Sunlight is a portion of the electromagnetic radiation given off by the Sun , in particular infrared , visible , and ultraviolet light. On Earth , sunlight is filtered through Earth’s atmosphere , and is visible as daylight when the Sun is above the horizon . When the direct solar radiation is not blocked by clouds , it is experienced as sunshine , a combination of bright light and radiant heat . When it is blocked by cloudsor reflects off other objects , it is experienced as diffused light. The World Meteorological Organization uses the term ” sunshine duration ” to mean the cumulative time During qui year area Receives Direct irradiance from the Sun of at least 120 watts per square meter . [1] Other sources indicate an average of 164 watts per square meter over a 24 hour day. [2]

The ultraviolet radiation in sunlight has both positive and negative health effects, as well as a principal source of vitamin D 3 and a mutagen .

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 particle would take between 10,000 and 170,000 years to get to the surface. [3]

Sunlight is a key factor in photosynthesis , the process used by plants and other autotrophic organisms to convert light energy , normally from the Sun, into chemical energy that can be used to fuel the organizations.


Researchers can measure the intensity of sunlight using a sunshine recorder , pyranometer , or pyrheliometer . To calculate the amount of sunlight reaching the ground, both Earth’s elliptical orbit and the attenuation by Earth’s atmosphere have to be taken into account. The extraterrestrial solar illuminance ( ext ), corrected for the elliptical orbit by the day of the year (dn), is given to a good approximation by [4]

{\ displaystyle E _ {\ rm {ext}} = E _ {\ rm {sc}} \ cdot \ left (1 + 0.033412 \ cdot \ cos \ left (2 \ pi {\ frac {{\ rm {dn}} – 3} {365}} \ right) \ right)}

where dn = 1 on January 1st; dn = 32 on February 1st; dn = 59 on March 1 (except for leap years, where dn = 60), etc. In this formula it is used, because in modern times Earth’s perihelion , the closest approach to the Sun and, therefore, the maximum ext occurs around January 3 each year. The value of 0.033412 is calculated according to the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.

The solar constant luminance ( sc ), is equal to 128 × 10  lx . The direct normal illuminance ( dn ), corrected for the attenuating effects of the atmosphere is given by:

{\ displaystyle E _ {\ rm {dn}} = E _ {\ rm {ext}} \, e ^ {- cm},}

Where c is the atmospheric extinction and m is the relative optical airmass . The atmospheric extinction brings the number of lux down to around 100,000.

The total amount of energy received 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 ), then the direct sunlight at Earth’s surface when the Sun is at the zenith is about 1050 W / m 2 , but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W / m 2 . [5] 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). [6] At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV), [7] with most of the ultraviolet ultraviolet. [8]

Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux . This is more than the efficacy of most artificial lighting (including fluorescent), which means using sunlight for illumination.

Multiplying the figure of 1050 watts per square meter by 93 lumens per watt indicates that bright illuminates an illuminance of approximately 98,000 lux ( lumens per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be much less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight in the South Pole (see insolation ).

Dividing the irradiance of 1050 W / m 2 by the size of the sun’s disk in steradians Gives year average radiance of 15.4 MW per square meter per steradian. (However, the radiance at the center of the sun is larger than the average at the limb darkness .) Multiplying this by π gives an upper bound to the irradiance which can be focused on a surface using mirrors: 48.5 MW / m 2 .

Composition and power

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at the end of wavelength range shown) but included very small amounts of Sun’s total output power.
See also: Ultraviolet , Infrared , and Light

The spectrum of the Sun’s solar radiation is close to That of a black body [9] [10] with a temperature of about 5,800 K . [11] The Sun emits EM radiation across most of the electromagnetic spectrum . Although the Sun produces gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization converts these super-high-energy photons to lower-energy photons before they reach the Sun’s surface and are emitted out of space. As a result, the Sun does not emit gamma rays from this process, but it does emit gamma rays from solar flares . [12]The Sun also emits X-rays , ultraviolet , visible light , infrared , and even radio waves ; [13] The only direct signature of the nuclear process is the emission of neutrinos .

Although the solar corona is a source of extreme ultraviolet 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 striking the Earth’s atmosphere ranges from 100 nm to about 1 mm (1,000,000 nm). citation needed ] This band of significant radiation may be divided into five regions in which wavelengths : [14]

  • 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). Due to absorption by the atmosphere very little reaches Earth’s surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
  • Ultraviolet B gold (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the Earth’s atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer . It directly damages DNA and causes sunburn , but is also required for vitamin D in the skin and fur of mammals. [15]
  • Ultraviolet A gold (UVA) spans 315 to 400 nm. This band was once when [ when? ] Held-to be less Damaging to DNA , and hence is used in cosmetic artificial sun tanning ( tanning boothsand tanning beds ) and PUVA therapy for psoriasis . However, UVA is known to cause indirect DNA damage via indirect routes (formation of free radicals and reactive oxygen species ), and can cause cancer. [16]
  • Visible range or light spans 380 to 780 nm. As the name suggests, this range is visible to the naked eye. It is also the strongest range of the Sun’s total irradiance spectrum.
  • Infrared range that spans 700 nm to 1,000,000 nm (1 mm ). It includes 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

Portland, Oregon, USA, United States of America, United States of America. appear on the web. [17]

Solar constant

Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber

The constant solar , a measure of flux density , is the amount of incoming solar electromagnetic radiation per unit area That Would Be incident on plane perpendicular to the rays, at a distance of one 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 . Its average value Was thought to be Approximately 1 366 W / m² [18] varying, Slightly with solar activity , recent recalibration purpose of the falling satellite observations indicate indication value closer to 1361 W / m² is more realistic. [19]

Total solar irradiance (TSI) and spectral solar irradiance (SSI) on Earth

Total solar irradiance (TSI) – NASA and ESA satellite experiments to be 1,361 kilo watts per square meter (kW / m²). [18] [20] [21] [22] TSI observations are continuing today with the ACRIMSAT/ ACRIM3, SOHO / VIRGO and SORCE / TIM satellite experiments. [23] Variation of TSI has been discovered on many timescales including the solar magnetic cycle [24] and many shorter periodic cycles. [25] TSI provides the energy that drives Earth’s climate, so continuation of the TSI time series is critical to understanding the role of solar variability in climate change.

Spectral solar irradiance (SSI) – the spectral distribution of the TSI – has been monitored since 2003 by the SORCE Spectral Irradiance Monitor (SIM). SSI at UV (ultraviolet) wavelength corresponds to a less clear, and probably more complicated fashion, with Earth’s climate responses more likely, fueling broad avenues of new research in “the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate “. [26]

Intensity in the Solar System

Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder .

Different bodies of the Solar System receive an intensity of inversely proportional to the square of their distance from Sun. A rough table comparing the amount of solar radiation received by each planet in the Solar System follows (from data in [1] ):

Planet gold dwarf planet distance ( AU ) Solar radiation (W / m²)
perihelion Aphelion maximum minimum
Mercury 0.3075 0.4667 14.446 6,272
Venus 0.7184 0.7282 2,647 2,576
Earth 0.9833 1.017 1,413 1,321
March 1.382 1,666 715 492
Jupiter 4,950 5458 55.8 45.9
Saturn 9048 10.12 16.7 13.4
Uranus 18.38 20.08 4.04 3.39
Neptune 29.77 30.44 1.54 1.47
Pluto 29.66 48.87 1.55 0.57

The actual brightness of sunlight that would be observed at the surface depends on the presence and composition of an atmosphere . For example, Venus’s thick atmosphere 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”. [27]

Sunlight on Mars would be more or less like daylight on Earth during the day, and there would be enough diffuse sky radiation that would not be particularly dark. Thus, it would give you perceptions and “feel” very much like Earth daylight. The spectrum on the surface is slightly redundant in the Martian atmosphere.

For comparison, sunlight on Saturn is slightly more than sunlight at sunset or sunrise (see daylight for comparison table). Even on Pluto, the sunlight would still be bright enough to match the average living room. To see sunlight as dim moon on Earth, a distance of about 500 AU (~ 69 light-hours ) is needed; There are only a handful of objects in the Solar System known to these orbit, among them 90377 Sedna and (87269) 2000 OO 67 .

Surface illumination

The spectrum of surface illumination depends on solar elevation due to atmospheric effects, with the dominant spectral component of sunrise and sunset, respectively, and dominating over sunrise and sunset. These effects are apparent in natural light photography where the main 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. “Ozone over the long horizon gives the zenith sky its blueness when the sun is near the horizon”. [28]

See also diffuse sky radiation for more details.

Spectral composition of sunlight at Earth’s surface

The Sun’s electromagnetic radiation is one of the Earth’s surface is predominantly light that falls within the range of wavelengths to which the visual systems of Earth’s surface are sensitive. The Sun may therefore be said to 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, [29] and given optimal conditions for absorption and scattering by Earth’s atmosphere produces illumination that approximates an equal-energy illuminant for most of this range. [30]The useful range for color vision in humans, for example, is approximately 450-650 nm. Aside from the effects of a sunrise and a sunrise, the spectral composition changes primarily in the respect of sunlight is able to illuminate. When illumination is indirect, Rayleigh scattering in the upper atmosphere will lead to blue wavelengths to dominate. Water vapor will also be absorbed by selective scattering and ozone, dust and water particles. [31] [32]

Spectrum of the visible wavelengths illuminated by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. The yellow line shows the spectrum of direct illumination under optimal conditions. The other enlightenment conditions are scaled to show their relationship to direct enlightenment. The units of spectral power are simply raw sensor values ​​(with a linear response at specific wavelengths).

Variations in solar irradiance

Seasonal and orbital variation

Further information: Insolation and Sunshine Duration

On Earth, the solar radiation varies with the angle of the sun above the horizon , with longer sunlight duration during the summer. 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 electromagnetic radiation in the form of heat .

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 and obliquity change with time (over Thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). As the orbital eccentricity exchange rates, the average distance from the sun (the semimajor axis does not Significantly vary, and so the total exposure over a year remains Almost constant due to Kepler’s second law ,

{\ displaystyle {\ tfrac {2A} {r ^ {2}}} dt = d \ theta,}

Where {\ displaystyle A}is the “areal velocity” invariant. That is, the integration over the orbital period (also invariant) is a constant.

{\ displaystyle \ int _ {0} ^ {T} {\ tfrac {2A} {r ^ {2}}} dt = \ int _ {0} ^ {2 \ pi} d \ theta = \ mathrm {constant} .}

If we assume the solar radiation power P as a constant over time and the solar irradiation by Given the inverse-square law , we également obtenir the average insolation as a constant.

But the seasonal and latitudinal distribution and intensity of solar radiation received by Earth’s surface does vary. [33] The effect of sun angle on climate change in solar energy in summer and winter. For example, at latitudes of 65 degrees, Earth’s orbital variation. Because changes in winter and summer tend to be offset, the change in the annual average inflation rate is very low, but the redistribution of energy is strongly affected by the intensity of seasonal cycles. Such exchange associated with the redistribution of solar energy are regarded A Likely causes for the coming and going of recent ice ages(see: Milankovitch cycles ).

Solar intensity variation

Further information: Solar variation

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many times scales, including the 11-year solar cycle sunspot. [24] 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. [34] [35] [36] [37] These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activitiy varies with longer cycles, such as the proposed 88 year ( Gleisberg cycle ), 208 year ( DeVries cycle ) and 1,000 year (Eddy cycle ).

Life on Earth

The existence of nearly all life on Earth is fueled by light from the Sun. Most autotrophs , such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars-a process known as photosynthesis . These sugars are then used as building blocks and in other synthetic pathways that allow the organism to grow.

Heterotrophs , such as animals, utilizing autotrophs, or 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 heterotrophic 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 have been allowed to be harvested by humans, and the population has begun to grow.

During the Neolithic Revolution , the domestication of plants and animals Fields devoted to crops have been enriched by inedible plant matter, providing sugars and nutrients for future harvests. They have been previously used by farmers, but they have been used for plowing their lives, fueled by grasses inedible to humans.

The most recent discoveries of coal , petroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed by sunlight and trapped within Earth for millions of years. Because these fossil fuels have accumulated over many millions of years, they have grown to increase the production and consumption of primary energy . As the amount of fossil fuel is large but finite, this can not continue indefinitely, and various theories exist as to what will follow this stage of human civilization (eg, alternative fuels , Malthusian disaster, new urbanism , peak oil ).

Cultural aspects

Many people find direct sunlight to be too bright for comfort, especially when reading directly from the sun. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses . Cars , many helmets and caps are equipped with a sun visor . Often Sunshine is blocked from Reviews entering buildings through the use of walls , window blinds , awnings , shutters , curtains , gold nearbyshade trees .

In colder countries, many people prefer sunnier days and often avoid the 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, umbrellas , and so on.

In Hinduism , the sun is considered to be a god, as it is the source of life and energy on earth.


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 nude . For some, an alternative to sunbathing is the use of a sunbed that generates ultraviolet light and can be used indoors regardless of weather conditions. Tanning beds have been banned in the world.

For many people with light skin, one of the targets for sunbathing is one of the skin ‘s colors (get a sun tan), as it is considered in some cultures to be attractive, associated with outdoor activities, vacations / holidays , and health. Some people prefer naked sunbathing so that an “all-over” or “even” can be obtained, sometimes as part of a specific lifestyle.

For people suffering from psoriasis , sunbathing is an effective way of healing symptoms.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes , and is an automatic response 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

Main article: Health effects of sunlight exposure

The ultraviolet radiation in sunlight has both positive and negative health effects, as well as a principal source of vitamin D 3 and a mutagen . [38] A dietary supplement can supply vitamin D without this mutagenic effect, [39] 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 [40] and possibly inhibiting the growth of some cancers. [41] [42] Sun exposure has been associated with the timing of melatoninsynthesis, maintenance of normal circadian rhythms , and reduced risk of seasonal affective disorder . [43]

Long-term sunlight exposure is known to be associated with the development of skin cancer , skin aging , immune suppression , and eye diseases such as cataracts and macular degeneration . [44] Short-term overexposure is the cause of sunburn , snow blindness , and solar retinopathy .

UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits, [45] and a number of public health organizations. little. [46] There is a general consensus that sunburn should always be avoided.

Epidemiological data shows that people have more exposure to the 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 benefit for over-all good health.” [47] A study found that there is no evidence that UV decreases lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure. [47]

Effect on plant genomes

Elevated solar UV- B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco ( Nicotiana tabacum ) plants. [48] These concentrations are accompanied by strong induction of an enzyme with recombinant repair of DNA damage. Thus the level of solar terrestrial UV-B radiation likely affects genome stability in plants.

See also

  • Color temperature
  • Coronal radiative losses
  • Diathermancy
  • Fraunhofer lines
  • List of cities by sunshine
  • Over-illumination
  • Photic sneeze reflex
  • Photosynthesis


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