Sunlight
When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat (atmospheric).
The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D3 synthesis and a mutagen.
[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.
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.
Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation).
[14] The quiet (non-flaring) Sun, including its corona, emits a broad range of wavelengths: X-rays, ultraviolet, visible light, infrared, and radio waves.
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).
[citation needed] This band of significant radiation power can be divided into five regions in increasing order of wavelengths:[20] 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] Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun.
[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.
The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet.
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
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).
Its average value was thought to be approximately 1,366 W/m2,[34] 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.
These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere.
Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate.
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.
Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests.
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 to painting, evidenced for instance in works of Édouard Manet and Claude Monet on outdoor scenes and landscapes.
Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, shutters, curtains, or nearby shade trees.
Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafes.
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.
Vitamin D has a wide range of positive health effects, which include strengthening bones[50] and possibly inhibiting the growth of some cancers.
[51][52] Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.
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] Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.
