The Sun, as seen from low Earth orbit overlooking theInternational Space Station. This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum.
The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite forvitamin D3 synthesis and amutagen.
Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun.[3] A photon starting at the center of the Sun and changing direction every time it encounters acharged particle would take between 10,000 and 170,000 years to get to the surface.[4]
Sunlight is a key factor inphotosynthesis, the process used by plants and otherautotrophic organisms to convertlight energy, normally from the Sun, intochemical energy that can be used to synthesize carbohydrates and fuel the organisms' activities.
Daylighting is the natural lighting of interior spaces by admitting sunlight.Solar irradiance is the solar energy available from sunlight.
Researchers can measure the intensity of sunlight using asunshine recorder,pyranometer, orpyrheliometer. To calculate the amount of sunlight reaching the ground, both theeccentricity of Earth'selliptic orbit and theattenuation byEarth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by[5]
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 timesEarth's perihelion, the closest approach to the Sun and, therefore, the maximumEext 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 (Esc), is equal to 128×103lux. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:
wherec is theatmospheric extinction andm is the relative opticalairmass. 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 1astronomical unit), then the direct sunlight at Earth's surface when the Sun is at thezenith 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.[6] In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm).[7] At the top of the atmosphere, sunlight is about 30% more intense, having about 8%ultraviolet (UV),[8] with most of the extra UV consisting of biologically damaging short-wave ultraviolet.[9]
Direct sunlight has aluminous efficacy of about 93 lumens per watt ofradiant flux. This is higher than the efficacy (of source) ofartificial lighting other thanLEDs, 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 anilluminance of approximately 98,000lux (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 theSouth Pole (seeinsolation).
Dividing theirradiance of 1,050 W/m2 by the size of the Sun's disk insteradians gives an averageradiance 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 tolimb 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.[10]
Thespectrum of the Sun's solar radiation can be compared tothat of a black body[11][12] with a temperature of about 5,800 K[13] (see graph). The Sun emits EM radiation across most of theelectromagnetic spectrum. Although the radiation created in the solar core consists mostly ofx rays, internal absorption and thermalization convert these super-high-energyphotons to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, thephotosphere of the Sun does not emit much X radiation (solar X-rays), although it does emit such "hard radiations" as X-rays and evengamma rays duringsolar flares.[14] The quiet (non-flaring) Sun, including itscorona, emits a broad rangeof wavelengths:X-rays,ultraviolet,visible light,infrared, andradio waves.[15] Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.[16]
There is also a flux of gamma rays from the quiescent sun, obeying apower law between 0.5 and 2.6TeV. Some gamma rays are caused bycosmic rays interacting with the solar atmosphere, but this does not explain these findings.[17][18][19]
The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interactingneutrinos.
Solarspectral irradiance (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).Spectral distribution of sunlight. The different curves reflect 3 different equally valid ways of characterizing the same sunlight. These curves have peaks at different wavelengths, which demonstrates that the notion of a location where the "peak" amount of sunlight is emitted is not meaningful, and is not a characteristic of the light itself (but is merely an artifact of how the spectrum is represented). Percentiles offer a way of thinking about the distribution of energy which is independent of the representation. 50 percent of solar irradiance is associated with wavelengths less than about 711 nm (based on approximating sunlight by the emissions of a 5775 K blackbody).
Although thesolar corona is a source ofextreme 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 solarelectromagnetic radiation striking theEarth's atmosphere spans a range of 100 nm to about 1 mm (1,000,000 nm).[citation needed] This band of significant radiation power can be divided into five regions in increasing order ofwavelengths:[20]
Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The termultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to thehuman eye). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiationhas germicidal properties, as used ingermicidal 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 thephotochemical reaction leading to the production of theozone layer. It directly damages DNA and causessunburn.[21] In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer,[22] but is also required forvitamin D synthesis in the skin of mammals.[21]
Visible range orlight spans 380 to 700 nm.[24] 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.[25]
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 at3.40×1014 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.[26]
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.[27]
The actual brightness of sunlight that would be observed at the surface also depends on the presence and composition of anatmosphere. 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".[29]
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 enoughdiffuse 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 fullmoonlight 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 them90377 Sedna and(87269) 2000 OO67.
On Earth, the solar radiation varies with the angle of the Sun above thehorizon, 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 assunshine. The warming of the ground (and other objects) depends on theabsorption of the electromagnetic radiation in the form ofheat.
The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth'sorbit andobliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to anorbital eccentricity of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the Sun (thesemimajor axis does not significantly vary, and so the totalinsolation over a year remains almost constant due toKepler's second law,
where is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.
If we assume the solar radiation power P as a constant over time and thesolar irradiation given by theinverse-square law, we obtain also the average insolation as a constant. However, theseasonal and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.[30] Theeffect of Sun angle on climate results in the change in solar energy in summer and winter. For example, atlatitudes 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 recentice ages (see:Milankovitch cycles).
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.[31] 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.[32][33][34][35] 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).
Thesolar constant is a measure offlux density, is the amount of incoming solarelectromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of oneastronomical unit (AU) (roughly the mean distance from the Sun to Earth). The "solar constant" includes all types of solar radiation, not just thevisible light. Its average value was thought to be approximately 1,366 W/m2,[36] varying slightly withsolar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1,361 W/m2 is more realistic.[37]
Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth
Since 1978, a series of overlapping NASA and ESA satellite experiments have measuredtotal 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).[36][38][39][40] TSI observations continue with theACRIMSAT/ACRIM3,SOHO/VIRGO andSORCE/TIM satellite experiments.[41] Observations have revealed variation of TSI on many timescales, including the solar magnetic cycle[31] and many shorter periodic cycles.[42] 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 monitoredSpectral 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".[43]
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 lightphotography where the principal source of illumination is sunlight as mediated by the atmosphere.
While the color of the sky is usually determined byRayleigh 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".[44]
Spectral composition of sunlight at Earth's surface
The Sun may be said toilluminate, 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,[45] and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates anequal-energy illuminant for most of this range.[46] 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.[47][48]
Spectrum of the visible wavelengths at approximately sea level; illumination by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. The yellow line shows the power spectrum of direct sunlight under optimal conditions. To aid comparison, the other illumination conditions are scaled by the factor shown in the key so they match at about 470 nm (blue light).
Sunlight penetrating through aforest canopy in Germany
The existence of nearly alllife on Earth is fueled by light from the Sun. Mostautotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known asphotosynthesis. 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 ascellular respiration.
Inprehistory, 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 theNeolithic 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 andnutrients 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 bygrasses 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.
The effect of sunlight is relevant topainting, evidenced for instance in works ofÉdouard Manet andClaude Monet on outdoor scenes and landscapes.
Téli verőfény ("Winter Sunshine") byLászló Mednyánszky, early 20th century
Many people find direct sunlight to be toobright for comfort; indeed, looking directly at the Sun can cause long-term vision damage.[49] To compensate for the brightness of sunlight, many people wearsunglasses.Cars, manyhelmets andcaps are equipped withvisors 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 ofwalls,window blinds,awnings,shutters,curtains, or nearbyshade trees. Sunshine exposure isneeded biologically for the production ofVitamin D in the skin, a vital compound needed to make strong bone and muscle in the body.
Sunbathing is a popularleisure 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 includebeaches, open airswimming pools,parks,gardens, andsidewalk cafes. Sunbathers typically wear limited amounts of clothing or some simply gonude. For some, an alternative to sunbathing is the use of asunbed that generatesultraviolet 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'sskin 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 prefernaked sunbathing so that an "all-over" or "even" tan can be obtained, sometimes as part of a specific lifestyle.
Controlledheliotherapy, or sunbathing, has been used as a treatment forpsoriasis and other maladies.
Skin tanning is achieved by an increase in the dark pigment inside skin cells calledmelanocytes, and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the Sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.
Theultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source ofvitamin D3 and amutagen.[50] A dietary supplement can supplyvitamin D without this mutagenic effect,[51] 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[52] and possibly inhibiting the growth of some cancers.[53][54] Sun exposure has also been associated with the timing ofmelatonin synthesis, maintenance of normalcircadian rhythms, and reduced risk ofseasonal affective disorder.[55]
UV rays, and therefore sunlight and sunlamps, are the only listedcarcinogens that are known to have health benefits,[57] 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.[58] 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".[59] 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.[59]
Elevated solarUV-B doses increase the frequency ofDNArecombination inArabidopsis thaliana and tobacco (Nicotiana tabacum) plants.[60] 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 affectsgenome stability in plants.
^Calculated from data in"Reference Solar Spectral Irradiance: Air Mass 1.5". National Renewable Energy Laboratory.Archived from the original on September 28, 2013. Retrieved2009-11-12. 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.
^See video referenced in the sentence "For more details about the comparison of the black body with the AM0 spectrum, see this video" atPietro Altermatt."The Extraterrestrial Spectrum".PV Lighthouse. PV Lighthouse Pty. Ltd.
^Naylor, Mark; Kevin C. Farmer (1995)."Sun damage and prevention".Electronic Textbook of Dermatology. The Internet Dermatology Society. Archived fromthe original on 2008-07-05. Retrieved2008-06-02.
^"The Unveiling of Venus: Hot and Stifling".Science News.109 (25):388–389. 1976-06-19.doi:10.2307/3960800.JSTOR3960800.100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds
^Buser, Pierre A.; Imbert, Michel (1992).Vision. MIT Press. p. 50.ISBN978-0-262-02336-8. Retrieved11 October 2013.Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.
^MacEvoy, Bruce (2008).color vision.Archived from the original on 24 September 2015. Retrieved27 August 2015.Noon sunlight (D55) has a nearly flat distribution...
^Wyszecki, Günter; Stiles, W. S. (1967).Color Science: Concepts and Methods, Quantitative Data and Formulas. John Wiley & Sons. p. 8.
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.
