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2 : Direct Sunlight

Sunlight 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, and is obvious 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. When blocked by clouds or reflected off other objects, sunlight is diffused. Sources estimate a global average of between 164 watts to 340 watts[1] per square meter over a 24-hour day;[2] this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance.

2 : Direct Sunlight

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]

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 (Eext), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by[5]

.mw-parser-output .vanchor>:target.vanchor-textbackground-color:#b1d2ffDirect 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).

The spectrum of the Sun's solar radiation can be compared to that 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 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, although it does emit such "hard radiations" as X-rays and even gamma rays during solar flares.[14] The quiet (non-flaring) Sun, including its corona, emits a broad rangeof wavelengths: X-rays, ultraviolet, visible light, infrared, and radio waves;[15] the only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting neutrinos. Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.[16]

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.[22]

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".[24]

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 (87269) 2000 OO67.

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 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".[39]

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.

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sunlight is directly shining. Indeed, looking directly at the Sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. 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.

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.

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.[44] A dietary supplement can supply vitamin D without this mutagenic effect,[45] 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[46] and possibly inhibiting the growth of some cancers.[47][48] Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.[49]

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.[50] 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,[51] 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.[52] 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".[53] 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.[53]

Midday is the best time to get vitamin D, as the sun is at its highest point and your body may manufacture it most efficiently around that time of day. This means you may need less time in the sunlight at midday. 041b061a72


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