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62-460: Sunlight is the term used for the Sun's starlight observed during daytime. During nighttime, albedo describes solar reflections from other Solar System objects, including moonlight , planetshine , and zodiacal light . Observation and measurement of starlight through telescopes is the basis for many fields of astronomy , including photometry and stellar spectroscopy . Hipparchus did not have

124-584: 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/m 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

186-440: A diverse range of pursuits including poetry , astronomy, and military strategy. The United States Army spent millions of dollars in the 1950s and onward to develop a starlight scope , that could amplify starlight, moonlight filtered by clouds, and the fluorescence of rotting vegetation about 50,000 times to allow a person to see in the night. In contrast to previously developed active infrared system such as sniperscope , it

248-562: 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". 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

310-431: A magnitude of extinction, so that the optical depth ~ 1. An optical depth of 1 corresponds to a mean free path, which is the distance, on average that a photon travels before scattering from a dust grain. So on average, a starlight photon is scattered from a single interstellar grain; multiple scattering (which produces circular polarization) is much less likely. Observationally, the linear polarization fraction p ~ 0.015 from

372-428: 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 , where A {\displaystyle A} is the "areal velocity" invariant. That is,

434-478: A need to understand the impact of moonlight . Starlight intensity has been observed to be a function of its polarization . Starlight becomes partially linearly polarized by scattering from elongated interstellar dust grains whose long axes tend to be oriented perpendicular to the galactic magnetic field . According to the Davis–Greenstein mechanism , the grains spin rapidly with their rotation axis along

496-463: 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

558-537: A risk factor for skin cancer, "sun avoidance may carry more of a cost than benefit for over-all good health". 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. These increases are accompanied by strong induction of an enzyme with

620-418: 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

682-408: A single scattering; circular polarization from multiple scattering goes as p 2 {\displaystyle p^{2}} , so we expect a circularly polarized fraction of q ∼ 2 × 10 − 4 {\displaystyle q\sim 2\times 10^{-4}} . Light from early-type stars has very little intrinsic polarization. Kemp et al. measured

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744-437: A telescope or any instrument that could measure apparent brightness accurately, so he simply made estimates with his eyes. He sorted the stars into six brightness categories, which he called magnitudes. He referred to the brightest stars in his catalog as first-magnitude stars and those so faint he could barely see them as sixth-magnitude stars. Starlight is also a notable part of personal experience and human culture , impacting

806-479: A treatment for psoriasis and other maladies. Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes , and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the Sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources. The ultraviolet radiation in sunlight has both positive and negative health effects, as it

868-442: Is diffused . Sources estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day; this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance . The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D 3 synthesis and a mutagen . Sunlight takes about 8.3 minutes to reach Earth from

930-490: 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/m , varying slightly with solar activity , but recent recalibrations of

992-537: Is a portion of the electromagnetic radiation given off by the Sun , in particular infrared , visible , and ultraviolet light. On Earth , sunlight is scattered and filtered through Earth's atmosphere as daylight when the Sun is above the horizon . When direct solar radiation is not blocked by clouds , it is experienced as sunshine , a combination of bright light and radiant heat (atmospheric). When blocked by clouds or reflected off other objects , sunlight

1054-410: 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

1116-437: Is both a principal source of vitamin D 3 and a mutagen . A dietary supplement can supply vitamin D without this mutagenic effect, 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 and possibly inhibiting the growth of some cancers. Sun exposure has also been associated with

1178-484: Is found in starlight. Serkowski, Mathewson and Ford measured the polarization of 180 stars in UBVR filters. They found a maximum fractional circular polarization of q = 6 × 10 − 4 {\displaystyle q=6\times 10^{-4}} , in the R filter. The explanation is that the interstellar medium is optically thin. Starlight traveling through a kiloparsec column undergoes about

1240-569: 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

1302-466: 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 (87269) 2000 OO 67 . 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

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1364-530: Is twisted grain alignment along the line of sight due to variation in the galactic magnetic field; another is the line of sight passes through multiple clouds. For these mechanisms the maximum expected CP fraction is q ∼ p 2 {\displaystyle q\sim p^{2}} , where p {\displaystyle p} is the fraction of linearly polarized (LP) light. Kemp & Wolstencroft found CP in six early-type stars (no intrinsic polarization), which they were able to attribute to

1426-422: The attenuation by Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance ( E ext ), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by 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 ,

1488-424: 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 . The quiet (non-flaring) Sun, including its corona , emits a broad range of wavelengths: X-rays , ultraviolet , visible light , infrared , and radio waves . Different depths in

1550-505: The attenuating effects of the atmosphere is given by: where c is the atmospheric extinction and 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

1612-415: 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

1674-422: 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

1736-445: 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

1798-437: The closest approach to the Sun and, therefore, the maximum 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 ( E sc ), is equal to 128×10   lux . The direct normal illuminance ( E dn ), corrected for

1860-881: 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). This band of significant radiation power can be divided into five regions in increasing order of wavelengths : 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

1922-443: 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

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1984-449: The dusty nebula. Circular polarization of zodiacal light and Milky Way diffuse galactic light was measured at wavelength of 550 nm by Wolstencroft and Kemp. They found values of q ∼ 5 × 10 − 3 {\displaystyle q\sim 5\times 10^{-3}} , which is higher than for ordinary stars, presumably because of multiple scattering from dust grains. Sunlight Sunlight

2046-593: 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/m , but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1,120 W/m . 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). At

2108-572: The first mechanism mentioned above. In all cases, q ∼ 10 − 4 {\displaystyle q\sim 10^{-4}} in blue light. Martin showed that the interstellar medium can convert LP light to CP by scattering from partially aligned interstellar grains having a complex index of refraction. This effect was observed for light from the Crab Nebula by Martin, Illing and Angel. An optically thick circumstellar environment can potentially produce much larger CP than

2170-399: The human eye's minimum illuminance (~0.1 mlx ) while moonlight coincides with the human eye's minimum colour vision illuminance (~50 mlx).   One of the oldest stars yet identified -  oldest but not most distant in this case - was identified in 2014: while "only" 6,000 light years away, the star SMSS J031300.36−670839.3 was determined to be 13.8 billion years old, or more or less

2232-435: 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 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. The effect of Sun angle on climate results in

2294-650: The interstellar medium. Martin suggested that LP light can become CP near a star by multiple scattering in an optically thick asymmetric circumstellar dust cloud. This mechanism was invoked by Bastien, Robert and Nadeau, to explain the CP measured in 6 T-Tauri stars at a wavelength of 768 nm. They found a maximum CP of q ∼ 7 × 10 − 4 {\displaystyle q\sim 7\times 10^{-4}} . Serkowski measured CP of q = 7 × 10 − 3 {\displaystyle q=7\times 10^{-3}} for

2356-503: The irradiance which can be focused on a surface using mirrors: 48.5 MW/m . The spectrum of the Sun's solar radiation can be compared to that of a black body with a temperature of about 5,800  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

2418-458: The light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700 nm, 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. The useful range for color vision in humans, for example, is approximately 450–650 nm. Aside from effects that arise at sunset and sunrise,

2480-442: The magnetic field. Light polarized along the direction of the magnetic field perpendicular to the line of sight is transmitted, while light polarized in the plane defined by the rotating grain is blocked. Thus the polarization direction can be used to map the galactic magnetic field . The degree of polarization is on the order of 1.5% for stars at 1,000 parsecs ' distance. Normally, a much smaller fraction of circular polarization

2542-464: The only listed carcinogens that are known to have health benefits, 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. 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

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2604-643: The optical polarization of the Sun at sensitivity of 3 × 10 − 7 {\displaystyle 3\times 10^{-7}} ; they found upper limits of 10 − 6 {\displaystyle 10^{-6}} for both p {\displaystyle p} (fraction of linear polarization) and q {\displaystyle q} (fraction of circular polarization). The interstellar medium can produce circularly polarized (CP) light from unpolarized light by sequential scattering from elongated interstellar grains aligned in different directions. One possibility

2666-405: 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 a sunshine recorder , pyranometer , or pyrheliometer . To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth's elliptic orbit and

2728-410: The past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done. 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). The solar constant

2790-410: The photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum. 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. The only direct signature of the nuclear processes in

2852-401: 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". The Sun may be said to illuminate , which is a measure of

2914-515: The red supergiant NML Cygni and q = 2 × 10 − 3 {\displaystyle q=2\times 10^{-3}} in the long-period variable M star VY Canis Majoris in the H band, ascribing the CP to multiple scattering in circumstellar envelopes . Chrysostomou et al. found CP with q of up to 0.17 in the Orion OMC-1 star-forming region, and explained it by reflection of starlight from aligned oblate grains in

2976-443: The redistribution of solar energy are considered a likely cause for the coming and going of recent ice 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. When going further back in time, one has to rely on irradiance reconstructions, using sunspots for

3038-581: The relevant satellite observations indicate a value closer to 1,361 W/m is more realistic. 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/m ). TSI observations continue with the ACRIMSAT /ACRIM3, SOHO /VIRGO and SORCE /TIM satellite experiments. Observations have revealed variation of TSI on many timescales, including

3100-500: 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 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 bright for comfort; indeed, looking directly at the Sun can cause long-term vision damage. To compensate for

3162-432: The same age as the universe itself. The starlight shining on Earth includes this star. Night photography includes photographing subjects that are lit primarily by starlight. Directly taking images of night sky is also a part of astrophotography . Like other photography, it can be used for the pursuit of science and/or leisure. Subjects include nocturnal animals . In many cases starlight photography may also overlap with

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3224-486: 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 is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there

3286-491: 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". 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

3348-592: The solar magnetic cycle and many shorter periodic cycles. 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

3410-408: 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. The existence of nearly all life on Earth

3472-605: 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

3534-486: 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 )

3596-440: The surface of the Sun. 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. Sunlight is a key factor in photosynthesis , the process used by plants and other autotrophic organisms to convert light energy , normally from the Sun, into chemical energy that can be used to synthesize carbohydrates and fuel

3658-474: The timing of melatonin synthesis, maintenance of normal circadian rhythms , and reduced risk of seasonal affective disorder . 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 . Short-term overexposure is the cause of sunburn , snow blindness , and solar retinopathy . UV rays, and therefore sunlight and sunlamps, are

3720-430: The top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV), with most of the extra UV consisting of biologically damaging short-wave ultraviolet. 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

3782-535: The web. 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: 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

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3844-502: Was a passive device and did not require additional light emission to see. The average color of starlight in the observable universe is a shade of yellowish-white that has been given the name Cosmic Latte . Starlight spectroscopy, examination of the stellar spectra, was pioneered by Joseph Fraunhofer in 1814. Starlight can be understood to be composed of three main spectra types, continuous spectrum , emission spectrum , and absorption spectrum . Starlight illuminance coincides with

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