's
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 ...

and
obliquity 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 mat ...
change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an
orbital eccentricity In astrodynamics Orbital mechanics or astrodynamics is the application of ballistics Ballistics is the field of mechanics Mechanics (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ε ...
of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the Sun (the
semimajor axis In 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 relative position ...
does not significantly vary, and so the total
insolation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...

over a year remains almost constant due to Kepler's second law, :$\tfracdt = d\theta,$ where $A$ is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant. :$\int_^ \tfracdt = \int_^ d\theta = \mathrm.$ If we assume the solar radiation power  as a constant over time and the solar irradiation given by the
inverse-square law 420px, S represents the light source, while r represents the measured points. The lines represent the flux emanating from the sources and fluxes. The total number of flux lines depends on the strength of the light source and is constant with in ...

, we obtain also the average insolation as a constant. But the
seasonal A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In ...
and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary. The effect of Sun angle on climate results in the change in solar energy in summer and winter. For example, at
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 ...

s of 65 degrees, this can vary by more than 25% as a result of Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent
ice age An ice age is a long period of reduction in the temperature 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 an ...

s (see:
Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the 's movements on its over thousands of years. The term is named for n and . In the 1920s, he hypothesized that variations in , , and resulted in cyclical variation in the r ...

).

## Solar intensity variation

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many time scales, including the 11-year sunspot solar cycle. When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done. These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activity varies with longer cycles, such as the proposed 88 year ( Gleisberg cycle), 208 year ( DeVries cycle) and 1,000 year (Eddy cycle).

## Solar constant

The solar constant is a measure of flux density, is the amount of incoming solar
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

per unit area that would be incident on a plane perpendicular to the rays, at a distance of one Astronomical unit, astronomical unit (AU) (roughly the mean distance from the Sun to Earth). The "solar constant" includes all types of solar radiation, not just the
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
. Its average value was thought to be approximately 1366 W/m2, varying slightly with solar cycle, solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1361 W/m2 is more realistic.

## Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth

Since 1978 a series of overlapping NASA and ESA satellite experiments have measured total solar irradiance (TSI) – the amount of solar radiation received at the top of Earth's atmosphere – as 1.365 kilo⁠watts per square meter (kW/m2). TSI observations continue with the ACRIMSAT/ACRIM3, Solar and Heliospheric Observatory, SOHO/VIRGO and SORCE/TIM satellite experiments. Observations have revealed variation of TSI on many timescales, including the solar magnetic cycle and many shorter periodic cycles. TSI provides the energy that drives Earth's climate, so continuation of the TSI time-series database is critical to understanding the role of solar variability in climate change. Since 2003 the SORCE Spectral Irradiance Monitor (SIM) has monitored Spectral solar irradiance (SSI) – the spectral distribution of the TSI. Data indicate that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth's climate responses than earlier assumed, fueling broad avenues of new research in "the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth's climate".

# Surface illumination and spectrum

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere. While the color of the sky is usually determined by Rayleigh scattering, an exception occurs at sunset and twilight. "Preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon". See
diffuse sky radiation upright=1.9, The visible spectrum, about 380 to about 740 nanometers (nm), shows the atmospheric water absorption band and the solar Fraunhofer lines. The blue sky spectrum presents across 450–485 nm, the wavelengths of the color blue. Diff ...
for more details.

## Spectral composition of sunlight at Earth's surface

The Sun may be said to Lighting, illuminate, which is a measure of the light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700 nm, and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates an Standard illuminant, equal-energy illuminant for most of this range. The useful range for color vision in humans, for example, is approximately 450–650 nm. Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate. When illumination is indirect, Rayleigh scattering in the upper atmosphere will lead blue wavelengths to dominate. Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb particular wavelengths.

# Life on Earth

The existence of nearly all Organism, life on Earth is fueled by light from the Sun. Most
autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
s, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

. These sugars are then used as building-blocks and in other synthetic pathways that allow the organism to grow. Heterotrophs, such as animals, use light from the Sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products, or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as cellular respiration. In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow. During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals that had previously provided humans with only meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans. Fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within Earth for millions of years.

# Cultural aspects

The effect of sunlight is relevant to painting, evidenced for instance in works of Édouard Manet and Claude Monet on outdoor scenes and landscapes. Many people find direct sunlight to be too brightness, bright for comfort, especially when reading from white paper upon which the sunlight is directly shining. Indeed, looking directly at the Sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Automobile, Cars, many helmets and caps are equipped with visors to block the Sun from direct vision when the Sun is at a low angle. Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, Window shutter, shutters, curtains, or nearby shade trees. Sunshine exposure is needed biologically for the production of Vitamin D in the skin, a vital compound needed to make strong bone and muscle in the body. In colder countries, many people prefer sunnier days and often avoid the Shade (shadow), shade. In hotter countries, the converse is true; during the midday hours, many people prefer to stay inside to remain cool. If they do go outside, they seek shade that may be provided by trees, Umbrella, parasols, and so on. In many world religions, such as Hinduism, the Sun is considered to be a god, as it is the source of life and energy on Earth. It also formed the basis for religion in Ancient Egypt.

## Sunbathing

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafes. Sunbathers typically wear limited amounts of clothing or some simply go Nudity, nude. For some, an alternative to sunbathing is the use of a Tanning bed, sunbed that generates
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

light and can be used indoors regardless of weather conditions. Tanning beds have been banned in a number of states in the world. For many people with light skin, one purpose for sunbathing is to darken one's Human skin color, skin color (get a sun tan), as this is considered in some cultures to be attractive, associated with outdoor activity, Vacation, vacations/holidays, and health. Some people prefer Naturism, naked sunbathing so that an "all-over" or "even" tan can be obtained, sometimes as part of a specific lifestyle. Controlled heliotherapy, or sunbathing, has been used as a treatment for
psoriasis Psoriasis is a long-lasting, noncontagious autoimmune disease An autoimmune disease is a condition arising from an abnormal immune response to a functioning body part. There are at least 80 types of autoimmune diseases. Nearly any body part c ...

and other maladies. Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes, and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the Sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

# Effects on human health

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of and a
mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of penta ...
. A dietary supplement can supply
vitamin D Vitamin D is a group of fat-soluble secosteroid 250px, The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield D vitamins. A secosteroid () is a type of steroid , hypothetical a steroid with ...
without this mutagenic effect, but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones and possibly inhibiting the growth of some cancers. Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder. Long-term sunlight exposure is known to be associated with the development of skin cancer, photoaging, skin aging, immune suppression, and eye diseases such as cataracts and macular degeneration. Short-term overexposure is the cause of
sunburn Sunburn is a form of radiation burn A radiation burn is a damage to the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. ...

, snow blindness, and solar retinopathy. UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits, and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little. There is a general consensus that sunburn should always be avoided. Epidemiological data shows that people who have more exposure to sunlight have less high blood pressure and cardiovascular-related mortality. While sunlight (and its UV rays) are a risk factor for skin cancer, "sun avoidance may carry more of a cost than benefit for over-all good health". A study found that there is no evidence that UV reduces lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure.

# Effect on plant genomes

Elevated solar ultraviolet, UV-B doses increase the frequency of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

homologous recombination, recombination in ''Arabidopsis thaliana'' and tobacco (''Nicotiana tabacum'') plants. These increases are accompanied by strong induction of an enzyme with a key role in recombinational repair of DNA damage. Thus the level of terrestrial solar UV-B radiation likely affects genome instability, genome stability in plants.

* Color temperature * Coronal radiative losses * Diathermancy * Fraunhofer lines * List of cities by sunshine duration * Over illumination * Photic sneeze reflex * Photosynthesis * Starlight * Moonlight

# References

* Thom Hartmann, Hartmann, Thom (1998). ''The Last Hours of Ancient Sunlight''. London: Hodder and Stoughton. .

*
Solar radiation – Encyclopedia of Earth
* [http://www.ngdc.noaa.gov/stp/solar/solarirrad.html Total Solar Irradiance (TSI) Daily mean data] at the website of the National Geophysical Data Center
Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present
by World Radiation Center, Physikalisch-Meteorologisches Observatorium Davos (pmod wrc)

Rivington et al. * [http://www.macaulay.ac.uk/LADSS/papers.html?2005 Evaluation of three model estimations of solar radiation at 24 UK stations], Rivington et al.
High resolution spectrum of solar radiation
from Paris Observatory, Observatoire de Paris