sunlight#Solar constant

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|E_ext}}), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by{{cite journal|author=C. KANDILLI|author2=K. ULGEN|name-list-style=amp|title=Solar Illumination and Estimating Daylight Availability of Global Solar Irradiance|journal=Energy Sources}}

:$E\_\{\backslash rm\; ext\}=\; E\_\{\backslash rm\; sc\}\; \backslash cdot\; \backslash left(1+0.033412\; \backslash cdot\; \backslash cos\backslash left(2\backslash pi\backslash frac\{\{\backslash rm\; dn\}-3\}\{365\}\backslash right)\backslash right),$

where dn=1 on January 1; dn=32 on February 1; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dn–3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, therefore, the maximum {{math|E_ext}} 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|E_sc}}), is equal to 128×10³ lux. The direct normal illuminance ({{math|E_dn}}), corrected for the attenuating effects of the atmosphere is given by:

:$E\_\{\backslash rm\; dn\}=E\_\{\backslash rm\; ext\}\backslash ,e^\{-cm\},$

where {{mvar|c}} is the atmospheric extinction and {{mvar|m}} is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100,000 lux.

The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1,367 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 1,050 W/m², but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1,120 W/m².{{cite web|title=Introduction to Solar Radiation|url= http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|publisher= Newport Corporation|url-status= live|archive-url= https://web.archive.org/web/20131029234117/http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|archive-date=October 29, 2013}} 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).Calculated from data in {{cite web|url=https://www.nrel.gov/grid/solar-resource/spectra.html |title=Reference Solar Spectral Irradiance: Air Mass 1.5|access-date=2009-11-12|url-status=live|archive-url=https://web.archive.org/web/20130928011257/http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.xls|archive-date=September 28, 2013|publisher=National Renewable Energy Laboratory}}
The first of each set of two figures is for total solar radiation reaching a panel aimed at the Sun (which is 42° above the horizon), whereas the second figure of each pair is the "direct plus circumsolar" radiation (circumsolar meaning coming from the part of the sky within a couple degrees of the Sun). The totals, from 280 to 4000 nm, are 1000.4 and 900.1 W/m² respectively. It would be good to have more direct figures from a good source, rather than summing thousands of numbers in a database. At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV),Calculated from the ASTM spectrum cited above. with most of the extra UV consisting of biologically damaging short-wave ultraviolet.

{{cite book |last=Qiang |first=Fu |chapter=Radiation (Solar) |chapter-url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |editor1-last=Holton |editor1-first=James R. |title=Encyclopedia of atmospheric sciences |volume=5 |publisher=Academic Press |location=Amsterdam |date=2003 |pages=1859–1863 |oclc=249246073 |isbn=978-0-12-227095-6 |url-status=live |archive-url=https://web.archive.org/web/20121101070344/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |archive-date=2012-11-01 }}

{{vanchor|Direct sunlight}} has a luminous efficacy of about 93 lumens per watt of radiant flux. This is higher than the efficacy (of source) of artificial lighting other than LEDs, which means using sunlight for illumination heats up a room less than fluorescent or incandescent lighting. Multiplying the figure of 1,050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides 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 considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation).

Dividing the irradiance of 1,050 W/m² by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. (However, the radiance at the center of the Sun's disk is somewhat higher than the average over the whole disk due to limb darkening.) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m².{{cite book | title=Introduction to Optics | publisher=Prentice Hall | last=Pedrotti & Pedrotti | date=1993 | isbn=0135015456 | url-access=registration | url=https://archive.org/details/introductiontoop00pedr }}

File:Solar spectrum compared to black-body.gif

{{See also|Ultraviolet|Infrared|Light}}

The spectrum of the Sun's solar radiation can be compared to that of a black body{{cite journal | doi = 10.1038/156534b0 | volume=156 | title=Departure of Long-Wave Solar Radiation from Black-Body Intensity | year=1945 | journal=Nature | pages=534–535 | last1 = Appleton | first1 = Edward V.| issue=3966 | bibcode=1945Natur.156..534A | s2cid=4092179 }}Iqbal, M., "An Introduction to Solar Radiation", Academic Press (1983), Chap. 3 with a temperature of about 5,800 K[http://solarsystem.nasa.gov/planets/profile.cfm?Display=Facts&Object=Sun NASA Solar System Exploration – Sun: Facts & Figures] {{webarchive|url=https://web.archive.org/web/20150703111716/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts |date=2015-07-03 }} retrieved 27 April 2011 "Effective Temperature ... 5777 K" (see graph). The Sun emits EM radiation across most of the electromagnetic spectrum. Although the radiation created in the solar core consists mostly of x rays, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, the photosphere of the Sun does not emit much X radiation (solar X-rays), although it does emit such "hard radiations" as X-rays and even gamma rays during solar flares.{{cite web|url=http://www.nasa.gov/mission_pages/GLAST/news/highest-energy.html|title=Fermi Detects Solar Flare's Highest-Energy Light|first=Rob|last=Garner|date=24 January 2017|access-date=25 January 2018|url-status=live|archive-url=https://web.archive.org/web/20170517060027/https://www.nasa.gov/mission_pages/GLAST/news/highest-energy.html|archive-date=17 May 2017}} The quiet (non-flaring) Sun, including its corona, emits a broad range

of wavelengths: X-rays, ultraviolet, visible light, infrared, and radio waves.{{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 |access-date= 2012-02-12 |url-status= live |archive-url= https://web.archive.org/web/20120229041535/http://www.windows2universe.org/sun/spectrum/multispectral_sun_overview.html |archive-date= 2012-02-29 }} Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.See video referenced in the sentence "For more details about the comparison of the black body with the AM0 spectrum, see this video" at {{cite web |last1=Pietro Altermatt |title=The Extraterrestrial Spectrum |url=https://pvlighthouse.com.au/cms/lectures/altermatt/solar_spectrum/blackbody-radiation |website=PV Lighthouse |publisher=PV Lighthouse Pty. Ltd.}}

There is also a flux of gamma rays from the quiescent Sun, obeying a power law between 0.5 and 2.6 TeV. Some gamma rays are caused by cosmic rays interacting with the solar atmosphere, but this does not explain these findings.{{cite journal |last1=Ryan Wilkinson |title=Record-Breaking Detection of Solar Photons |journal=Physics |date=Aug 3, 2023 |volume=16 |doi=10.1103/Physics.16.s107 |bibcode=2023PhyOJ..16.s107W |s2cid=260763644 |url=https://physics.aps.org/articles/v16/s107|doi-access=free }}{{cite journal |last1=Leah Crane |title=Astronomers have spotted inexplicably bright light coming from the sun |journal=New Scientist |date=Aug 3, 2023 |url=https://www.newscientist.com/article/2386042-astronomers-have-spotted-inexplicably-bright-light-coming-from-the-sun/}}{{cite journal |last1=A. Albert |title=Discovery of Gamma Rays from the Quiescent Sun with HAWC |journal=Phys. Rev. Lett. |date=Aug 3, 2023 |volume=131 |issue=5 |page=051201 |doi=10.1103/PhysRevLett.131.051201|pmid=37595214 |arxiv=2212.00815 |bibcode=2023PhRvL.131e1201A |s2cid=254221151 }}

The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting neutrinos.

File:Solar spectrum en.svg (watts per square metre per nanometre) above atmosphere (yellow) and at surface (red). Extreme UV and X-rays are produced (left of wavelength range) but comprise very small amounts of the Sun's total output power (area under the curve).]]

File:Spectral Distribution of Sunlight.svg

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 spans a range of 100 nm to about 1 mm (1,000,000 nm).{{Citation needed|reason=Specific information with no sourcing|date=March 2017}} This band of significant radiation power can be divided into five regions in increasing order of wavelengths:{{cite web

|last = Naylor

|first = Mark

|author2 = Kevin C. Farmer

|title = Sun damage and prevention

|work = Electronic Textbook of Dermatology

|publisher = The Internet Dermatology Society

|date = 1995

|url = http://www.telemedicine.org/sundam/sundam2.4.1.html

|access-date = 2008-06-02

|url-status = dead

|archive-url = https://web.archive.org/web/20080705111726/http://telemedicine.org/sundam/sundam2.4.1.html

|archive-date = 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). 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.{{cite journal | title = Sunlight and Vitamin D: A global perspective for health. | vauthors = Wacker M, Holick, MF | date = 2013 | journal = Dermato-Endocrinology | volume = 5 | issue = 1 | pages = 51–108 | doi = 10.4161/derm.24494 | pmid = 24494042 | pmc=3897598}} In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer,{{Cite web|title=Radiation: Ultraviolet (UV) radiation

| date = 9 March 2016

| author= World Health Organization

|url=https://www.who.int/news-room/questions-and-answers/item/radiation-ultraviolet-(uv)|access-date=2023-02-08|language=en}} but is also required for vitamin D synthesis in the skin of mammals.

• 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.{{cite journal|title=Ultraviolet Radiation Exposure and Its Impact on Skin Cancer Risk|date=1 August 2017|pmc = 5036351|last1 = Watson|first1 = M.|last2 = Holman|first2 = D. M.|last3 = Maguire-Eisen|first3 = M.|journal = Seminars in Oncology Nursing|volume = 32|issue = 3|pages = 241–254|doi = 10.1016/j.soncn.2016.05.005|pmid = 27539279}}
• Visible range or light spans 380 to 700 nm.{{Cite web | url=https://science.nasa.gov/ems/09_visiblelight |title = Visible Light {{pipe}} Science Mission Directorate| date=10 August 2016 }} As the name suggests, this range is visible to the naked eye.
• Infrared range that spans 700 nm to 1,000,000 nm (1 mm). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
• Infrared-A: 700 nm to 1,400 nm
• Infrared-B: 1,400 nm to 3,000 nm
• Infrared-C: 3,000 nm to 1 mm.

The sunlight reaching Earth's surface is 49.4% infrared, 42.3% visible, and 8% ultraviolet.{{cite web |title=Solar Radiation and Photosynethically Active Radiation |url=https://www.fondriest.com/environmental-measurements/parameters/weather/solar-radiation/ |publisher=Fondriest Environmental |access-date=7 March 2025 |date=March 21, 2014}}

It is sometimes asserted that the Sun's maximum output is in the visible range. However, this statement is a misconception based on only seeing the solar spectral irradiance plotted on a per-wavelength basis. When plotted that way, the power spectral density of sunlight peaks at a wavelength of about 501 nm, which is in the visible range. However, the solar spectral irradiance can with equal validity be calculated on a per-frequency basis, in which case the maximum is at {{val|3.40e14|u=Hz}}, corresponding to a wavelength of about 882 nm, which is in the near infrared (Infrared-A) range. Counterintuitively, it is not meaningful to assert that the solar output is greatest at some precise location in the spectrum.{{cite web |last1=Mobley |first1=Curtis |title=A Common Misconception |url=https://www.oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception |website=Ocean Optics |access-date=6 March 2025}}

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.

{{cite web

|url = https://www.fs.usda.gov/pnw/pubs/pnw_rp142.pdf

|title = Direct Solar Radiation On Various Slopes From 0 To 60 Degrees North Latitude

|author = John Buffo

|author2 = Leo J. Fritschen

|author3 = James L. Murphy

|publisher = Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA

|date = 1972

|access-date = 15 Jan 2014

|url-status = live

|archive-url = https://web.archive.org/web/20131127124158/http://www.fs.fed.us/pnw/pubs/pnw_rp142.pdf

|archive-date = 2013-11-27

}}

File:Mars sunset PIA00920.jpg. ]]

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from the Sun.

A table comparing the amount of solar radiation received by each planet in the Solar System at the top of its atmosphere:{{cite web|archive-url=https://web.archive.org/web/20091122194548/http://starhop.com/library/pdf/studyguide/high/SolInt-19.pdf|archive-date=2009-11-22|url=http://starhop.com/library/pdf/studyguide/high/SolInt-19.pdf|url-status=dead|title=Solar Intensity|publisher=McAuliffe-Shepard Discovery Center}}

The actual brightness of sunlight that would be observed at the surface also 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".{{cite journal |title=The Unveiling of Venus: Hot and Stifling |journal=Science News |volume=109 |issue=25 |pages=388–389 |date=1976-06-19 |quote=100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds |jstor=3960800 |doi=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. 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; only a handful of objects in the Solar System have been discovered that are known to orbit farther than such a distance, among them 90377 Sedna and {{mpl|(87269) 2000 OO|67}}.

{{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,

:$\backslash tfrac\{2A\}\{r^2\}dt\; =\; d\backslash theta,$

where $A$ is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.

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

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. However, the seasonal and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.{{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 |access-date=2012-02-12 |url-status=live |archive-url=https://web.archive.org/web/20120112043906/http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html |archive-date=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).

{{Further|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 time scales, including the 11-year sunspot solar cycle. When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years.

Such reconstructions have been done.{{cite journal | title = Modeling the Sun's Magnetic Field and Irradiance since 1713 | last1 = Wang | display-authors = etal | date = 2005 | journal = The Astrophysical Journal | volume = 625 | issue = 1| pages = 522–538 | doi = 10.1086/429689 | bibcode=2005ApJ...625..522W| doi-access = free }}{{cite journal | title = Total solar irradiance since 1996: is there a long-term variation unrelated to solar surface magnetic phenomena? | last1 = Steinhilber | display-authors = etal | date = 2009 | url =https://www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A6539/datastream/PDF/view | journal = Geophysical Research Letters | volume = 36 | page = L19704 | doi = 10.1051/0004-6361/200811446 | bibcode=2010A&A...523A..39S| doi-access = free }}{{cite journal | title = Evolution of the solar irradiance during the Holocene | last1 = Vieira | display-authors = etal | date = 2011 | journal = Astronomy & Astrophysics | volume = 531 | page = A6 | doi = 10.1051/0004-6361/201015843 | bibcode=2011A&A...531A...6V|arxiv = 1103.4958 | s2cid = 119190565 }}{{cite journal | title = 9,400 years of cosmic radiation and solar activity from ice cores and tree rings | last1 = Steinhilber | display-authors = etal | date = 2012 | journal = Proceedings of the National Academy of Sciences | volume = 109| issue = 16| pages = 5967–5971| doi = 10.1073/pnas.1118965109 |bibcode = 2012PNAS..109.5967S | pmid=22474348 | pmc=3341045| url = http://epic.awi.de/30297/1/PNAS-2012-Steinhilber-1118965109.pdf| doi-access = free }} These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activity varies with longer cycles, such as the proposed 88 year (Gleisberg cycle), 208 year (DeVries cycle) and 1,000 year (Eddy cycle).

{{Main|Solar irradiance}}

File:Solar irradiance spectrum 1992.gif]]

{{Main|Solar constant}}

The solar constant is 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 1,366 W/m²,{{cite web |url=http://acrim.com/TSI%20Monitoring.htm |title=Satellite observations of total solar irradiance |publisher=Acrim.com |access-date=2012-02-12 |url-status=live |archive-url=https://web.archive.org/web/20030204191816/http://acrim.com/TSI%20Monitoring.htm |archive-date=2003-02-04 }} varying slightly with solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1,361 W/m² is more realistic.{{cite journal|last1=G. Kopp|author2=J. Lean |title=A new, lower value of total solar irradiance: Evidence and climate significance|journal=Geophys. Res. Lett.|date=2011|pages=L01706|doi=10.1029/2010GL045777|bibcode = 2011GeoRL..38.1706K|first1=Greg|volume=38 |issue=1 |doi-access=free}}

{{Anchor|Total Solar Irradiance|TSI}}

Since 1978, a series of overlapping NASA and ESA satellite experiments have measured total solar irradiance (TSI) – the amount of solar radiation received at the top of Earth's atmosphere – as 1.365 kilo⁠watts per square meter (kW/m²).{{cite journal | last1 = Willson | first1 = R. C. | last2 = Mordvinov | first2 = A. V. | year = 2003 | title = Secular total solar irradiance trend during solar cycles 21–23 | journal = Geophys. Res. Lett. | volume = 30 | issue = 5 | page = 1199 | doi = 10.1029/2002GL016038 | bibcode = 2003GeoRL..30.1199W | doi-access = free }}{{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 |access-date= 2005-10-05 |url-status= dead |archive-url= https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant |archive-date= 2011-08-30 }}{{cite web|url= http://www.acrim.com/index.htm|title= Current Projects|website= www.acrim.com|access-date= 25 January 2018|url-status= live|archive-url= https://web.archive.org/web/20171016112445/http://www.acrim.com/Index.htm|archive-date= 16 October 2017}} TSI observations continue with the ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM satellite experiments.{{cite web|url= http://www.acrim.com/Comparison%20of%20TSI%20Results.htm|website= ACRIM.com|title= Comparison: ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM results|access-date= 25 January 2018|url-status= live|archive-url= https://web.archive.org/web/20171016112535/http://www.acrim.com/Comparison%20of%20TSI%20Results.htm|archive-date= 16 October 2017}} Observations have revealed variation of TSI on many timescales, including the solar magnetic cycle{{cite web |url= http://www.acrim.com/TSI%20Monitoring.htm |title= Graphics Gallery |publisher= Acrim.com |access-date= 2014-04-21 |url-status= live |archive-url= https://web.archive.org/web/20140502013019/http://acrim.com/TSI%20Monitoring.htm |archive-date= 2014-05-02 }} and many shorter periodic cycles.{{cite web |url= http://www.acrim.com/Comparison%20of%20TSI%20Results.htm |title= Comparison: ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM results |website= ACRIM.com |access-date= 2013-03-14 |url-status= live |archive-url= https://web.archive.org/web/20130530223744/http://www.acrim.com/Comparison%20of%20TSI%20Results.htm |archive-date= 2013-05-30 }} TSI provides the energy that drives Earth's climate, so continuation of the TSI time-series database is critical to understanding the role of solar variability in climate change.

Since 2003, the SORCE Spectral Irradiance Monitor (SIM) has monitored Spectral solar irradiance (SSI) – the spectral distribution of the TSI. Data indicate that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth's climate responses than earlier assumed, fueling broad avenues of new research in "the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth's climate".

{{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

|access-date= 2012-02-12 |url-status= dead

|archive-url= http://archive.wikiwix.com/cache/20110920123747/http://atmospheres.gsfc.nasa.gov/climate/index.php?section=136

|archive-date= 2011-09-20

}}

{{see also|Diffuse sky radiation}}

File:Sun_over_Lake_Hawea,_New_Zealand.jpgs, giving rise to crepuscular rays]]

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere.

While the color of the sky is usually determined by Rayleigh scattering, an exception occurs at sunset and twilight. "Preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".{{cite web|url=http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf|title=Atmospheric Optics|author=Craig Bohren|author-link=Craig Bohren|url-status=live|archive-url=https://web.archive.org/web/20131206175627/http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf|archive-date=2013-12-06}}

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 400–700 nm,{{cite book|last1=Buser|first1=Pierre A.|last2=Imbert|first2=Michel|title=Vision|url=https://archive.org/details/vision0000buse|url-access=registration|access-date=11 October 2013|date=1992|publisher=MIT Press|isbn=978-0-262-02336-8|page=[https://archive.org/details/vision0000buse/page/50 50]|quote=Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.}} 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.{{cite book|last1=MacEvoy|first1=Bruce|title=color vision|date=2008|url=http://www.handprint.com/HP/WCL/color1.html|access-date=27 August 2015|quote=Noon sunlight (D55) has a nearly flat distribution...|url-status=live|archive-url=https://web.archive.org/web/20150924024814/http://www.handprint.com/HP/WCL/color1.html|archive-date=24 September 2015}} The useful range for color vision in humans, for example, is approximately 450–650 nm. Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate. When illumination is indirect, Rayleigh scattering in the upper atmosphere will lead blue wavelengths to dominate. Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb particular wavelengths.{{cite book|last1=Wyszecki|first1=Günter|last2=Stiles|first2=W. S.|title=Color Science: Concepts and Methods, Quantitative Data and Formulas|date=1967|publisher=John Wiley & Sons|page=8}}{{cite book|last1=MacAdam|first1=David L.|title=Color Measurement: Theme and Variations|url=https://archive.org/details/colormeasurement00ddav|url-access=limited|edition=Second Revised|date=1985|isbn=0-387-15573-2|publisher=Springer|pages=[https://archive.org/details/colormeasurement00ddav/page/n46 33]–35}}

File:Spectrum of Sunlight en.svg

File:Dülmen, Göversheide -- 2015 -- 7718-22.jpg in Germany]]

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, use light from the Sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products, or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as cellular respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals that had previously provided humans with only meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans. Fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within Earth for millions of years.

File:Edouard Manet - Luncheon on the Grass - Google Art Project.jpg: Le déjeuner sur l'herbe (1862–63)]]

The effect of sunlight is relevant to painting, evidenced for instance in works of Édouard Manet and Claude Monet on outdoor scenes and landscapes.

File:Winter Sunshine.jpg, early 20th century]]

Many people find direct sunlight to be too bright for comfort; indeed, looking directly at the Sun can cause long-term vision damage.{{Cite journal |last1=Chawda |first1=Dishita |last2=Shinde |first2=Pranaykumar |date=2022-10-29 |title=Effects of Solar Radiation on the Eyes |journal=Cureus |volume=14 |issue=10 |pages=e30857 |language=en |doi=10.7759/cureus.30857 |doi-access=free |issn=2168-8184 |pmc=9709587 |pmid=36465785}} 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 production of Vitamin D in the skin, a vital compound needed to make strong bone and muscle in the body.

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. The Sun was also considered to be a god in Ancient Egypt.

===Sunbathing===

{{Main|Sun tanning}}

{{Unreferenced section|date=January 2015}}

File:YBF 2010 ja Bikini Bar.jpg

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{{Citation |last=Nguyen |first=Tien V. |title=Ultraviolet Therapy for Psoriasis |date=2014 |work=Advances in Psoriasis |pages=91–110 |editor-last=Weinberg |editor-first=Jeffrey M. |url=https://link.springer.com/chapter/10.1007/978-1-4471-4432-8_8 |access-date=2025-04-23 |place=London |publisher=Springer London |language=en |doi=10.1007/978-1-4471-4432-8_8 |isbn=978-1-4471-4431-1 |last2=Koo |first2=John Y. M. |editor2-last=Lebwohl |editor2-first=Mark|url-access=subscription }} and other maladies.{{Cite journal |last=Alora |first=M. B. T. |last2=Fitzpatrick |first2=T. B. |last3=Taylor |first3=C. R. |date=1997-10-12 |title=Total body heliotherapy |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1600-0781.1997.tb00225.x |journal=Photodermatology, Photoimmunology & Photomedicine |language=en |volume=13 |issue=5-6 |pages=178–180 |doi=10.1111/j.1600-0781.1997.tb00225.x |issn=0905-4383|url-access=subscription }}

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.{{Cite book |last=Ahmad |first=Shamim I. |title=Ultraviolet Light in Human Health, Diseases and Environment |date=2017 |publisher=Springer International Publishing AG |isbn=978-3-319-56017-5 |series=Advances in Experimental Medicine and Biology |location=Cham}} Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

{{Main|Health effects of sunlight exposure}}

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D₃ and a mutagen.{{cite journal | pmid = 12174089 | volume=147 | issue=2 | title=Vitamin D and systemic cancer: is this relevant to malignant melanoma? |date=August 2002 | journal=Br. J. Dermatol. | pages=197–213 | author=Osborne JE | author2=Hutchinson PE | doi = 10.1046/j.1365-2133.2002.04960.x| s2cid=34388656 }} A dietary supplement can supply vitamin D without this mutagenic effect,{{cite web|url=http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp|title=Dietary Supplement Fact Sheet: Vitamin D|publisher=Office of Dietary Supplements, National Institutes of Health|url-status=live|archive-url=https://web.archive.org/web/20070716065832/http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp|archive-date=2007-07-16}} 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{{cite journal | display-authors = 4| author = Cranney A| author2 = Horsley T| author3 = O'Donnell S| author4 = Weiler H| author5 = Puil L| author6 = Ooi D| author7 = Atkinson S| author8 = Ward L| author9 = Moher D| author10 = Hanley D| author11 = Fang M| author12 = Yazdi F| author13 = Garritty C| author14 = Sampson M| author15 = Barrowman N| author16 = Tsertsvadze A| author17 = Mamaladze V | title = Effectiveness and safety of vitamin D in relation to bone health | journal = Evidence Report/Technology Assessment | issue = 158 | pages = 1–235 | date = August 2007 | pmid = 18088161 | pmc=4781354}} and possibly inhibiting the growth of some cancers.{{cite journal | display-authors = 4| author = John E| author2 = Schwartz G| author3 = Koo J| author4 = Van Den Berg D| author5 = Ingles S | title = Sun Exposure, Vitamin D Receptor Gene Polymorphisms, and Risk of Advanced Prostate Cancer | journal = Cancer Research | volume = 65 | issue = 12 | pages = 5470–5479 | date = June 15, 2005 | doi=10.1158/0008-5472.can-04-3134 | pmid=15958597| doi-access = free }}{{cite journal | author = Egan K | author2 = Sosman J | author3 = Blot W | title = Sunlight and Reduced Risk of Cancer: Is The Real Story Vitamin D? | journal = J Natl Cancer Inst | volume = 97 | issue = 3 | pages = 161–163 | date = February 2, 2005 | doi = 10.1093/jnci/dji047 | pmid = 15687354 | doi-access = free }} Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.{{cite journal |author=Mead MN |title=Benefits of sunlight: a bright spot for human health |journal=Environmental Health Perspectives |volume=116 |issue=4 |pages=A160–A167 |date=April 2008 |pmid=18414615 |pmc=2290997 |doi=10.1289/ehp.116-a160}}

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.{{cite journal |author=Lucas RM|author2=Repacholi MH|author3=McMichael AJ |title=Is the current public health message on UV exposure correct? |journal=Bulletin of the World Health Organization |volume=84 |issue=6 |pages=485–491 |date=June 2006 |pmid=16799733 |pmc=2627377 |doi=10.2471/BLT.05.026559}} 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,{{cite web |url=http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |title=13th Report on Carcinogens: Ultraviolet-Radiation-Related Exposures |publisher=National Toxicology Program |date=October 2014 |access-date=2014-12-22 |url-status=dead |archive-url=https://web.archive.org/web/20141222172504/http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |archive-date=2014-12-22 }} and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little.{{cite web |url=http://www.cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf |title=Risks and Benefits |access-date=2010-05-13 |url-status=live |archive-url=https://web.archive.org/web/20101120224439/http://cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf |archive-date=2010-11-20 }} There is a general consensus that sunburn should always be avoided.

Epidemiological data shows that people who have more exposure to sunlight have less high blood pressure and cardiovascular-related mortality. While sunlight (and its UV rays) are a risk factor for skin cancer, "sun avoidance may carry more of a cost than benefit for over-all good health".{{cite journal|last1=Weller|first1=RB|title=Sunlight Has Cardiovascular Benefits Independently of Vitamin D.|journal=Blood Purification|date=2016|volume=41|issue=1–3|pages=130–4|doi=10.1159/000441266|pmid=26766556|hdl=20.500.11820/8f7d93d4-db22-418d-a1cc-3dbf9ddad8c3|s2cid=19348056|hdl-access=free}} A study found that there is no evidence that UV reduces lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure.

Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.{{cite journal |vauthors=Ries G, Heller W, Puchta H, Sandermann H, Seidlitz HK, Hohn B |title=Elevated UV-B radiation reduces genome stability in plants |journal=Nature |volume=406 |issue=6791 |pages=98–101 |year=2000 |pmid=10894550 |doi=10.1038/35017595 |bibcode=2000Natur.406...98R |s2cid=4303995 |url=https://publikationen.bibliothek.kit.edu/1000016569 }} 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.

• Color temperature
• Coronal radiative losses
• Diathermancy
• Fraunhofer lines
• List of cities by sunshine duration
• Moonlight
• Light pollution
• Photic sneeze reflex
• Photosynthesis
• Starlight
• {{annotated link|Sunbeam}}

{{Reflist|30em}}

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

• {{Commons category-inline|Sunlight}}
• [http://www.eoearth.org/article/Solar_radiation Solar radiation – Encyclopedia of Earth]

• [https://www.ncei.noaa.gov/products/space-weather/legacy-data/total-solar-irradiance Total Solar Irradiance (TSI) Daily mean data] at the website of the National Geophysical Data Center
• [https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present] by World Radiation Center, Physikalisch-Meteorologisches Observatorium Davos (pmod wrc)
• [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.
• [http://www.macaulay.ac.uk/LADSS/papers.html?2005 Evaluation of three model estimations of solar radiation at 24 UK stations], Rivington et al.
• [http://bass2000.obspm.fr/solar_spect.php High resolution spectrum of solar radiation] from Observatoire de Paris
• [https://web.archive.org/web/20040213233926/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.
• [https://web.archive.org/web/20060423090912/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.
• [https://web.archive.org/web/20070305050844/http://www.ecy.wa.gov/programs/eap/models/solrad.zip An Excel workbook] with a solar position and solar radiation time-series calculator; by [https://web.archive.org/web/20070525051436/http://www.ecy.wa.gov/programs/eap/models.html Greg Pelletier]
• [http://www.astm.org/Standards/G173.htm ASTM Standard] for solar spectrum at ground level in the US (latitude ~37 degrees).
• [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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