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sunlight
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{{redirect|Sunshine}}{{about||natural lighting of interior spaces by admitting sunlight|Daylighting|solar energy available from sunlight|Solar irradiance|other uses|Sunlight (disambiguation)}}(File:ÐÑ–Ð¶Ð½Ð¸Ð¹_Ñ€Ð°Ð½ÐºÐ¾Ð²Ð¸Ð¹_ÑÐ²Ñ–Ñ‚Ð»Ð¾.jpg|thumb|right|Sunlight through trees in the National park Sviati Hory, Ukraine.)
missing image!
- Apollo 7 Florida.jpg -
Sunrise over the Gulf of Mexico and Florida from Apollo 7.

## Measurement

Researchers can measure the intensity of sunlight using a sunshine recorder, pyranometer, or pyrheliometer. To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth's elliptic orbit and the attenuation by Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance ({{math|Eext}}), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation byJOURNAL, C. KANDILLI, K. ULGEN, yes, Solar Illumination and Estimating Daylight Availability of Global Solar Irradiance, Energy Sources,
E_{rm ext}= E_{rm sc} cdot left(1+0.033412 cdot cosleft(2pifrac{{rm dn}-3}{365}right)right),
where dn=1 on January 1st; dn=32 on February 1st; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dnâ€“3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, therefore, the maximum {{math|Eext}} 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 ({{math|Esc}}), is equal to 128Ã—103 lux. The direct normal illuminance ({{math|Edn}}), corrected for the attenuating effects of the atmosphere is given by:
E_{rm dn}=E_{rm ext},e^{-cm},

## Composition and power

, Naylor
, Mark
, Kevin C. Farmer
, Sun damage and prevention
, Electronic Textbook of Dermatology
, The Internet Dermatology Society
, 1995
, 2008-06-02
, yes
, 2008-07-05
,
,
• 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). Ultra means beyond. 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 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 reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn, but is also required for vitamin D synthesis in the skin and fur of mammals.JOURNAL, Sunlight and Vitamin D: A global perspective for health., Wacker M, Holick, MF, 2013, Dermato-Endocrinology, 5, 1, 51â€“108, 10.4161/derm.24494, 24494042, 3897598,
• Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once{{when|date=December 2016}} held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and can cause cancer.WEB,weblinkweblink" title="web.archive.org/web/20130131164352weblink">weblink yes, 31 January 2013, Artificial Tanning Booths and Cancer - National Cancer Institute, 31 January 2013, 25 January 2018,
• 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 output range of the Sun's total irradiance spectrum.
• Infrared range that spans 700 nm to 1,000,000 nm (1 mm). Infra means below. 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.WEB
, Direct Solar Radiation On Various Slopes From 0 To 60 Degrees North Latitude
, John Buffo
, Leo J. Fritschen
, James L. Murphy
, Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA
, 1972
, 15 Jan 2014
, no
, 2013-11-27
,
,

## Solar constant

File:Solar irradiance spectrum 1992.gif|thumb|Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumberwavenumberThe solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a 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 1366 W/mÂ²,WEB,weblink Satellite observations of total solar irradiance, Acrim.com, 2012-02-12, no,weblink" title="web.archive.org/web/20030204191816weblink">weblink 2003-02-04, varying slightly with solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1361 W/mÂ² is more realistic.JOURNAL, G. Kopp, J. Lean, A new, lower value of total solar irradiance: Evidence and climate significance, Geophys. Res. Lett., 2011, L01706, 10.1029/2010GL045777, 2011GeoRL..38.1706K, Greg, 38,

## Intensity in the Solar System

File:Mars sunset PIA00920.jpg|thumb|right|Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars PathfinderMars PathfinderDifferent bodies of the Solar System receive light of an intensity 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 weblink" title="https:/-/web.archive.org/web/20091122194548weblink">weblink):{| class="wikitable"! rowspan=2 |Planet or dwarf planet! colspan=2 |distance (AU)! colspan=2 |Solar radiation (W/mÂ²)
Perihelion>|Aphelion|minimum
Mercury (planet)>Mercury| 0.4667| 6,272
| Venus
| 0.7282| 2,576
| Earth
| 1.017| 1,321
| Mars
| 1.666| 492
| Jupiter
| 5.458| 45.9
| Saturn
| 10.12| 13.4
| Uranus
| 20.08| 3.39
| Neptune
| 30.44| 1.47
Pluto>| 0.57
The actual brightness of sunlight that would be observed at the surface depends also 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".JOURNAL, The Unveiling of Venus: Hot and Stifling, Science News, 109, 25, 388â€“389, 1976-06-19, 100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds, 3960800, 10.2307/3960800, 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 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 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 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 orbit farther than such a distance, among them 90377 Sedna and {{mpl|(87269) 2000 OO|67}}.

## Surface illumination

File:Sun_over_Lake_Hawea,_New_Zealand.jpg|thumb|300px|right|Sunlight shining through clouds, giving rise to crepuscular rayscrepuscular raysThe 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".WEB,weblink Atmospheric Optics, Craig Bohren, Craig Bohren, no,weblink" title="web.archive.org/web/20131206175627weblink">weblink 2013-12-06, See diffuse sky radiation for more details.

### Spectral composition of sunlight at Earth's surface

The Sun may 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 400â€“700 nm,BOOK, Buser, Pierre A., Imbert, Michel, Vision,weblink 11 October 2013, 1992, MIT Press, 978-0-262-02336-8, 50, Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mÎ¼), or 4000 to 7000 Ã…., and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates an equal-energy illuminant for most of this range.BOOK, MacEvoy, Bruce, color vision, 2008,weblink 27 August 2015, Noon sunlight (D55) has a nearly flat distribution..., no,weblink" title="web.archive.org/web/20150924024814weblink">weblink 24 September 2015, The useful range for color vision in humans, for example, is approximately 450â€“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 selective wavelengths.BOOK, Wyszecki, GÃ¼nter, Stiles, W. S., Color Science: Concepts and Methods, Quantitative Data and Formulas, 1967, John Wiley & Sons, 8, BOOK, MacAdam, David L., Color Measurement: Theme and Variations, Second Revised, 1985, 0-387-15573-2, Springer, 33â€“35, (File:Spectrum of Sunlight en.svg|thumb|center|upright=3.65|Spectrum of the visible wavelengths at approximately sea level; illumination 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 illumination conditions are scaled to show their relation to direct illumination. The units of spectral power are simply raw sensor values (with a linear response at specific wavelengths).)

### Seasonal and orbital variation

{{Further|Insolation|Sunshine duration}}On Earth, the solar radiation varies with the angle of the Sun above the horizon, 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.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 changes, the average distance from the Sun (the semimajor axis does not significantly vary, and so the total insolation over a year remains almost constant due to Kepler's second law,
tfrac{2A}{r^2}dt = dtheta,
where A is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.
int_{0}^{T} tfrac{2A}{r^2}dt = int_{0}^{2pi} dtheta = mathrm{constant}.
If we assume the solar radiation power {{mvar|P}} as a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a constant.But the seasonal and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.WEB,weblink Graph of variation of seasonal and latitudinal distribution of solar radiation, Museum.state.il.us, 2007-08-30, 2012-02-12, no,weblink" title="web.archive.org/web/20120112043906weblink">weblink 2012-01-12, The effect of Sun angle on climate results in the change in solar energy in summer and winter. For example, at latitudes 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 ages (see: Milankovitch cycles).

## 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 sugarsâ€”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, 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.The more 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 using energy from sunlight and then trapped within Earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g., alternative fuels, Malthusian catastrophe, new urbanism, peak oil).

## Cultural aspects

{{Unreferenced section|date=February 2011}}File:Edouard Manet - Luncheon on the Grass - Google Art Project.jpg|thumb|upright=1.35|left|Eduard Manet: Le dÃ©jeuner sur l'herbeLe dÃ©jeuner sur l'herbeThe effect of sunlight is relevant to painting, evidenced for instance in works of Eduard Manet and Claude Monet on outdoor scenes and landscapes.File:Winter Sunshine.jpg|thumb|upright=1.35|right|TÃ©li verÅ‘fÃ©ny ("Winter Sunshine") by LÃ¡szlÃ³ MednyÃ¡nszkyLÃ¡szlÃ³ MednyÃ¡nszkyMany people find direct sunlight to be too 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. 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, shutters, curtains, or nearby shade trees. Sunshine exposure is needed biologically for the creation 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. 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, 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

{{Unreferenced section|date=January 2015}}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 a number of states in the world.For many people with light skin, one purpose for sunbathing is to darken one's skin color (get a sun tan), as this is considered in some cultures to be attractive, associated with outdoor activity, vacations/holidays, and health. Some people prefer 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 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.

## Effect on plant genomes

Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.JOURNAL, Ries G, Heller W, Puchta H, Sandermann H, Seidlitz HK, Hohn B, Elevated UV-B radiation reduces genome stability in plants, Nature, 406, 6791, 98â€“101, 2000, 10894550, 10.1038/35017595, 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 stability in plants.

{{div col|colwidth=30em}} {{div col end}}

## References

{{Reflist|30em}}

• Hartmann, Thom (1998). The Last Hours of Ancient Sunlight. London: Hodder and Stoughton. {{ISBN|0-340-82243-0}}.

{{The Sun}}{{Natural resources}}{{Star}}

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