Sunlight - Wiki Article

Sunlight Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic
radiation given off by the Sun, particularly infrared, visible, and ultraviolet light.
On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is
obvious as daylight when the Sun is above the horizon. When the direct solar radiation
is not blocked by clouds, it is experienced as sunshine, a combination of bright light
and radiant heat. When it is blocked by the clouds or reflects off of other objects, it
is experienced as diffused light. The World Meteorological Organization uses the term
"sunshine duration" to mean the cumulative time during which an area receives direct
irradiance from the Sun of at least 120 watts per square meter. Sunlight may be recorded
using a sunshine recorder, pyranometer or pyrheliometer. Sunlight takes about 8.3 minutes
to reach the Earth. On average, it takes energy between 10,000 and 170,000 years to leave
the sun's interior and then be emitted from the surface as light. Direct sunlight has
a luminous efficacy of about 93 lumens per watt of radiant flux. Bright sunlight provides
illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface.
Sunlight's composition at ground level, per square meter, with the sun at the zenith,
is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of
ultraviolet radiation. At the top of the atmosphere sunlight is about 30% more intense with more
than three times the fraction of ultraviolet (UV), with most of the extra UV consisting
of biologically-damaging shortwave ultraviolet. Sunlight is a key factor in photosynthesis,
a process vital for many living beings on Earth. Composition and power The spectrum
of the Sun's solar radiation is close to that of a black body with a temperature of about
5,800 K. 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, these super high
energy photons are converted to lower energy photons before they reach the Sun's surface
and are emitted out into space. As a result, the Sun doesn't give off any gamma rays. The
Sun does, however, emit X-rays, ultraviolet, visible light, infrared, and even radio waves.
When ultraviolet radiation is not absorbed by the atmosphere or other protective coating,
it can cause damage to the skin known as sunburn or trigger an adaptive change in human skin
pigmentation. The spectrum of electromagnetic radiation striking the Earth's atmosphere
spans a range of 100 nm to about 1 mm. This can be divided into five regions in increasing
order of wavelengths: 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). Owing to absorption by the
atmosphere very little reaches the Earth's surface (Lithosphere). This spectrum of radiation
has germicidal properties, and is used in germicidal lamps. Ultraviolet B or (UVB) range
spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible
for the photochemical reaction leading to the production of the ozone layer. Ultraviolet
A or (UVA) spans 315 to 400 nm. It has been traditionally held as less damaging to DNA,
and hence used in tanning and PUVA therapy for psoriasis. Visible range or light spans
380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye.
Infrared range that spans 700 nm to 10 to the 6 nm (1 millimeters). It is responsible
for an important part of the electromagnetic radiation that reaches the Earth. It is also
divided 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. Sunlight in space at the top of
Earth's atmosphere at a power of 1366 watts/m squared is composed (by total energy) of about
50% infrared light, 40% visible light, and 10% ultraviolet light. At ground level this
decreases to about 1120-1000 watts/m squared , and by energy fractions to 44% visible light,
3% ultraviolet (with the Sun at the zenith, but less at other angles), and the remainder
infrared. Thus, sunlight's composition at ground level, per square meter, with the sun
at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and
32 watts of ultraviolet radiation. Calculation To calculate the amount of sunlight reaching
the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphere
have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the
elliptical orbit by using the day number of the year (dn), is given by where dn=1 on January
1; dn=2 on January 2; dn=32 on February 1, 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 128×10 cubed lx. The direct normal illuminance (Edn), corrected for the attenuating effects
of the atmosphere is given by: where c is the atmospheric extinction coefficient and
m is the relative optical airmass. Solar constant The solar constant, 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 the 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 kW/m², varying slightly with solar activity, but recent recalibrations of the
relevant satellite observations indicate a value closer to 1.361 kW/m² is more realistic.
This radiation is about 50% infrared, 40% visible, and about 10% ultraviolet at the
top of the atmosphere. Total (TSI) and spectral solar irradiance (SSI) upon Earth Total Solar
Irradiance upon Earth (TSI) was earlier measured by satellite to be roughly 1.366 kilowatts
per square meter (kW/m²), but most recently NASA cites TSI as "1361 W/m² as compared
to ~1366 W/m² from earlier observations Kopp et al., 2005", based on regular readings from
NASA's Solar Radiation and Climate Experiment(SORCE) satellite, active since 2003, noting that
this "discovery is critical in examining the energy budget of the planet Earth and isolating
the climate change due to human activities." Furthermore the Spectral Irradiance Monitor
(SIM) has found in the same period that spectral solar irradiance (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".
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 rough table comparing
the amount of solar radiation received by each planet in the Solar System follows (from
data in 1): The actual brightness of sunlight that would be observed at the surface depends
also on the presence and composition of an atmosphere. For example Venus' 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". Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses,
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. For comparison purposes, sunlight on Saturn is slightly brighter
than Earth sunlight at the average sunset or sunrise (see daylight for comparison table).
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 the Earth, a distance of about
500 AU (~69 light-hours) is needed; there are only a handful of objects in the solar
system known to orbit farther than such a distance, among them 90377 Sedna and (87269)
2000 OO67. Surface illumination The spectrum of surface illumination depends upon solar
elevation due to atmospheric effects, with the blue spectral component from atmospheric
scatter 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. According to Craig Bohren, "preferential absorption of sunlight by ozone over long
horizon paths gives the zenith sky its blueness when the sun is near the horizon". See diffuse
sky radiation for more details. Climate effects Further information: Solar variation, Solar
dimming, and Insolation On Earth, solar radiation is obvious as daylight when the
sun is above the horizon. This is during daytime, and also in summer near the poles at night,
but not at all in winter near the poles. When the direct radiation is not blocked by clouds,
it is experienced as sunshine, combining the perception of bright white light (sunlight
in the strict sense) and warming. The warming on the body, the ground and other objects
depends on the absorption (electromagnetic radiation) 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. The 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%). The total insolation remains almost constant due to Kepler's second law, 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
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 the Earth's surface also varies. For example,
at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more
than 25% as a result of the 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). Past variations in solar irradiance Space-based observations of solar irradiance
started in 1978. These measurements show that the solar constant is not constant. It varies
with the 11-year sunspot solar cycle. 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. These studies show that solar
irradiance does vary with distinct periodicities such as: 11 years (Schwabe), 88 years (Gleisberg
cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle). 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 which 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 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 which had previously
only provided humans with meat and tools once they were killed were now used for labour
throughout their lives, fueled by grasses inedible to humans. The more recent discoveries
of coal, petroleum and natural gas are modern extensions of this trend. These fossil fuels
are the remnants of ancient plant and animal matter, formed using energy from sunlight
and then trapped within the earth for millions of years. Because the stored energy in these
fossil fuels has accumulated over many millions of years, they have allowed modern humans
to massively increase the production and consumption of primary energy. As the amount of fossil
fuel is large but finite, this cannot continue indefinitely, and various theories exist as
to what will follow this stage of human civilization (e.g. alternative fuels, Malthusian catastrophe,
new urbanism, peak oil). Cultural aspects The effect of sunlight is relevant to painting,
evidenced for instance in works of Claude Monet on outdoor scenes and landscapes. Many
people find direct sunlight to be too bright for comfort, especially when reading from
white paper upon which the sun 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 or curtains, or
by nearby shade trees. In colder countries, many people prefer sunnier days and often
avoid the shade. In hotter countries the converse is true; during the midday hours many people
prefer to stay inside to remain cool. If they do go outside, they seek shade which may be
provided by trees, parasols, and so on. In Hinduism the sun is considered to be a god
as it is the source of life and energy on earth. 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 cafés. 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
outdoor weather conditions and amount of sunlight. For many people with pale or brownish 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 beautiful, 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. For people suffering from psoriasis,
sunbathing is an effective way of healing the symptoms. Skin tanning is achieved by
an increase in the dark pigment inside skin cells called melanocytes and it is actually
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 Risks and benefits
of sun exposure The body produces vitamin D from sunlight (specifically from the UVB
band of ultraviolet light), and excessive seclusion from the sun can lead to deficiency
unless adequate amounts are obtained through diet. Sunburn can have mild to severe inflammation
effects on skin; this can be avoided by using a proper sunscreen cream or lotion or by gradually
building up melanocytes with increasing exposure. Another detrimental effect of UV exposure
is accelerated skin aging (also called skin photodamage), which produces a difficult to
treat cosmetic effect. Some people are concerned that ozone depletion is increasing the incidence
of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.
A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal
affective disorder (SAD), a serious form of the "winter blues". SAD occurrence is more
prevalent in locations further from the tropics, and most of the treatments (other than prescription
drugs) involve light therapy, replicating sunlight via lamps tuned to specific wavelengths
of visible light, or full-spectrum bulbs. A recent study indicates that more exposure
to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS)
later in life.