Sunlight
Sunlight is a portion of the
The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for
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 a charged particle would take between 10,000 and 170,000 years to get to the surface.[4]
Sunlight is a key factor in
Daylighting is the natural lighting of interior spaces by admitting sunlight. Solar irradiance is the solar energy available from sunlight.
Measurement
Researchers can measure the intensity of sunlight using a
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 Eext occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.
The solar illuminance constant (Esc), is equal to 128×103 lux. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:
where c is the
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.[6] 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).[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 a
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.[10]
Composition and power
The
, emits a broad range of wavelengths:There is also a flux of gamma rays from the quiescent sun, obeying a
The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting neutrinos.
Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all solar electromagnetic radiation striking the Earth's atmosphere 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 of wavelengths:[20]
- 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.[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 for vitamin D synthesis in the skin of mammals.[21]
- Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once[free radicals and reactive oxygen species), and can cause cancer.[23]
- Visible range or light spans 380 to 700 nm.[24]As the name suggests, this range is visible to the naked eye. It is also the strongest output range of the Sun's total irradiance spectrum.
- 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.
Published tables
Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.[25]
Intensity in the Solar System
Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun.
A table comparing the amount of solar radiation received by each planet in the Solar System at the top of its atmosphere:[26]
Planet or dwarf planet | distance (AU) | Solar radiation (W/m2) | ||
---|---|---|---|---|
Perihelion |
Aphelion
|
maximum | minimum | |
Mercury | 0.3075 | 0.4667 | 14,446 | 6,272 |
Venus | 0.7184 | 0.7282 | 2,647 | 2,576 |
Earth | 0.9833 | 1.017 | 1,413 | 1,321 |
Mars | 1.382 | 1.666 | 715 | 492 |
Jupiter | 4.950 | 5.458 | 55.8 | 45.9 |
Saturn | 9.048 | 10.12 | 16.7 | 13.4 |
Uranus | 18.38 | 20.08 | 4.04 | 3.39 |
Neptune | 29.77 | 30.44 | 1.54 | 1.47 |
Pluto | 29.66 | 48.87 | 1.55 | 0.57 |
The actual brightness of sunlight that would be observed at the surface 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".[27]
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
Variations in solar irradiance
Seasonal and orbital variation
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
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 the
Solar intensity 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.[29] 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.[30][31][32][33] 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).
Solar irradiance
Solar constant
The solar constant is a measure of
Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth
Since 1978, a series of overlapping NASA and ESA satellite experiments have measured
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".[41]
Surface illumination and spectrum
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".[42]
See diffuse sky radiation for more details.
Spectral composition of sunlight at Earth's surface
The Sun may be said to illuminate, which is a measure of the light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700 nm,[43] 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.[44] 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.[45][46]
Life on Earth
The existence of nearly all life on Earth is fueled by light from the Sun. Most autotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building-blocks and in other synthetic pathways that allow the organism to grow.
Heterotrophs, such as animals, 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
Cultural aspects
The effect of sunlight is relevant to painting, evidenced for instance in works of Édouard Manet and Claude Monet on outdoor scenes and landscapes.
Many people find direct sunlight to be too
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
Sunbathing is a popular
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
Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes, and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the Sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.
Effects on human health
The
Long-term sunlight exposure is known to be associated with the development of
UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits,[55] 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.[56] 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".[57] 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.[57]
Effect on plant genomes
Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.[58] 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.
See also
- Color temperature
- Coronal radiative losses
- Diathermancy
- Fraunhofer lines
- List of cities by sunshine duration
- Moonlight
- Overillumination
- Photic sneeze reflex
- Photosynthesis
- Starlight
- Sunbeam – Rays of sunlight that appear to radiate from the point in the sky where the sun is located
References
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- ^ "Basics of Solar Energy". Archived from the original on 2016-11-28. Retrieved 2016-12-06.
- ^ "The 8-minute travel time to Earth by sunlight hides a thousand-year journey that actually began in the core". SunEarthDay.NASA.gov. NASA. Archived from the original on 2012-01-22. Retrieved 2012-02-12.
- ^ C. KANDILLI & K. ULGEN. "Solar Illumination and Estimating Daylight Availability of Global Solar Irradiance". Energy Sources.
- ^ "Introduction to Solar Radiation". Newport Corporation. Archived from the original on October 29, 2013.
- ^ Calculated from data in "Reference Solar Spectral Irradiance: Air Mass 1.5". National Renewable Energy Laboratory. Archived from the original on September 28, 2013. Retrieved 2009-11-12.
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. - ^ Calculated from the ASTM spectrum cited above.
- ^ Qiang, Fu (2003). "Radiation (Solar)" (PDF). In Holton, James R. (ed.). Encyclopedia of atmospheric sciences. Vol. 5. Amsterdam: Academic Press. pp. 1859–1863. (PDF) from the original on 2012-11-01.
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- ^ Garner, Rob (24 January 2017). "Fermi Detects Solar Flare's Highest-Energy Light". Archived from the original on 17 May 2017. Retrieved 25 January 2018.
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- ^ "Solar Intensity" (PDF). McAuliffe-Shepard Discovery Center. Archived from the original (PDF) on 2009-11-22.
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100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds
- ^ "Graph of variation of seasonal and latitudinal distribution of solar radiation". Museum.state.il.us. 2007-08-30. Archived from the original on 2012-01-12. Retrieved 2012-02-12.
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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. Retrieved 27 August 2015.
Noon sunlight (D55) has a nearly flat distribution...
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Further reading
- ISBN 0-340-82243-0.
External links
- Media related to Sunlight at Wikimedia Commons
- Solar radiation – Encyclopedia of Earth
- Total Solar Irradiance (TSI) Daily mean data at the website of the National Geophysical Data Center
- Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present by World Radiation Center, Physikalisch-Meteorologisches Observatorium Davos (pmod wrc)
- A Comparison of Methods for Providing Solar Radiation Data to Crop Models and Decision Support Systems, Rivington et al.
- Evaluation of three model estimations of solar radiation at 24 UK stations, Rivington et al.
- High resolution spectrum of solar radiation from Observatoire de Paris
- Measuring Solar Radiation : A lesson plan from the National Science Digital Library.
- 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.
- An Excel workbook with a solar position and solar radiation time-series calculator; by Greg Pelletier
- ASTM Standard for solar spectrum at ground level in the US (latitude ~37 degrees).
- Detailed spectrum of the Sun at Astronomy Picture of the Day.