Watt Hour Meter Measures

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The Sun, as seen from low Earth orbit overlooking the International Space Station.

Cultural aspects[edit]

This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum. Sunlight shining upon two different sides of the U.S. state of New Jersey. Sunrise on the Jersey Shore at Spring Lake, Monmouth County (above), and sunset on the Shore at Sunset Beach, Cape May County (below). Both are filtered through high stratus clouds. Sunrise over the Gulf of Mexico and Florida. Taken on 20 October 1968 from Apollo 7. 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. 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.

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 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 to fuel the organisms' activities. 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]. 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 128×103lux. The direct normal illuminance (Edn), corrected for 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).

Composition and power[edit]

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Multiplying the figure of 1050 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). Dividing the irradiance of 1050 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]. Solar irradiance spectrum above atmosphere (yellow) and at surface (red). Extreme UV and X-rays are produced (at left of wavelength range) but comprise very small amounts of the Sun's total output power (= area under the curve).

The spectrum of the Sun's solar radiation is close to that of a black body[11][12] with a temperature of about 5,800 K.[13] The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces gamma rays as a result of the nuclear-fusion process, 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 Sun does not emit gamma rays from this process, but it does emit gamma rays from solar flares.[14] The Sun also emits X-rays, ultraviolet, visible light, infrared, and even radio waves;[15] the only direct signature of the nuclear process is the emission of 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:[16].

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.

External links[edit]

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.[17] In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer,[18] but is also required for vitamin D synthesis in the skin of mammals.[17].

Further reading[edit]

Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once[when?] held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and can cause cancer.[19]. Visible range or light spans 380 to 700 nm.[20] 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. 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 nmInfrared-B: 1,400 nm to 3,000 nmInfrared-C: 3,000 nm to 1 mm. 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. 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.[21]. Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder. 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:[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".[23].

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 (see daylight for comparison table).

Example values[change | change source]

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 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 a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%).

See also

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, 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. But the seasonal and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.[24] The effect of Sun angle on climate results in 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 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.[25] 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.[26][27][28][29] 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 spectrum at top of atmosphere, on a linear scale and plotted against wavenumber. The solar constant 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.