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.[26] 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.[37][38]seasonal and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.[36] 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.[26] 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.[37][38][39][40] 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).
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 energ
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 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 the
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.
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 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).
The effect of sunlight is relevant to painting, evidenced for instance in works of Eduard Manet and 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 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 creation of Vitamin D in the skin, a vital compound needed to make strong bone and muscle in the body.
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 that may be provided by trees, parasols, and so on.
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. It also formed the basis for religion in Ancient Egypt.
Sunbathing
Main article:
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 creation of
Vitamin D in the skin, a vital compound needed to make strong bone and muscle in the body.
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 that may be provided by trees, parasols, and so on.
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. It also formed the basis for religion in Ancient Egypt.
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.[4For 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 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 is both a principal source of vitamin D3 and a mutagen.[41] A dietary supplement can supply vitamin D without this mutagenic effect,[42] 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[43] and possibly inhibiting the growth of some cancers.[44][45] Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.[46]
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.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.[47] 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,[48] 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.[49] 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."[50] 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.[50]
Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.[51] These increases are accompanied by strong induction of an enzyme with a key role in recombinational repair of DNA damage. Thus the level of terrestrial solar UV-B radiation likely affects genome stability in plants.
See also