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| {{redirect|Sunshine}}
| | Understandably, numerous anxiousness victims acquire clinical depression as properly, and others are prone to produce alcohol and drug dependencies. Most of the time your dog acts out in destructive ways not because you are gone but because he is bored. Anxiety attacks and its debilitating symptoms really do affect a person's life, their work, and virtually every part of their day to day life. Keep an eye out for my article, Panic Attacks and Anxiety: Adios. To some, these new regulations may seem intrusive, but they were instilled with the best of intentions. <br><br>The education or Spoken English Courses should certainly include love, understanding together with proper care. When you leave your home for whatever reason does your dog become panicked and start barking or whining when you leave. Social phobias or anxiety can lead a teen into seclusion, isolation and emotional torment. It also helps to settle nerves and creates an overall calming effect. Many people think that when they are anxious they would get heat attack, but it is not true. <br><br>Phobias can be two major types: social phobia and specific phobia. A job interview is an employer's chance to asses you , together with your occasion where you can take a deeper look at the organization. The most effective collections mix three or additional exciting and closely relevant pieces. Again, I ask that you refer to my article Rescue Dog Training - Crate Training is Kind not Cruel which will explain how to acclimate your dog to spending time in the crate. If there is nothing you can do, will yourself to stop worrying and even thinking about whatever worries you. <br><br>It's very important for school counselors to educate students about test anxiety. Eventually, it all bottles up, because they fail to seek help and TALK out their problems. Social Anxiousness is often just the starting of an anxiety problem and will frequently conclude up in a state of depression. On the net anxiety forums you'll notice a full variety of sections. Instead of thinking the worst will happen, try to change your thought process into something that is positive. <br><br>This high motivational need for achievement is where the quest for self-actualization is realized which embodies what is considered a continual yearning and eagerness to survive. A big issue with many being affected by the common anxiety dysfunction is that with all of their thinking and being troubled, they can cause an all out attack. Use only a select couple of items of essential furnishings, and retain a lot of open space. The child may also experience shortness of breath, choking or smothering sensations, pounding heartbeat, chest pain, nausea, lightheadedness, trembling and shaking, and fear of losing their mind. In fact, more often than not, anxiety is not the cause of high blood pressure.<br><br>If you liked this article and you would such as to get even more facts pertaining to [http://www.stress-lavoro-correlato.info/sitemap/ medicine for social anxiety] kindly visit our internet site. |
| {{about||natural lighting of interior spaces by admitting sunlight|Daylighting|solar energy available from sunlight|Insolation|other uses|Sunlight (disambiguation)}}
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| [[File:Sunshine at Dunstanburgh.JPG|thumb|right|240px|Sunlight shining through [[cloud]]s, giving rise to [[crepuscular rays]].]]
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| '''Sunlight''' is a portion of the [[electromagnetic radiation]] given off by the [[Sun]], in particular [[infrared]], [[visible light|visible]], and [[ultraviolet]] light. On [[Earth]], sunlight is [[Filter (optics)|filter]]ed through the [[Atmosphere of Earth|Earth's atmosphere]], and is obvious 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 the clouds or [[diffuse reflection|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 which an area receives direct [[irradiance]] from the Sun of at least 120 [[watt]]s per [[square meter]].<ref>{{cite web | url=http://www.wmo.int/pages/prog/www/IMOP/publications/CIMO-Guide/CIMO%20Guide%207th%20Edition,%202008/Part%20I/Chapter%208.pdf |format=PDF| title=Chapter 8 – Measurement of sunshine duration | work=CIMO Guide | publisher=[[World Meteorological Organization]] | accessdate=2008-12-01 }}</ref> Sunlight on the skin is an effective source of [[vitamin D]].
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| ==Summary==
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| Sunlight may be recorded using a [[sunshine recorder]], [[pyranometer]], or [[pyrheliometer]].
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| Sunlight takes about 8.3 minutes to reach the Earth. On average, it takes energy between 10,000 and 170,000 years to leave the sun's interior and then be emitted from the surface as light.<ref>{{cite web|url=http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |title=NASA: The 8-minute travel time to Earth by sunlight hides a thousand-year journey that actually began in the core |publisher=NASA, ''sunearthday.nasa.gov'' |date= |accessdate=2012-02-12}}</ref>
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| The total amount of energy received at ground level from the sun at the zenith is 1004 watts per square meter, which is composed of 527 watts of infrared radiation, 445 watts of [[visible light]], and 32 watts of [[ultraviolet]] radiation. At the top of the atmosphere sunlight is about 30% more intense, with more than three times the fraction of [[ultraviolet]] (UV), with most of the extra UV consisting of biologically-damaging shortwave ultraviolet.<ref name="Solar radiation">
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| {{cite book |last=Qiang |first=Fu |chapter=Radiation (Solar) |chapterurl=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |format=PDF |editor1-last=Holton |editor1-first=James R. |title=Encyclopedia of atmospheric sciences |volume=''5 ''[''Rad - S'']'' |publisher=Academic Press |location=Amsterdam |year=2003 |pages=1859–1863 |oclc=249246073 |isbn=978-0-12-227095-6 }}
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| </ref><ref name="Solar constant at ground level">[http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx Solar constant at ground level]</ref><ref name="rredc.nrel.gov">{{cite web|url=http://rredc.nrel.gov/solar/spectra/am1.5/ |title=Reference Solar Spectral Irradiance: Air Mass 1.5|accessdate=2009-11-12}}</ref>
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| Direct sunlight has a luminous efficacy of about 93 [[lumen (unit)|lumen]]s per watt of [[radiant flux]]. Multiplying the figure of 1004 watts per square metre by 93 lumens per watt indicates that bright sunlight provides an [[illuminance]] of approximately 93,000 [[lux]] or [[lumen (unit)|lumen]]s 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.
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| Sunlight is a key factor in [[photosynthesis]], the process used by plants and other [[autotroph]]ic organisms to convert [[light]] energy, normally from the [[sun]], into chemical energy that can be used to fuel the organisms' activities.
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| == Composition and power ==
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| [[File:Solar Spectrum.png|250px|thumb|Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.]]
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| {{See also|Ultraviolet|Infrared|Light}}
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| The [[frequency spectrum|spectrum]] of the Sun's solar radiation is close to [[black body radiation|that of a black body]] with a temperature of about 5,800 [[Kelvin|K]].<ref>[http://solarsystem.nasa.gov/planets/profile.cfm?Display=Facts&Object=Sun NASA Solar System Exploration - Sun: Facts & Figures] retrieved 27 April 2011 "Effective Temperature ... 5777 K"</ref> The Sun emits EM radiation across most of the [[electromagnetic spectrum]]. Although the Sun produces [[Gamma ray]]s as a result of the [[nuclear fusion]] process, these super-high-energy [[photon]]s are converted 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. The Sun does, however, emit [[Xray|X-rays]], [[ultraviolet]], [[visible light]], [[infrared]], and even [[radio wave]]s.<ref>{{cite web|url=http://www.windows2universe.org/sun/spectrum/multispectral_sun_overview.html |title=The Multispectral Sun, from the National Earth Science Teachers Association |publisher=Windows2universe.org |date=2007-04-18 |accessdate=2012-02-12}}</ref>
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| 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 [[Atmosphere of Earth|Earth's atmosphere]] spans a range of 100 [[Nanometer|nm]] to about 1 [[Millimeter|mm]]. This band of significant radiation power can be divided into five regions in increasing order of [[wavelength]]s:<ref>{{cite web
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| | last = Naylor
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| | first = Mark
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| | coauthors = Kevin C. Farmer
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| | title = Sun damage and prevention
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| | work = Electronic Textbook of Dermatology
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| | publisher = The Internet Dermatology Society
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| | year = 1995
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| | url = http://www.telemedicine.org/sundam/sundam2.4.1.html
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| | accessdate = 2008-06-02}}</ref>
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| * '''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]]). Owing to absorption by the atmosphere very little reaches the Earth's surface<!-- Ocean's surface is not lithosphere. Do not insert junk, please. -->. This spectrum of radiation [[Ultraviolet germicidal irradiation|has germicidal properties]], and is used in [[germicidal lamp]]s.
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| * '''Ultraviolet B''' or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the [[photochemical reaction]] leading to the production of the [[ozone layer]]. It directly damages DNA and causes [[sunburn]].
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| * '''Ultraviolet A''' or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to [[DNA]], and hence is used in cosmetic artificial [[sun tanning]] ([[tanning booth]]s and [[tanning bed]]s) 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 is able to cause cancer.<ref>[http://web.archive.org/web/20130131164352/http://www.cancer.gov/newscenter/entertainment/tipsheet/tanning-booths tanning booth cancer] </ref>
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| * '''[[Visible light|Visible]] range''' or '''light''' spans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye. It is also the strongest output range of the sun's total irradiance spectrum.
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| * '''[[Infrared]]''' range that spans 700 nm to 10<sup>6</sup> nm (1 [[Millimeter|mm]]). It is responsible for an important part of the electromagnetic radiation that reaches the Earth. It is also divided into three types on the basis of wavelength:
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| ** Infrared-A: 700 nm to 1,400 nm
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| ** Infrared-B: 1,400 nm to 3,000 nm
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| ** Infrared-C: 3,000 nm to 1 mm.
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| Sunlight in space at the top of [[Atmosphere of Earth|Earth's atmosphere]] at a power of 1366 watts/m<sup>2</sup> is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.<ref name="Solar radiation"/> At ground level, this decreases to about 1120–1000 watts/m<sup>2</sup>, and by energy fractions to 44% visible light, 3% ultraviolet (with the Sun at the zenith, but less at other angles), and the remainder infrared.<ref name="Solar constant at ground level"/> Thus, sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of [[visible light]], and 32 watts of [[ultraviolet]] radiation.<ref name="rredc.nrel.gov"/>
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| ===Published tables===
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| Tables of direct solar radiation on various slopes from 0 to 60 degrees North Latitude, in calories per square centimetre were issued in 1972 are published by Pacific Northwest Forest and Range Experiment Station|Forest Service, U.S. Department of Agriculture, Portland, Oregon,USA
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| <ref> {{Cite web
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| |url=http://www.fs.fed.us/pnw/pubs/pnw_rp142.pdf
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| |title=Direct Solar Radiation On Various Slopes From 0 To 60 Degrees North Latitude
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| |author= John Buffo, Leo J. Fritschen, James L. Murphy
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| |publisher= Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon,USA
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| |date=1972
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| |accessdate=15 Jan 2014}}</ref>
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| == Calculation ==
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| To calculate the amount of sunlight reaching the ground, both the [[Earth's orbit|elliptical orbit of the Earth]] and the [[Atmospheric extinction|attenuation by the Earth's atmosphere]] have to be taken into account. The extraterrestrial solar illuminance ({{math|''E''<sub>ext</sub>}}), corrected for the elliptical orbit by using the day number of the year (dn), is given by<ref>{{cite journal|last=C. KANDILLI and K. ULGEN|title=Solar Illumination and Estimating Daylight Availability of Global Solar Irradiance|journal=Energy Sources}}</ref>
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| :<math>E_{\rm ext}= E_{\rm sc} \cdot \left(1+0.033412 \cdot \cos\left(2\pi\frac{{\rm dn}-3}{365}\right)\right),</math>
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| where dn=1 on January 1; dn=2 on January 2; dn=32 on February 1, etc. In this formula dn-3 is used, because in modern times [[Apsis#Earth's perihelion and aphelion|Earth's perihelion]], the closest approach to the Sun and, therefore, the maximum {{math|''E''<sub>ext</sub>}} 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.
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| The solar illuminance constant ({{math|''E''<sub>sc</sub>}}), is equal to 128×10<sup>3</sup> [[Lux|lx]]. The direct normal illuminance ({{math|''E''<sub>dn</sub>}}), corrected for the attenuating effects of the atmosphere is given by:
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| :<math>E_{\rm dn}=E_{\rm ext}\,e^{-cm},</math>
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| where {{mvar|c}} is the atmospheric [[Complex index of refraction|extinction coefficient]] and {{mvar|m}} is the relative optical [[airmass]].
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| ==Solar constant==
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| {{Main|Solar constant}}
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| [[File:Solar irradiance spectrum 1992.gif|thumb|Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against [[wavenumber]].]]
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| The '''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|astronomical unit (AU)]] (roughly the mean distance from the Sun to the 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 kW/m²,<ref name=TSI20>{{cite web|url=http://acrim.com/TSI%20Monitoring.htm |title=Satellite observations of total solar irradiance |publisher=Acrim.com |date= |accessdate=2012-02-12}}</ref> varying slightly with [[solar cycle|solar activity]], but recent recalibrations of the relevant satellite observations indicate a value closer to 1.361 kW/m² is more realistic.<ref name=KoppLean11>{{cite journal|last=G. Kopp|coauthors=J. Lean|title=A new, lower value of total solar irradiance: Evidence and climate significance|journal=Geophys. Res. Lett.|year=2011|pages=L01706|doi=10.1029/2010GL045777|bibcode = 2011GeoRL..3801706K|first1=Greg|volume=38 }}</ref> This radiation is about 50% infrared, 40% visible, and about 10% ultraviolet, at the top of the atmosphere.<ref name="Solar radiation"/>
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| ==Total (TSI) and spectral solar irradiance (SSI) upon Earth==
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| {{Anchor|Total Solar Irradiance|TSI}}
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| '''Total Solar Irradiance''' (TSI) – the amount of solar radiation received at the top of the Earth’s atmosphere – has been measured since 1978 by series of overlapping NASA and ESA satellite experiments to be 1.361 [[Kilo-|kilo]]watts per square meter (kW/m²). <ref name=TSI20 /><ref>Willson, R. C., and A. V. Mordvinov (2003), Secular total solar irradiance trend during solar cycles 21–23, Geophys. Res. Lett., 30(5), 1199, {{doi|10.1029/2002GL016038}} [http://www.acrim.com/Reference%20Files/Secular%20total%20solar%20irradiance%20trend%20during%20solar%20cycles%2021%E2%80%9323.pdf ACRIM]</ref><ref name=TSI>{{cite web |title=Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present |url=http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant | accessdate = 2005-10-05}}</ref><ref>http://www.acrim.com/index.htm Current Projects<!-- Bot generated title -->]</ref> TSI observations are continuing today with the [[ACRIMSAT]]/ACRIM3, [[Solar and Heliospheric Observatory|SOHO]]/VIRGO and [[SORCE]]/TIM satellite experiments.<ref>[http://www.acrim.com/Comparison%20of%20TSI%20Results.htm Graphics Gallery<!-- Bot generated title -->]</ref> Variation of TSI has been discovered on many timescales including the solar magnetic cycle <ref name="acrim">http://www.acrim.com/TSI%20Monitoring.htm</ref> and many shorter periodic cycles. <ref>http://www.acrim.com/Comparison%20of%20TSI%20Results.htm</ref> TSI provides the energy that drives the Earth's climate, so continuation of the TSI time series database is critical to understanding the role of solar variability in climate change.
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| '''Spectral Solar Irradiance''' (SSI) - The spectral distribution of the TSI - has been monitored since 2003 by the [[SORCE]] [[Spectral Irradiance Monitor]] (SIM) and has found 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”.<ref name=goddard>{{cite web|url=http://atmospheres.gsfc.nasa.gov/climate/index.php?section=136 |title=NASA Goddard Space Flight Center: Solar Radiation |publisher=Atmospheres.gsfc.nasa.gov |date=2012-02-08 |accessdate=2012-02-12}}</ref>
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| ==Intensity in the Solar System==
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| [[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 Pathfinder]]. <!-- To compensate for lower levels of sunlight, researchers often enhance images taken on the planet. -- rubbish? -->]]
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| Different 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 [http://starhop.com/library/pdf/studyguide/high/SolInt-19.pdf]):
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| {| class="wikitable"
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| |-
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| ! rowspan=2 |[[Planet]]
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| ! colspan=2 |distance ([[Astronomical unit|AU]])
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| ! colspan=2 |Solar radiation (W/m²)
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| |-
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| |[[Perihelion]]||[[Aphelion]]
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| |maximum||minimum
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| |-
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| | [[Mercury (planet)|Mercury]]
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| | 0.3075 || 0.4667
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| | 14,446 || 6,272
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| |-
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| | [[Venus]]
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| | 0.7184 || 0.7282
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| | 2,647 || 2,576
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| |-
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| | [[Earth]]
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| | 0.9833 || 1.017
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| | 1,413 || 1,321
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| |-
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| | [[Mars]]
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| | 1.382 || 1.666
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| | 715 || 492
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| |-
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| | [[Jupiter]]
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| | 4.950 || 5.458
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| | 55.8 || 45.9
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| |-
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| | [[Saturn]]
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| | 9.048 || 10.12
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| | 16.7 || 13.4
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| |-
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| | [[Uranus]]
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| | 18.38 || 20.08
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| | 4.04 || 3.39
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| |-
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| | [[Neptune]]
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| | 29.77 || 30.44
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| | 1.54 || 1.47
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| |}
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| The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an [[atmosphere]]. For example, [[Atmosphere of Venus|Venus' 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".<ref>{{cite journal |title=The Unveiling of Venus: Hot and Stifling |journal=Science News |volume=109 |issue=25 |date=1976-06-19 |page=388 |quote=100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds |jstor=3960800 |doi=10.2307/3960800}}</ref>
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| Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses, 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.
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| For comparison purposes, 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-hour]]s) 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 [[(87269) 2000 OO67]].
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| == Surface illumination ==
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| 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.
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| 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".<ref>{{cite web |url=http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf |title=Atmospheric Optics|author=[[Craig Bohren]]}}</ref>
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| See [[diffuse sky radiation]] for more details.
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| ==Climate effects ==
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| {{Further|Solar variation|Solar dimming|Insolation|Sunshine duration}}
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| On Earth, solar radiation is obvious as daylight when the sun is above the [[horizon]]. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as ''sunshine'', combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body, the ground, and other objects depends on the [[absorption (electromagnetic radiation)]] of the [[electromagnetic radiation]] in the form of [[heat]]. | |
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| The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The 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%). The total [[insolation]] remains almost constant due to [[Kepler's second law]],
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| :<math>\tfrac{2A}{r^2}dt = d\theta,</math>
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| where <math>A</math> is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant. | |
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| :<math>\int_{0}^{T} \tfrac{2A}{r^2}dt = \int_{0}^{2\pi} d\theta = \mathrm{constant}.</math>
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| 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.
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| But the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies.<ref>{{cite web|url=http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html |title=Graph of variation of seasonal and latitudinal distribution of solar radiation |publisher=Museum.state.il.us |date=2007-08-30 |accessdate=2012-02-12}}</ref> For example, at [[latitude]]s of 65 degrees, the change in solar energy in summer and winter can vary by more than 25% as a result of the 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]]s (see: [[Milankovitch cycles]]).
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| ==Past variations in solar irradiance==
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| Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many periodic cycles including the 11-year sunspot solar cycle. <ref name="acrim" /> 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.
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| Such reconstructions have been done.<ref>Wang et al. (2005). The Astrophysical Journal, Volume 625, issue 1, pages 522–538, [http://dx.doi.org/10.1086/429689 dx.doi.org/10.1086/429689].</ref><ref>Steinhilber et al. (2009), Geophysical Research Letters, Volume 36, L19704, http://dx.doi.org/10.1051/0004-6361/200811446</ref><ref>Vieira et al. (2011), Astronomy&Astrophysics, Volume 531, A6, http://dx.doi.org/10.1051/0004-6361/201015843</ref><ref>Steinhilber et al.(2012), Proceedings of the National Academy of Sciences, Early Edition http://dx.doi.org/10.1073/pnas.1118965109</ref> These studies show that solar irradiance does vary with distinct periodicities such as: 11 years (Schwabe), 88 years (Gleisberg cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle).
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| ==Life on Earth==
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| The existence of nearly all [[Organism|life on Earth]] is fueled by light from the sun. Most [[autotroph]]s, 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.
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| [[Heterotroph]]s, 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]].
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| 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.
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| 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.
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| The more recent discoveries of [[coal]], [[petroleum]] and [[natural gas]] are modern extensions of this trend. These [[fossil fuel]]s are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the 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]]).
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| ==Cultural aspects==
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| {{Unreferenced section|date=February 2011}}
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| [[File:Monet dejeunersurlherbe.jpg|thumb|[[Claude Monet]]: ''Le déjeuner sur l'herbe'']]
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| The effect of sunlight is relevant to [[painting]], evidenced for instance in works of [[Claude Monet]] on outdoor scenes and landscapes.
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| [[File:Winter Sunshine.jpg|thumb|right|200px|Winter sunshine]]
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| Many people find direct sunlight to be too [[brightness|bright]] for comfort, especially when reading from white paper upon which the sun 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 [[helmet]]s and [[cap]]s are equipped with [[visor]]s 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 [[wall]]s, [[window blind]]s, [[awning]]s, [[Window shutter|shutter]]s, [[curtain]]s, or nearby [[shade tree]]s.
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| 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|parasol]]s, and so on.
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| In [[Hinduism]] the sun is considered to be a god as it is the source of life and energy on earth.
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| ===Sunbathing===<!-- This section is linked from [[Naturism]] -->
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| 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 [[beach]]es, open air [[swimming pool]]s, [[park]]s, [[garden]]s, and [[sidewalk cafe]]s. 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]] light and can be used indoors regardless of outdoor weather conditions and amount of sunlight.
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| For many people with pale or brownish skin, one purpose for sunbathing is to darken one's [[Human skin color|skin color]] (get a [[Sun tanning|sun tan]]), as this is considered in some cultures to be beautiful, 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.
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| For people suffering from [[psoriasis]], sunbathing is an effective way of healing the symptoms.
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| [[Sun tanning|Skin tanning]] is achieved by an increase in the dark [[pigment]] inside skin cells called [[melanocyte]]s, and it is actually an automatic response mechanism of the body to sufficient exposure to [[ultraviolet]] radiation from the sun or from artificial [[Tanning bed#Tanning lamps|sunlamp]]s. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.
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| ==Effects on human health==
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| {{main|Health effects of sun exposure}}
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| The body produces [[vitamin D]] from sunlight (to be specific, from the UVB band of [[ultraviolet]] light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet.
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| [[Sunburn]] can have mild to severe [[inflammation]] effects on skin; this can be avoided by using a proper [[sunscreen]] cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called [[Photoaging|skin photodamage]]), which produces a [[Aesthetics|cosmetic]] effect that is difficult to treat. Some people are concerned that [[ozone depletion]] is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.<ref>[http://www.theozonehole.com/consequences.htm Ozone Hole Consequences] retrieved 30 October 2008</ref>
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| A lack of sunlight, on the other hand, is considered one of the primary causes of [[seasonal affective disorder]] (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) involve [[light therapy]], replicating sunlight via lamps tuned to specific wavelengths of visible light, or full-spectrum bulbs.
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| A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from [[multiple sclerosis]] (MS) later in life.<ref>{{cite web|url=http://www.neurology.org/cgi/content/abstract/69/4/381?etoc |title=NEUROLOGY 2007;69:381-388 |publisher=Neurology.org |date=2007-07-24 |accessdate=2012-02-12}}</ref>
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| In January 2014, British researchers found that sunlight may lower blood pressure, a dangerous factor for heart attacks and stroke. It was reported that 20 minutes of ultraviolet A (UVA) sunlight lowered blood pressure by a small but significant amount by dilating blood vessels and easing hypertension. The ''Journal of Investigative Dermatology'' tested 24 volunteers and found that the sun increases nitric oxide levels, a chemical linked to blood flow, and results in lowered blood pressure. This research supports the claim of [[Richard Weller]] of the [[University of Edinburgh]] and [[Martin Feelisch]] of the [[University of Southampton]], who found that people who live in the darker north have higher rates of heart disease. They concluded, "We are concerned that well-meaning advice to reduce the comparatively low numbers of deaths from skin cancer may inadvertently increase the risk of death from far higher prevalent cardiovascular disease and stroke, and goes against epidemiological data showing that sunlight exposure reduces all cause and cardiovascular mortality."<ref name="Fox-healthy sunlight">{{cite news|last=Fox|first=Maggie|title=Ahhh. Sunlight may lower your blood pressure|url=http://www.today.com/health/ahhh-sunlight-may-lower-your-blood-pressure-2D11947404|accessdate=20 January 2014|newspaper=TODAY Health|date=20 January 2014}}</ref><ref name="sunlight lowering blood pressure">{{cite news|title=Sunlight lowers blood pressure and may reduce risk of heart attack and stroke, study finds|url=http://www.heraldsun.com.au/lifestyle/health/sunlight-lowers-blood-pressure-and-may-reduce-risk-of-heart-attack-and-stroke-study-finds/story-fni0dgux-1226806300823|accessdate=20 January 2014|newspaper=Herald Sun|date=20 January 2014}}</ref><ref name=Reinberg>{{cite news|last=Reinberg|first=Steven|title=Sunlight Might Be Good for Your Blood Pressure: Study|url=http://consumer.healthday.com/circulatory-system-information-7/blood-pressure-news-70/sunlight-may-be-good-for-your-blood-pressure-683990.html|accessdate=20 January 2014|newspaper=Health Day|date=20 January 2014}}</ref>
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| ==See also==
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| {{Div col|cols=3}}
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| * [[Color temperature]]
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| * [[Coronal radiative losses]]
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| * [[Crepuscular rays]], rays of sunlight that appear to radiate from a single point in the sky.
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| * [[Daylighting]]
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| * [[Diathermancy]]
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| * [[Atmosphere of Earth]]
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| * [[Diffuse sky radiation]]
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| * [[Effect of sun angle on climate]]
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| * [[Electromagnetic radiation and health]]
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| * [[Fraunhofer lines]]
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| * [[Illuminance]]
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| * [[Insolation]]
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| * [[Irradiance]]
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| * [[Light]]
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| * [[Light beam]]
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| * [[List of light sources]]
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| * [[Moonlight]]
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| * [[Over-illumination]]
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| * [[Ozone hole]]
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| * [[Ozone layer]]
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| * [[Photic sneeze reflex]]
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| * [[Health effects of sun exposure]]
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| * [[Season]]
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| * [[Solar variation]]
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| * [[Sunshine recorder]]
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| {{Div col end}}
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| ==References==
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| {{reflist|30em}}
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| ==Further reading==
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| * [[Thom Hartmann|Hartmann, Thom]] (1998). The Last Hours of Ancient Sunlight. London: Hodder and Stoughton. ISBN 0-340-82243-0.
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| ==External links==
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| {{Commons category}}
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| *[http://www.eoearth.org/article/Solar_radiation Solar radiation - Encyclopedia of Earth]
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| <!-- *[ http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarirradiance.html http://www.ngdc.noaa.gov/stp/solar/solarirrad.html Total solar irradiance data archive 1978–2007] not found -->
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| *[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]]
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| *[http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present] by World Radiation Center, Physikalisch-Meteorologisches Observatorium Davos (pmod wrc)
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| *[http://www.macaulay.ac.uk/LADSS/papers.html?2002 A Comparison of Methods for Providing Solar Radiation Data to Crop Models and Decision Support Systems], Rivington et al.
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| *[http://www.macaulay.ac.uk/LADSS/papers.html?2005 Evaluation of three model estimations of solar radiation at 24 UK stations], Rivington et al.
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| *[http://bass2000.obspm.fr/solar_spect.php High resolution spectrum of solar radiation] from [[Paris Observatory|Observatoire de Paris]]
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| * [http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Solar_Radiation Measuring Solar Radiation] : A lesson plan from the National Science Digital Library.
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| * [http://websurf.nao.rl.ac.uk/surfbin/first.cgi Websurf astronomical information]: Online tools for calculating Rising and setting times of Sun, Moon or planet, Azimuth of Sun, Moon or planet at rising and setting, Altitude and azimuth of Sun, Moon or planet for a given date or range of dates, and more.
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| * [http://www.ecy.wa.gov/programs/eap/models/solrad.zip An Excel workbook] with a solar position and solar radiation time-series calculator; by [http://www.ecy.wa.gov/programs/eap/models.html Greg Pelletier]
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| * [http://rredc.nrel.gov/solar/spectra/am1.5/ DOE information] about the ASTM standard solar spectrum for PV evaluation.
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| * [http://www.astm.org/Standards/G173.htm ASTM Standard] for solar spectrum at ground level in the US (latitude ~ 37 degrees).
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| * [http://apod.nasa.gov/apod/ap100627.html Detailed spectrum of the sun] at [http://apod.nasa.gov/apod/archivepix.html Astronomy Picture of the Day].
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| *[http://sunmetrix.com/discover Solar Radiation Resource]
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| {{The Sun}}
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| {{Natural resources}}
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| [[Category:Sun]]
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| [[Category:Atmospheric radiation]]
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| [[Category:Climate forcing]]
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| [[Category:Natural resources]]
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| [[Category:Light]]
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| [[Category:Photovoltaics]]
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| [[Category:IARC Group 1 carcinogens]]
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| [[Category:Light sources]]
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