Sun - SpeedyLook encyclopedia

General presentation

The Sun is a dwarf star which is composed of 74% of Hydrogène, of 25% of Hélium and a fraction of heavier elements. The Sun is of spectral Type G2-V. “G2” means that it is hotter (5770 [[Kelvin]] S surfaces some approximately) and more brilliance that the average, with a yellow color drawing on the white. Its spectrum contains bands of ionized and neutral metals, as well as weak hydrogen bands. The suffix “V” indicates that it currently evolves/moves, like the majority of stars, on the principal sequence of the diagram of Hertzsprung-Russell: it draws its energy from reactions of nuclear Fusion which transform, in its core, the Hydrogène in Hélium, and is in a hydrostatic state of balance, not undergoing neither contraction, nor dilation continual. There exists in our galaxy more than one hundred million stars of the spectral type identical, which makes Sun a star altogether rather banal. Although it is in fact more brilliance that 85% of the stars of the galaxy, which are as a majority of the dwarf reds.

The Sun revolves around the center of the Milky Way of which it is distant approximately 25 with: 28000 light-years. Its period of galactic revolution is approximately 220 million years, and its speed of 217 km S -1, equivalent at one light-year every 1400 years (approximately), and a astronomical Unité every 8 days.

In this galactic revolution, the Sun, like other stars of the disc, has an oscillating movement around the galactic Plan: the solar galactic orbit presents sinusoidal undulations perpendicular to its plan of revolution. The Sun would approximately cross this plan all the 30 million years, a side then other - galactic North-South direction, then conversely - and would move away from there to the maximum of 230 light-years approximately, while remaining in the galactic disc. Mass of the galactic disc attracting the stars which would have a different plan of revolution.

The Sun also rotates, with one 27 days period terrestrial approximately. Actually, not being a solid object, it undergoes a differential Rotation: it turns more quickly to the equator (25 days) that with the poles (35 days). The Sun is also in rotation around the Barycentre of the solar system, this last being located at close to a solar Rayon of the center of star, because mainly of the colossal mass of Jupiter (approximately thousandths of the solar mass).

Natural history of the Sun

See also: Evolution of the stars, Formation and evolution of the solar system

The Sun is a star of currently old Population I of 4,6 billion years approximately, that is to say about half of its way on the principal sequence. It is generally admitted that it was formed under the effect of the shock waves produced by a Supernova.

In its actual position, the heart of the Sun transforms each second more than four million tons of matter (of mass) into energy which is transmitted to the roadbases of the star and is emitted in space in the form of radiation electromagnetic (Lumière, solar Rayonnement) and of flow of particles (Solar wind). In the five billion years to come, the Sun will exhaust its hydrogen reserves gradually; its brightness will increase approximately 7% per billion by years. When it is old of approximately 10 billion years, the hydrostatic balance will be broken. The core will start to contract and to be heated while the surface layers, dilated by the heat flux and thus partially released from the gravitational effect, will be gradually pushed back: the Sun will dilate and be transformed into Géante red. At the end of this process, the diameter of the Sun will be approximately hundred times superior with the current one; it will exceed the orbit of Mercure and Venus. The Earth, if it still remains, will be nothing any more but one calcined desert.

The mass of the Sun is not sufficient so that it explodes in Supernova. Approximately 250 million years later, when the heart reaches some 100 million Kelvin, the core will crumble on itself while the surface layers will be ejected in space and will give rise to a Nébuleuse planet gear. The remainders of star will form a white Naine then which will be able to still survive several billion years during which it will cool before dying out definitively. This scenario is characteristic of stars of low mass to average.

History of solar exploration

Development of the modern scientific approach

The Greek philosopher Anaxagore was one of the first Westerners to propose a scientific theory on the Sun, advancing which it was about an incandescent mass larger than the Peloponnese and not the carriage of Hélios. This audacity was worth to him to be imprisoned and condemned to death, even if it were released later thanks to the intervention of Périclès. Two centuries later, Ératosthène is undoubtedly the first to have estimated with precision the distance Ground-Sun (approximately 149 million kilometers), at the third century before Jesus-Christ.

At the 16th century, Copernic emitted the theory which the Earth turned around the Sun, and not the reverse as one had always believed. At the beginning of the 17th century Galileo inaugurated the telescopic observation Sun, observed the sunspots, suspecting that they were located at the surface of the star and that they were not objects passing between the Sun and the Earth. Nearly one hundred years later, Newton broke up solar light by means of a prism, revealing the visible spectrum, while in 1800 William Herschel discovered the rays Infrarouge S. the 19th century saw considerable advances, in particular in the field of the spectroscopic observation of the Sun under the impulse of Joseph von Fraunhofer, which observed the absorption lines solar spectrum, to which it gave its name.

The source of solar energy was the principal enigma of the first years of the modern scientific era. Initially several theories were proposed, but none proved really satisfactory. Lord Kelvin proposed a model suggesting that the Sun was a liquid body which cooled gradually while radiating starting from a reserve of heat stored in its center. Kelvin and Helmholtz tried to explain the production of solar energy by the theory known under the name of Mécanisme of Kelvin-Helmholtz. Unfortunately, the estimated age of the Sun according to this mechanism did not exceed 20 million years, which was much lower than than let suppose the Géologie. In 1890 Joseph Norman Lockyer, the discoverer of the Helium, proposed a theory meteoritic on the formation and the evolution of the Sun.

It was necessary to await 1904 and work of Ernest Rutherford so that finally a plausible assumption is offered. Rutherford supposed that energy was produced and maintained by an internal source of heat and that the Radioactivité was with the source of this energy. While showing the relation between the mass and energy (E=mc ²), Albert Einstein brought an essential component to the comprehension of the generator of solar energy. In 1920 Sir Arthur Eddington proposed the theory according to which the center of the Sun was the seat of Pression S and Température S extremes, allowing reactions of nuclear Fusion which transformed the Hydrogène into Hélium, releasing from energy proportionally to a reduction in the mass. This ideal model was supplemented in the years 1930 by work of the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe, which described in detail the two principal nuclear reactions energy producers in the middle of the Sun. To finish in 1957, an article entitled Synthèse of the Elements in the Stars brought the final demonstration that the majority of the elements met in the Univers were formed under the effect of nuclear reactions in the middle of stars such as the Sun.

Solar space missions

The first probes designed to observe the Sun since interplanetary space were launched by NASA between 1959 and 1968: they were the missions Pioneer 5,6,7,8 and 9 . In orbit around the Sun at a similar distance to that of the terrestrial orbit, they allowed the first detailed analyzes of the Solar wind and the solar magnetic field. Pioneer 9 remained operational particularly a long time and sent information until in 1987.

In the Years 1970, two missions brought to the scientists capital information on the solar wind and the solar crown. The probe germano - American Helios 1 studied the solar wind since the Périhélie of an orbit smaller than that of Mercure. The American station Skylab , launched in 1973, comprised a solar module of observation baptized Apollo Telescope Mount and ordered by the Spationaute S embarked in the station. Skylab made the first observations of the zone of transition between the chromosphere and the crown and of the ultraviolet emissions from the solar crown. The mission also allowed the first observations of ejections of mass coronale and coronaux holes, phenomena which one knows today that they are closely related to the solar wind.

In 1980 NASA launched the satellite Solar Maximum Mission (more known under the name of SolarMax ), conceived for the observation of the rays gamma, X and Ultraviolet S emitted by the solar eruptions during the time of strong Solar activity. Unfortunately a few months after its launching, an electronic dysfunction placed the satellite in mode standby , and the apparatus remained inactive the three following years. In 1984 however the mission STS-41-C of the program Space Shuttle Challenger intercepted the satellite and allowed a repair and a restarting. SolarMax could then carry out thousands of observations of the solar Couronne and sunspots until its destruction in June 1989.

The satellite Japan board Yohkoh ( Sun ray ), launched in 1991, observed the solar eruptions with the wavelengths of x-rays. the data brought back by the mission made it possible to the scientists to identify various types of eruptions, and showed that the crown beyond the areas of peaks of activity was much more dynamic and active that it before had been supposed. Yohkoh followed a solar Cycle whole but broke down following an annular eclipse from Sun on December 14th, 2001. It was destroyed while penetrating in the atmosphere in 2005.

One of the most important solar missions to date is the Solar and Heliospheric Observatory or SoHO, impetus jointly by the European space agency and NASA on December 2nd, 1995. Envisaged at the beginning for two years, the SoHO mission is always active. It proved so powerful that a mission of prolongation baptized Solar Dynamics Observatory is considered for 2008. Located with the Not of Lagrange between the Earth and the Sun (to which the attraction force of these two celestial bodies is equal), SoHO permanently sends images of the Sun to various wavelengths. In addition to this direct observation of the Sun, SoHO allowed the discovery of a great number of Comet S, mainly of very small comets effleurant the Sun and destroyed at the time of their passage.

All the observations recorded by these satellites are taken since the plan of the ecliptic . Consequently, they could observe in detail only the only equatorial areas of the Sun. In 1990 however the probe Ulysses was launched to study the polar regions of the Sun. It travelled initially towards Jupiter and used its gravitational Assistance to separate from the plan of the ecliptic. By chance it was ideally placed to observe, in July 1994, the collision between the Comet Shoemaker-Levy 9 and Jupiter. Once on the orbit envisaged, Ulysses studied the solar wind and the force of the magnetic field to high solar Latitude S, discovering that the solar wind with the poles slower than was envisaged (750 km/s approximately) and that important magnetic waves emerged from it, taking part in the dispersion of the cosmic rays.

The mission Genesis was launched by NASA in 2001 with an aim of capturing pieces of solar wind in order to obtain a direct measurement of the composition of the solar matter. It was severely damaged at the time of its return to earth, on September 10th, 2004, but part of sampling could be saved and is currently being analyzed.

The STEREO mission (Solar TErrestrial Observatories Relation) launched on October 25th, 2005 by NASA allowed for the first time the three-dimensional observation of our star since space. Composed of two almost identical satellites, this mission must allow a better comprehension of the relations Sun-Ground, in particular by allowing the observation of the CME (Ejections of Coronale Mass) until the terrestrial electromagnetic environment.

Structure and operation of the Sun

Although the Sun is a star of intermediate size, it only represents with him more than 99% of the mass of the Solar system. Its form is almost perfectly spherical, with a Aplatissement with the poles estimated at nine millionth, which means that its polar diameter is smaller than its equatorial diameter of only ten kilometers.

Contrary to the telluric objects, the Sun does not have well defined external limit: the density of its gases falls so as to little close exponential as one moves away from his center. On the other hand its internal structure is well defined, as described low. The ray of the Sun is measured of its center until the Photosphère. Photosphere is the layer below which the gases are condensed enough to be opaque and beyond which they become transparent. Photosphere is thus most readily visible with the naked eye. The major part of the solar mass concentrates with 0,7 ray center. The internal structure of the Sun is of course not observable directly, and the Sun itself being radiopaque, no visual instrument can bore its internal composition. But in the same way that the Sismologie allowed, by the study of the waves produced by the earthquakes, to determine the internal structure of the Ground, the Héliosismologie uses the solar pulsations to measure and visualize indirectly the internal structure of the Sun. The data-processing Simulation is also used as theoretical tool to probe the deepest layers.

The heart or core

It is considered that the heart of the Sun extends from the center with approximately 0,2 solar Rayon. Its density is higher than 150000 kg m -3 (150 times the density of water on Earth) and its temperature approaches the 15 million Kelvins (what contrasts clearly with the temperature of surface of the Sun, which borders them: 6000 Kelvins). It is in the heart that occurs the thermonuclear reactions exothermic (nuclear fusion) which transforms mainly the Hydrogène into Hélium, but also helium in Carbone, carbon in Fer (see also: Reaction nucléaire#Le Sun).

Approximately 8,9 Proton S (hydrogen cores) are converted into helium each second, releasing energy at a rate of 4,26 million tons of matter consumed a second, producing 383 yottajoules (383 joules) a second, that is to say the equivalent of the explosion of 9,15 ton S of TNT. The nuclear rate of fusion is proportional to the density of the core, so that nuclear fusion within the heart is a self-regulated process : very light increase in the rate of fusion causes a warming and a Dilatation of the heart which reduces in return the rate of fusion. Conversely, any light reduction in the rate of fusion cools and densified the heart, which makes return the level of fusion to its starting point.

The heart is the only part of the Sun which produces a notable quantity of Chaleur by fusion: the remainder of star draws its heat only from the energy which comes from it. The totality of the energy which is produced there must cross many successive layers until photosphere, before escaping in solar space in the form of Rayonnement or of flow of particles.

The Photon S of high energy (rays X and gamma) released during the reactions of fusion spend a considerable time to reach the surface of the Sun, slowed down by the interaction with the matter and the permanent phenomenon of absorption and réémission with lower energy in the solar coat. It is estimated that the time of transit of a photon of the heart on the surface is between: 17000 and 50 million years. After having crossed the layer of convection and having reached the Photosphere, the photons escape in space, mainly in the form of visible Lumière. Each gamma ray produced in the center of the Sun is finally transformed into several million luminous photons before escaping in space. Neutrino S are also released by the reactions of fusion, but contrary to the photons they interact little with the matter and are thus released immediately. During years, the number of neutrinos produced by the Sun was measured weaker of a third than the theoretical value: it was the problem of the solar neutrinos , which was recently solved (in 1998) thanks to a better comprehension of the phenomenon of Oscillation of the neutrino.

The zone of radiation

The zone of radiation or radiative zone ranges roughly between 0,2 and 0,7 solar Rayon. The solar matter is so hot and so dense there that the transfer of the heat of the center towards the most external layers is made by only the thermal radiation. Ionized hydrogen and helium emit Photon S which travel on a short distance before being reabsorbed by other ions. In this zone, there is no thermal convection because although the matter cools while moving away from the heart, the heat gradient remains lower than the adiabatic thermal Gradient. The temperature decreases there to two million Kelvins.

The zone of convection

The zone of convection or convective zone extends from 0,7 solar Rayon center on the visible surface of the Sun. It is separated from the zone of radiation by a thick layer from approximately: 3000 kilometers, the Tachocline, which according to the recent studies could be the seat of powerful magnetic fields and would play a big role in the solar Dynamo. In the zone of convection the matter is any more neither rather dense nor enough heat to evacuate heat by radiation: it is thus by Convection, according to a vertical movement, that heat is led towards photosphere. The temperature passes 2 million there to: 6000 Kelvins. The matter arrived on the surface, cooled, plunges again until the base of the zone of convection to receive the heat of the upper part of the zone of radiation, etc gigantic the cells of convection thus formed are responsible for the solar granulations observable on the surface of the star. Turbulences occurring in this zone produce a $dynamo effect responsible for the North-South magnetic polarity on the surface of the Sun.

Photosphere

The Photosphère is an external part of the star which produces the visible Lumière amongst other things. It is more or less wide: of less 1 % of the ray for dwarf stars (a few hundred kilometers) with a few tens of pourcent ray of star for giant. The light which is produced there contains all information on the temperature, the gravity of surface and the chemical composition of star. For the Sun, photosphere has a thickness of approximately 400 kilometers. Its average temperature is of 6000 K. It makes it possible to define the effective temperature which for the Sun is of 5781 K. On the image of solar photosphere one can see the darkening center-edge which is one of the characteristics of photosphere. The analysis of the spectrum of solar photosphere is very rich in information in particular on the chemical composition of the Sun which is very close to that of the meteorites.

Solar atmosphere

Beyond photosphere the structure of the Sun is generally known under the name of Solar atmosphere . It includes/understands three principal zones: the Chromosphere, the crown and the Heliosphere. The chromosphere is separated from photosphere by the zone of temperature minimum and from the crown by a zone of transition . The heliosphere extends to the borders from the solar system where it is limited by the Héliopause. For a reason still badly elucidated, the chromosphere and the crown are hotter than the surface of the Sun. Although it can be studied in detail by the spectroscopic Télescope S , the solar atmosphere is never as accessible as at the time of Sun the total eclipses.

Chromosphere

The zone of temperature minimum which separates photosphere from the chromosphere offers a sufficiently low temperature (: 4000 Kelvins) so that molecules are found there simple (Carbon monoxide, Eau), detectable by their absorption spectrum. The Chromosphère itself is thick of approximately: 2000 kilometers. Its temperature increases gradually with altitude, to reach a maximum of: 100000 Kelvin at its top. Its spectrum is dominated by band S of emission and absorption. Its name, which comes from the Greek root chroma (color), was given to him because of the constant pink flash which it lets foresee at the time of the total Sun eclipses.

The crown

The zone of transition between the chromosphere and the crown is the seat of a fast rise in temperature, which can approach a million Kelvins. This rise is related to a Transition from phase during which the Hélium becomes completely ionized under the effect of the very high temperatures. The zone of transition does not have a clearly defined altitude. Coarsely, it forms a halation overhanging the chromosphere under the appearance of spicules and filaments. It is the seat of a chaotic and permanent movement. Difficult to perceive since the Earth in spite of the use of coronographes, it is more easily analyzed by the space instruments sensitive to the radiations ultraviolet extremes of the spectrum.

Much vaster than the Sun itself, the solar Couronne itself extends starting from the zone from transition and disappears gradually in space, fray with the Héliosphère by the solar winds. The lower crown, nearest to the surface of the Sun, has a particulate density ranging between 1 {{x10 14}} m −3 and 1 {{x10 16}} m −3, is less than one billionth of the particulate density of the terrestrial atmosphere to the sea level. Its temperature, which can reach the 5 million Kelvins, contrasts clearly with the temperature of photosphere. Although no theory explains this difference yet completely, part of this heat could come from a magnetic process of reconnexion.

Heliosphere

Beginner with approximately 20 solar rays (0.1 [[astronomical Unit ua]]) of the center of the Sun, the Héliosphère extends to the borders from the Solar system. It is admitted that it begins when the flow of Solar wind becomes faster than the Ondes of Alfven (flow is then known as superalfvenic ): dynamic turbulences and forces occurring beyond this border do not have an influence on the structure of the solar crown, because information can move only at the speed of the Ondes of Alfven. The solar wind moves then uninterrupted through the heliosphere, giving to the solar magnetic field the form of a Spirale of Parker until its meeting with the Héliopause, with more than 50 [[astronomical Unit ua]] of the Sun. In December 2004, Voyager 1 became the first probe to cross the héliopause. Each of the two Voyager probes detected important energy levels with the approach of this border.

The solar activity

The solar magnetic field

The Sun is a magnetically active star. All the solar matter being in the form of Gaz and of plasma because of the extremely high temperatures, the Sun turns more quickly to the equator (approximately twenty-five days for a turn) that with the poles (thirty-five days for a turn). This differential Rotation of the solar latitudes gives to the solar Magnetic field a form of Spirale in perpetual rotation, the lines of field being gotten mixed up the ones with the others during time. This tangle would be at least partly responsible for the solar Cycle, periodic phenomenon being spread out over 11,2 years on average with an alternation of minima and maximum every eleven six-month periods approximately. At the end of a solar cycle the magnetic field was reversed compared to the end of the precedent. The most spectacular demonstrations in period of intense magnetic activity are the appearance of sunspots and protuberances.

Sunspots

Although all the details on the genesis of the sunspots are not elucidated yet, it was shown that they are the resultant of intense a magnetic activity within the zone of convection, if powerful that it slows down the Convection and limit the thermal contribution surfaces some with the Photosphère. They are thus less hot of: 1500 to 2000 Kelvins that areas close, which is enough to explain why they appear to us, in contrast, much darker than the remainder of photosphere. However if they were isolated from the remainder of photosphere, sunspots, where reign despite everything a temperature close to the 4500 Kelvins, would seem to us ten times more brilliant than full moon, that is to say more than a Electric arc. The space probe SoHO made it possible to show that the sunspots answer a mechanism close to that of the cyclones on Earth. One distinguishes two parts within the sunspot: the central remote region (approximately 2000 Kelvins) and the peripheral zone of half-light (approximately 2700 Kelvins). The diameter of the sunspots smallest is usually more twice the higher than that of the Earth. In working life it is sometimes possible to observe them with the naked eye on the setting Sun, with an adapted ocular protection.

The monitoring of the sunspots is an excellent means to control the solar activity and to predict its terrestrial repercussions. A sunspot has one two weeks average lifespan. The German astronomer Heinrich Schwabe, at the 18th century, was the first to hold a methodical cartography of the sunspots, which enabled him to evaluate their periodicity. The later studies fixed to them period at 11,2 years, each half-period being alternatively characterized by a maximum of activity (where the spots multiply) and a minimum of activity. The last maximum of activity was recorded in 2001, with a group of spots particularly marked (image). The next minimum of activity is planned for first half of the year of 2007.

See also: Amorce=Pour more details on the sunspots, to see the article, solar Cycle

Solar eruptions

See also: Amorce=Pour more details, to see the articles, Solar eruption, solar Start

Terrestrial effects of the solar activity

The terrestrial effects of the solar activity are multiple, most spectacular is the phenomenon of the polar lights.

The Ground has a Magnétosphère which protects it from the solar winds, but when those are more intense, they deform the magnetosphere and of the solar radioactive particles cross it while following the lines of fields. These particles excite or ionizes the particles of the upper atmosphere. The result of these reactions is the creation of ionized clouds which reflect the Onde S of which the Lumière, which causes the formation of the polar lights.

The solar winds can also disturb the means of communication and of navigations using of the satellites, satellite in-effect the at low altitude can be damaged by the ionization of the Ionosphère.

The solar system

See also: Formation and evolution of the solar system

With him only, the sun accounts for 99.86% of the total mass of the Solar system, the 0.14% remainders including the Planet S (especially Jupiter), of which the Ground.

Symbolic system

The sun is a very powerful Symbole for the men. It occupies a dominant place in each culture.

Generally, it is a male, active principle. However, certain wandering people of Central Asia regarded it as a female principle (the Mother sun); in fact also the case of the Japanese, for the Sun is the kami Amaterasu, the lady sun, wife of the lord the Moon. In the Scandinavian Mythologie, the children of Mundilfari and Glaur are Sol (goddess of the Sun) and Mani (god of the Moon), an idea that J.R.R. Tolkien imported in its work.

Often, the Sun represents the capacity. This star gives the life. If the Sun had suddenly disappeared, or even if its rays did not reach us any more, the life would die out on Earth, from where the symbol of life (donor of life).

In the ancient Egypt, Râ (or Re) is the god Sun (he was one of the most important gods, even most important) and Akhénaton will make of it its single god under the name of Aton. In the Greek Pantheon it is Apollon, wire of Zeus and titanium Léto. Let us quote also Hélios which is the personification of the Sun itself. The Aztèques called it Huitzilopochtli, god of the Sun and the war, the Master of the world. If it is not associated with a god, people associated it with themselves like the King de France Louis XIV called the Sun king (crowned of God). The Japanese imperial family prides itself to go down from Amaterasu, goddess of the Sun.

In Alchemy, the symbol of the Sun and gold are a Cercle with a center point: . It represents the interior with all that revolves around. In Astronomy as in Astrology, the symbol is the same one.

Observation of the sun and dangers to the eye

Observation with the naked eye

To briefly look at the sun with the naked eye can be painful and even dangerous.

A glance towards the sun involves Cécité S partial and temporary (dark tasks in the vision). At the time of this action, approximately four milliwatts of light strike the Rétine, heating it a little, and possibly deteriorating it. The cornea can also be reached.

The general exposure to solar light can also be a danger. Indeed, with the passing of years, the exposure to UV yellows the Cristallin or reduces its transparency and can contribute to the formation of cataracts.

Observation with an optical device

To look at the sun through the magnifying optical devices - for example of the Twin , a teleobjective, a telescope or a telescope - deprived of filter adapted (solar filter) is extremely dangerous and can quickly cause irrevocable damage with the retina, the crystalline lens and the cornea.

With binoculars, approximately 500 times more energy strike the retina, which can destroy the cells rétinales almost instantaneously and involve a permanent blindness.

A method to look without danger the sun is to project its image on a screen by using a Télescope with removable Oculaire (the other types of telescopes can be deteriorated by this treatment).

The filters used to observe the sun must be especially manufactured for this use. Certain filters let pass the UV or Infrarouge S, which can wound the eye. The filters must be placed on the lens of the objective or the opening, but never on the eyepiece because its own filters can break under the action of heat.

The over-exposed photographic films - and thus blacks - are not sufficient to observe the sun in full safety because it let pass too much from infra-reds. It is recommended to use special Mylar glasses, black plastic which lets pass only one very weak fraction of the light.

Particular case of the eclipses

The partial solar eclipses S are particularly dangerous because the Pupille dilates according to the total light of the field of view and not according to the point more shining present in the field. During an eclipse, the major part of the light is blocked by the moon, but the not hidden parts of photosphere are always also brilliant. Under these conditions, the pupil dilates to reach two to six millimetres and each cell exposed to the solar radiation receives approximately ten times more light than by looking at the sun without eclipse! This can damage or even kill these cells what creates small blind points in the vision.

The eclipses are even more dangerous for the inexperienced observers and the children because there is not perception of pain at the time of this destruction of cells. The observers can not realize that them vision is being made destroy.

Particular cases of the rising and to lay down sun

During the paddle and the dawn, the solar radiation is attenuated by the Rayleigh scatter and the Diffusion of Mie due to a longer passage in the terrestrial atmosphere, so much so that the sun can be observed with the naked eye without much danger. On the other hand, it is necessary to avoid looking at it when its light is attenuated by clouds or the fog, because its luminosity could grow very quickly as soon as it would leave there. A misty time, the airborne dusts and nebulosity are as many factors which contribute to attenuate the radiation.

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