Sun

Of all of the astronomical objects, the Sun is the most important to human beings. Since the dawn of civilization, knowing the daily and annual behavior of the Sun has meant the difference between life and death for people learning when to plant crops and when to harvest. Ancient mythologies preserved this knowledge in story form. These were often picturesque descriptions of the Sun's behavior—for example, the Chinese interpretation of a solar eclipse as a dragon chasing and eating the Sun. Sometimes the stories included precise enough details for predicting solar behavior—for instance, in the version from India, the dragon is sliced into two invisible halves. When the position in the sky of one of these halves is lined up with the Sun and the Moon, an eclipse occurs.

Solar Eclipses

Over centuries of observations and study, a scientific understanding of the Sun has grown out of these myths. The invisible dragon halves were a way of describing the serendipitous arrangement of the relative locations and sizes of Earth, the Moon, and the Sun. In order for a solar eclipse to happen, the Moon not only has to be in new phase (between the Sun and Earth) but also has to line up exactly with the disk of the Sun. Since the Moon's orbit around Earth is tilted with respect to Earth's orbit around the Sun, this happens about twice a year instead of once a month. Solar eclipses are not visible all over Earth, but only under the moving shadow of the Moon. In areas not completely covered by the Moon's shadow, observers see a "partial eclipse," which looks like a bite has been taken out of the Sun. Or, if the Moon is in the far reaches of its orbit it might not be quite big enough to cover the Sun's disk. Then observers would see the Sun shining in a thin, bright ring around the Moon in what is known as an "annular eclipse," even if they are perfectly lined up. Total eclipses of the Sun are rarely seen, because the timing and geometry have to be just right to position a large enough Moon-shadow right over a particular location. When this happens, observers in that location have an opportunity to observe parts of the Sun that are usually impossible to see.

Solar Corona

It is when the Sun is totally eclipsed that the solar corona is visible. "Corona" means "crown," and indeed the outer atmosphere of the Sun appears to encircle its blacked-out disk in an extended pearly crown. Ordinarily, the corona is so much dimmer than the bright disk of the Sun that it cannot be seen—even during a partial or annular eclipse. There is another way to see the corona, however, even without an eclipse. Although the part of the Sun seen with the naked eye normally outshines it, the corona is actually the brightest part of the Sun when observed with an X-ray telescope. The Sun emits light at a wide range of frequencies , or colors. Most of the light it emits is in the range visible to human eyes—the colors that make up a rainbow. Human eyes have actually adapted to be sensitive to the frequencies at which the majority of the sunlight shines. X rays are light emitted at much higher frequencies than humans can see, in the same way as a dog whistle blows at a frequency that is beyond the sensitivity of the human ear. An X-ray telescope filters out all the light from the Sun except X rays, and what is left is mostly the solar corona.

Because the corona shines in X rays we know it is very hot. This is strange. It means that although the temperature of the Sun decreases from its center out to its surface (from several million degrees Celsius down to several thousand), it increases again in the corona (up again to several million degrees). How and why the corona gets heated is one of the big mysteries of solar physics. It probably has to do with the energy that comes from magnetic fields generated inside the Sun, which is dumped into the corona, heating it up.

Sunspots and Magnetic Fields

Besides the more obvious daily and annual variations of the Sun, an approximately eleven-year cycle was discovered once people started observing with telescopes. This was first seen by counting the number of sunspots on the Sun. Sunspots are dark regions on the solar surface that are fairly infrequent during the minimum phase of the eleven-year solar cycle, but that become more and more common during the maximum phase. They are dark because they are cooler than their surroundings, and they are cool because they are regions of very strong magnetic field where less heat escapes the solar surface.

Sunspots are not the only solar features that are most abundant at solar cycle maximum. Explosive flashes known as "solar flares," and massive eruptions of material out from the Sun known as coronal mass ejections also become more and more frequent. The material that is hurled outward in a coronal mass ejection can affect us here on Earth, damaging satellites and even power stations, and potentially causing blackouts or disrupting satellite TV or cell phone transmissions. Like sunspots, flares and coronal mass ejections are related to solar magnetic fields. In general, magnetic activity increases at solar cycle maximum.

Magnetic fields are an important part of almost everything that is observed about the Sun. So where do they come from? The motions of sunspots provide a clue. Like Earth, the Sun is spinning so it has its own north pole, south pole, and equator. As they move around as the Sun spins, sunspots near the solar equator return to their starting point in about twenty-five days. Sunspots near the north and south pole of the Sun, however, take about thirty-five days to spin all the way around. The reason for this difference is that the Sun is not solid like a baseball, but fluid—more like a water balloon. Just below the surface this fluid is vigorously boiling and churning around, and this motion causes different parts of the Sun to spin around at different speeds. Furthermore, all this churning and spinning creates a magnetic field that is pointing one way near the north pole of the Sun and the opposite way near the south pole, like a giant bar magnet. Every eleven years, this magnet flips upside down so that in twenty-two years it has flipped over twice and is back where it started. Solar minimum happens when the magnet is pointing either due north or due south, and solar maximum occurs while it is in the process of flipping over.

Inside the Sun

When we look at the Sun, we see only the outside; how do we know what is happening below the surface? It turns out we can use techniques that are similar to those used in studying earthquakes. The surface of the Sun is continuously vibrating like a never-ending earthquake or a bell that is constantly being rung. By looking at the pattern of these vibrations and their frequency (like the tone of the bell), we can figure out what the inside of the Sun must be like. Thanks in part to these vibrations, we can confidently say that the churning motions below the surface not only create magnetic fields and make the Sun spin at different speeds, but they also move heat from the center of the Sun to the surface, where it is radiated away as light.

Near the center of the Sun the churning motions stop and the fluid becomes very dense and hot. Hydrogen atoms fly around at incredible speeds and when they collide they can stick together, creating helium atoms. This process, which is called fusion, provides the energy that causes stars to shine. In some stars, fusion can convert hydrogen and helium into heavier elements, such as carbon, oxygen, and nitrogen, which can in turn be combined to make still heavier elements, such as iron, lead, and even gold! In fact, everything on Earth—air, water, dirt, rocks, buildings, cars, trees, dogs, and even people—is made of elements that were created in stars by fusion.

The Evolution of the Sun

As exciting as it is, the Sun is often referred to as an "ordinary" star. This means that the information gained from the vast array of solar observations can be applied to understanding many of the stars in the sky. Furthermore by studying similar stars at various stages of their lifetimes, astronomers can tell how the Sun formed and how it will eventually die.

The Sun and the solar system began as a huge clump of gas in space, mostly made of hydrogen with some helium and only a relatively small amount of everything else (carbon, oxygen, iron, etc.). This clump slowly condensed and heated up due to gravity, and eventually it became dense and hot enough that fusion began and it started to shine. Not all of the gas fell into the young Sun; some of it stayed behind and was flattened into a pancake-like disk because it was spinning (just as a skilled pizza cook can flatten a clump of dough by tossing and spinning it). This disk then broke up into smaller clumps, which eventually became Earth and the other planets. Meanwhile, the Sun settled down to a quieter life, slowly converting hydrogen into helium by fusion and shining the energy away into space. That was about 5 billion years ago and the Sun is still going strong.

The Sun's Future

But that is not the end of the story. Eventually, there will not be any hydrogen left in the center of the Sun to make helium. Gravity will then cause the center part of the Sun to collapse in on itself, and the energy given off by this implosion will cause the outer part to inflate. So, while the inner part of the Sun shrinks, the outer part will expand, and it will become so big that it will envelop Mercury, Venus, and even Earth.

The Sun will then continue its life as a red giant star, but not for long. As its last hydrogen is used up, the center of the Sun will heat up and start to convert helium into other elements in a last-ditch effort to keep fusion going and to keep shining. The available helium will be used up relatively quickly, however, and before long all fusion in the center will stop. The outer part of the Sun will then slowly expand and dissipate into space while the inner part will become a white dwarf, a relatively small, inactive lump of matter, which will slowly cool down as it radiates all its remaining energy into space. Life on Earth would not survive these events—but as this terrible fate is not due to happen for another 5 billion years, we have plenty more time to study the Sun in all its splendor!

see also Cosmic Rays (volume 2); Solar Particle Radiation (volume 2); Solar Wind (volume 2); Space Environment, Nature of (volume 2); Stars (volume 2); Weather, Space (volume 2).

Sarah Gibson and Mark Miesch

Bibliography

Golub, Leon, and Jay M. Pasachoff. The Solar Corona. Cambridge, UK: Cambridge University Press, 1997.

Krupp, Edwin. C. Echoes of Ancient Skies: Astronomy of Lost Civilizations. New York:Harper and Row, 1983.

Phillips, Kenneth J. H. Guide to the Sun. Cambridge, UK: Cambridge University Press, 1992.

Strong, Keith. T., et al., eds. The Many Faces of the Sun: A Summary of the Results from NASA's Solar Maximum Mission. New York: Springer-Verlag, 1999.

Taylor, Peter O. and Nancy L. Hendrickson. Beginner's Guide to the Sun. Waukesha, WI: Kalmbach Publishing Company, 1995.

Taylor, Roger. J. The Sun as a Star. Cambridge, UK: Cambridge University Press,1997.

Internet Resources

Mr. Eclipse. <http://www.MrEclipse.com/Special/SEprimer.html>.

Solar and Heliospheric Observatory. <http://sohowww.nascom.nasa.gov/>.

The Stanford Solar Center. <http://solar-center.stanford.edu>.

Sun

The Sun, the star at the center of our solar system, is an average-sized, middle-aged star. It is a gas ball made mostly of hydrogen and helium, with a small amount of carbon, nitrogen, oxygen, and trace amounts of heavy metals. The Sun is roughly 865,000 miles (1,392,000 kilometers) in diameter, about 109 times the diameter of Earth. The Sun, so large that more than 1.3 million Earths could fit inside of it, accounts for about 99.8 percent of the mass of the solar system.

Because the Sun is a gas ball, the rate of its rotation about its axis varies—it spins faster around its equator than around its poles. At its equator, it completes one rotation in about 25 Earth days. At its poles, one rotation takes place about every 35 Earth days. The Sun's surface gravity is almost 28 times that of Earth. Its gravitational attraction holds all the planets, comets, and other solar system bodies in their orbits.

The solar core

The Sun's core is located about 312,000 miles (502,000 kilometers) below the surface. With a diameter of 240,000 miles (386,160 kilometers), the core accounts for only about 3 percent of the Sun's volume. Yet it is so dense that it contains about 60 percent of the Sun's mass.

The temperature in this dense area is an incredible 27,000,000°F (15,000,000°C). It is here that nuclear fusion, the Sun's heat-producing process, takes place. Under tremendous pressure and heat, two hydrogen nuclei are combined to form one helium nucleus, releasing a tremendous amount of energy in the process. The amount of helium found in the Sun indicates that the fusion of hydrogen to helium must have been going on for about 4.5 billion years. Scientists estimate that the Sun has enough hydrogen to continue producing energy for about 5 billion more years.

Words to Know

Chromosphere: Glowing layer of gas that makes up the middle atmospheric layer of the Sun.

Convection zone: Outermost one-third of the solar interior where heat is transferred from the core toward the surface via slow-moving gas currents.

Core: Central region of the Sun where thermonuclear fusion reactions take place.

Corona: Outermost and hottest layer of the solar atmosphere.

Flare: Temporary bright spot that explodes on the Sun's surface.

Granules: Earth-sized cells covering the Sun's surface that transfer hot gas from the Sun's interior to its outer atmospheric layers.

Nuclear fusion: Nuclear reactions that fuse two or more smaller atoms into a larger one, releasing huge amounts of energy in the process.

Photosphere: Innermost layer of solar atmosphere that constitutes the Sun's surface and where most of the visible light is emitted.

Plages: Bright hydrogen clouds on the surface of the Sun that are hotter than their surrounding area.

Prominence: High-density cloud of gas projecting outward from the Sun's surface.

Radiative zone: Central two-thirds of the solar interior.

Solar wind: Electrically charged subatomic particles that flow out from the Sun.

Sunspot: Cool area of magnetic disturbance that forms a dark blemish on the surface of the Sun.

Enveloping the core is a region called the radiative zone, in which heat is dispersed into the surrounding hot plasma (a substance made of ions [electrically charged particles] and electrons). Above the radiative zone is the convection zone, where heat is carried toward the surface by slow-moving gas currents. The temperature at the surface of the Sun is about 6,000°F (3,315°C).

The Sun's atmosphere

The atmosphere of the Sun consists of three general layers: the photosphere, the chromosphere, and the corona. Since these layers are composed of gases, no sharp boundaries mark the beginning of one layer and the end of another.

Photosphere. The photosphere, the innermost layer of the Sun's atmosphere, is a few hundred miles thick and has a temperature of about 10,800°F (6,000°C). When gas currents in the convection zone reach the photosphere, they release the heat they carry, then cycle back toward the center of the Sun to be reheated. The photosphere is covered with cells in which this heat transfer occurs. These cells, called granules, are Earth-sized chunks that constantly change size and shape.

Another feature of the photosphere is the presence of sunspots, dark areas that may exceed Earth in size. A sunspot has two components: a small, dark featureless core (the umbra) and a larger, lighter surrounding region (the penumbra). Sunspots vary in size and tend to be clustered in groups. They are magnetic storms caused by the transfer of heat stirring up the weak magnetic field lying beneath them. They are dark because they are 2,700°F (1,500°C) cooler than the surrounding area.

Chromosphere. Beyond the photosphere lies the chromosphere, another region through which heat and light pass from the inner layers to space. It is around 1,200 to 1,900 miles (1,930 to 3,060 kilometers) thick. At its greatest distance from the Sun's surface, the chromosphere has a temperature of about 180,000°F (100,000°C). This atmospheric layer is punctuated with plages and flares. Plages are bright patches that are hotter than their surroundings. Solar flares are sudden, temporary outbursts of light that extend from the outer edge of the chromosphere into the corona, the next layer. They produce an incredible amount of energy in only five to ten minutes. A flare can accelerate solar particles to nearly the speed of light. The largest flares generate enough energy to supply the United States's needs for 100,000 years.

Corona. The chromosphere merges into the outermost part of the Sun's atmosphere, the corona. The weak light emitted by the corona (about one-half the light of a full moon) is usually overpowered by the light of the photosphere and therefore is not detectable. During a solar eclipse, however, the Moon blocks the light of the photosphere and the corona can be seen shining around it.

The corona is the thinnest part of the atmosphere. It consists of low-density gas and is peppered with prominences. Prominences are high-density clouds of gas projecting outward from the Sun's surface into the inner corona. They can be more than 100,000 miles (161,000 kilometers) long and maintain their shape for several months before breaking down. The corona extends out into space for millions of miles. As its distance from the Sun increases, so does its temperature, to an incredible 3,600,000°F (2,000,000°C). Astronomers believe that the corona's energy may emanate from spectacular pillars of fiery gas near the Sun's surface, at the bottoms of looping arches of magnetic fields (like those produced by a bar magnet) that stretch for hundreds of thousands of miles above the surface. Hot gases seem to explode upward along the magnetic fields and heat the rest of the corona.

At its farthest reaches, the corona becomes the solar wind, a stream of charged particles (mainly free protons and electrons) that flows throughout the solar system and beyond. When the solar wind reaches Earth, the protons and electrons are flowing along at speeds up to 620 miles (1,000 kilometers) per second. Little of the solar wind reaches Earth's atmosphere because the charged particles are deflected by the planet's magnetic field. The particles that do get through spiral down toward the north and south magnetic poles where they collide with oxygen and nitrogen molecules present in the upper atmosphere. As a result of this collision, the molecules become ionized (electrically charged) and emit the shimmering, green or red curtains of light known as auroras (aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere).

Solar activity cycle

The solar activity cycle is the periodic variation in active features such as sunspots, prominences, and flares in the Sun's atmosphere and

on its visible surface. Sunspot activity generally follows an 11-year cycle from the time when the number of sunspots is at a maximum to the next. During Solar Max or solar maximum, the Sun's magnetic north and south poles flip or reverse. Also accompanying the variations in sunspot number are corresponding changes in prominences and flares. An increase in all of these solar activities increases the solar wind and other matter ejected by the Sun. This, in turn, increases the appearances of auroras in Earth's atmosphere and also causes radio communication interference.

In April 2001, at the peak of the Sun's solar activity cycle, a solar flare erupted from the surface of the Sun near a giant sunspot that was 14 times as large as Earth. According to scientists, the flare was more powerful than any detected in the previous 25 years. Most of the blast was directed away from Earth. Still, two days after the flare erupted, luminous arcs, streamers, veils, rays, and curtains of light were seen in the night sky above Earth. Fortunately, sensitive electrical and communications systems were spared.

The Sun's end

About 5 billion years from now, the Sun will have used up all of its hydrogen fuel and will swell into a red giant, taking on a reddish color as its temperature begins to drop. Because the Sun will shed a great deal of its mass, Earth may be lucky enough to escape being swallowed up in its outer atmosphere, a 3,000°F (1,650°C) plasma. Even though Earth's orbit will be pushed slowly out into the solar system, the oceans will boil off, the atmosphere will evaporate, and the crust may melt. Earth will be a burnt ember. Eventually, the Sun's atmosphere will float away, leaving only a glowing core called a white dwarf that will cool for eternity.

[See also Nuclear fusion; Solar system; Star; Stellar magnetic fields ]

Sun

The Sun is located in the "suburbs" of the Milky Way galaxy, around 30,000 light-years from the center and within one of its spiral arms. It revolves around the galaxy's center at an average speed of 155 miles per second, taking 225 million years to complete each circuit.

Although the Sun is just one star among an estimated 400 billion stars in the Milky Way galaxy, it is the closest star to Earth. More importantly, it is the only star that provides Earth with enough light, heat, and energy to sustain life. Also, the strong gravitational pull of the Sun holds the Earth in orbit within its solar system.

Many people forget that the Sun is a star because it looks so big and different when compared to other stars and because the Sun appears in the sky during the day, whereas other stars only appear at night. Because the Sun is so close to the Earth, its luminosity (brightness) overwhelms the brightness of other stars, drowning out their daytime light.*

*Because of its relative nearness, the Sun appears about 10 billion times brighter than the next brightest star.

Size and Distance

Archimedes (287 b.c.e.–212 b.c.e.) placed the Sun at the center of the solar system. Observations of Galileo (1564–1642) supported this heliocentric theory, nullifying the ancient belief that the Earth was the center of the solar system.

Before the Sun's distance was known, Aristarchus (310 b.c.e.–230 b.c.e.) knew that the Moon shines by reflected sunlight. Aristarchus decided that if he measured the angle between the Moon and the Sun when the Moon is half-illuminated, he could then compute the ratio of their distances from the Earth. Aristarchus estimated that this angle was 87, resulting in the ratio of their distances at sin 3°.

Before the invention of trigonometry, Aristarchus used a similar method to calculate the inequality , reasoning that the Sun was 18 to 20 times farther away from the Earth than the Moon. As calculations were refined, the angle between Moon and Sun was shown to be 89°50′. Astronomers learned that the Sun is actually 400 times farther away from the Earth than the Moon.

Scientists now know that the Earth-Sun distance is 93 million miles. This distance was discovered when radar signals were bounced off Venus's surface to determine the Earth-to-Venus distance. At a speed of 500 mph, a journey from the Earth to the Sun would take 21 years.

Ancient civilizations thought that the Sun and Moon were the same size. Yet the Sun's diameter is really 864,338 miles across, which is more than 400 times the Moon's diameter and about 109 times the Earth's diameter. The Sun has a volume 1.3 million times the Earth's volume. Thus, a million Earths could be packed within the Sun. Although enormous compared to Earth, the Sun is an average-sized star.

German mathematician Johannes Kepler (1531–1630) devised his laws of planetary motion while studying the motion of Mars around the Sun. The Sun's mass can be calculated from his third law with the equation T ² where T is the period of Earth's revolution (3.15 ×10⁷ seconds), r is the radius of Earth's revolution (1.5×10¹¹m), and G is the planetary constant (6.67 ×10¹¹ Newton's m²/kg). To find M _s (Sun's mass) insert these values into the equation and solve

M_s = (π)(1.5 × 10¹¹m)³/(6.67 × 10⁻¹¹N m²/kg)(3.15 × 10⁷s )²
       = 2.0 × 10³⁰ kilograms.

Therefore, the Sun's mass is about 300,000 times the Earth's mass. Yet with respect to other stars, the Sun's mass is just average.

Time and Temperature

The Sun's rotation is similar to the Earth's rotation (one rotation every day), but because the Sun is gaseous not all parts rotate at the same speed. Galileo first noticed that the Sun rotates when he observed sunspots moving across the disk. He found that a particular spot took 27 days to make a complete circuit. Later observations found that the Sun at its equator rotates in slightly over 24.5 days. At locations two-thirds of the distance above and below the equator, the rotation take nearly 31 days.

In order to support its large mass, the Sun's interior must possess extremely large pressures and temperatures. The force of gravity at the core's surface is about 250 million times as great as Earth's surface gravity. No solids or liquids exist under these conditions, so the Sun's body primarily consists of the gases hydrogen (73 percent) and helium (25 percent).

Within the Sun's core, nuclear fusion reactions release huge amounts of energy. About 5 billion kilograms of hydrogen convert to helium, releasing energy each second. The core temperature is about 15 million kelvin, with a density of 160 grams per cubic centimeter. Based on mathematical calculations, the solar core is approximately the size of Jupiter, or approximately 75,000 to 100,000 miles in diameter. The amount of hydrogen within the Sun's core should sustain fusion for another 7 billion years.

For now, the Sun is bathing the Earth with just the right amount of heat. High-energy gamma rays (the particles created by fusion reactions) travel outward from the core and ultimately through the outer layers of the photosphere , chromosphere , and corona , losing most of their energy in the process. Along the way to the Sun's surface, the temperature of the gamma rays has dropped from 15 million to 6,000 kelvin. Yet even at these temperatures, the Sun is in the middle range of stellar surface temperatures.

Measuring the Sun

Information available about the Sun has increased with revolutionary scientific discoveries. Early telescopic observations allowed scientific study to begin, showing that the Sun is a dynamic, changing body. Later developments within spectroscopy, and the discovery of elementary particles and nuclear fusion, allowed scientists to further understand its composition and the processes that fuel it.*

*Astronomers now believe that the Sun is about 4.6 billion years old and will shine for another 7 billion years.

Recent developments of artificial satellites and other spacecraft now allow scientists to continuously study the Sun. Among the advances that have significantly influenced solar physics are the spectroheliograph, which measures the spectrum of individual solar features; the coronagraph, which permits study of the solar corona without an eclipse; and the magnetograph, which measures magnetic-field strength over the solar surface. Space instruments have revolutionized solar study and continue to add to increased, but still incomplete, knowledge about the Sun.

see also Archimedes; Galileo Galilei; Solar System Geometry, History of; Solar System Geometry, Modern Understandings of.

William Arthur Atkins with

Philip Edward Koth

Bibliography

Nicolson, Iain. The Sun. New York: Rand McNally and Company, 1982.

Noyes, Robert W. The Sun: Our Star. Cambridge, MA: Harvard University Press, 1982.

Washburn, Mark. In the Light of the Sun. New York: Harcourt Brace Jovanovich, 1981.

Internet Resources

"The Sun." NASA's Goddard Space Flight Center. <http://www-istp.gsfc.nasa.gov/Education/Isun.html>.

"Today from Space: Sun and Solar System." Science at NASA. <http://www.science.nasa.gov/newhome/pad/sun_today.htm#anchor1452757>.

sun the star round which the earth orbits and from which it receives light and warmth; it is the central body of the solar system and provides the light and energy that sustains life on earth, and its changing position relative to the earth's axis determines the terrestrial seasons.

In the ancient and medieval world, it was believed (in accordance with the Ptolemaic system) that the earth is the stationary centre of the universe. The heliocentric theory was proposed by the Polish astronomer Copernicus (1473–1543) in De Revolutionibus Orbium Coelestium (1543), and later supported by Galileo (1564–1642); although he was forced to recant by the Inquisition, the theory continued to gain ground.

The sun has been an object of worship in a number of religions, and has thus been personified as a male being, sometimes identified with a particular god, especially Apollo, who in classical mythology was believed to drive his chariot across the sky.

Proverbially the sun is a type of brightness and clearness, and in literary and poetic usage often stands for a person or thing regarded as a source of glory, inspiration, or understanding; the word may also be used with reference to someone's success or prosperity.

Recorded from Old English (in form sunne), the word is of Germanic origin, and comes ultimately from an Indo-European root shared by Greek hēlios and Latin sol.
never let the sun go down on your anger proverbial saying, mid 17th century, recommending a swift reconciliation after a quarrel; originally with biblical allusion to Ephesians 4:26, ‘Let not the sun go down on your wrath’.
sun in splendour in heraldry, the sun as heraldically blazoned, depicted with rays and often a human face; it was an emblem of the House of York.
Sun King a designation of Louis XIV of France, a translation of French roi soleil.
the sun loses nothing by shining into a puddle proverbial saying, early 14th century, of classical origin, meaning that something which is naturally clear and radiant cannot be tainted or diminished by association. The comment ‘the sun shines into dung but is not tainted’ is attributed to the Greek philosopher Diogenes, and Tertullian has, ‘the sun spreads his rays even into the sewer, and is not stained’.
Sun of Righteousness an epithet of Jesus Christ, after Malachi 4:2.
when the sun is over the yardarm originally in nautical usage, the time of day (noon) when it is permissible to begin drinking; the earlier variant when the sun is over the foreyard dates from the mid, and this from the late, 19th century.

See also happy is the bride that the sun shines on, make hay while the sun shines, nothing new under the sun, place in the sun.

Sun (symbol ⨀) The central body of the Solar System, and by far the nearest star—the only one that can be studied in great detail. It is classified as a G2V star: a yellowish star with an effective temperature of 5770 K (spectral type G2) and a main-sequence dwarf (luminosity class V). Its apparent visual magnitude is −26.7, but its absolute magnitude is only +4.82. The Sun is largely hydrogen (71% by mass), with some helium (27%) and heavier elements (total 2%). Its age is estimated to be about 4.6 billion years. The energy produced by nuclear reactions at its core is transferred to the surface layer, or photosphere, by radiation through the inner two-thirds of its radius and thence by convection for the outer one-third. The transfer of energy from the core to the surface takes 10 million years. At its centre the temperature is calculated to be 15.6 million K and the density 148 000 kg/m³. The vast majority of its energy escapes to space from the photosphere. The photosphere is marked by granulation and supergranulation, both of which are relatively small-scale convection currents, and features related to solar activity, such as sunspots and faculae, which are associated with regions of strong magnetic field. The Sun's sidereal rotation period is about 25 days at the equator, and 27–28 days at latitude 40°; near the poles the period is 33.5 days. The adopted mean value, corresponding to latitude 17°, is 25.38 days.

sun: physical data

387. Sun

See also 25. ASTRONOMY ; 85. CLIMATE ; 100. COSMOLOGY ; 143. EQUATOR ; 245. LIGHT ; 318. PLANETS ; 417. WEATHER .

celidography

Archaic. a description of the surface markings on a planet or the spots on the sun.

coronagraphy

the observation of the corona of the sun by use of a telescope modifled to simulate an eclipse. —coronagraphic, adj.

heliodon

an instrument used in astronomy to show the apparent movement of the sun.

heliography

1. the measurement of the duration and intensity of sunlight.

2. the system or process of signaling by reflecting the sun’s rays in a mirror.

3. an early photographic process involving coated metal plates exposed to sunlight. —heliographer, n. —heliographic, heliographical, adj.

heliolatry

the worship of the sun. — heliolator, n.

heliology

Archaic. the science of the sun. — heliologist, n.

heliomania

an abnormal love of the sun.

heliophobia

1. an abnormal fear of sunlight.

2. an avoidance of sunlight.

helioseismology

the study of motions of the solar surf ace.

heliotherapy

a method of treating illness by exposure to the rays of the sun.

pyrheliometer

an instrument for measuring the intensity of the sun’s radiation. —pyrheliometric, adj.

radiometer

an instrument for measuring the intensity of radiant energy, composed of vanes which rotate at speeds proportionate to the intensity of the energy source. —radiometric, adj.

radiometry

the measurement of radiant energy by means of a radiometer. —radiometric, adj.

radiophony

the transformation of radiant energy into sound.

siriasis

1. sunstroke.

2. Obsolete, a sun bath or exposure to the sun for curative purposes.

solarism

1. the explanation of myths by reference to the sun or the personifi-cation of the sun, as the hero as sunfigure.

2. an overreliance on this method of interpretation. —solarist, n.

solarium

a room designed and situated so as to receive the maximum amount of sunlight.

sun / sən/ • n. 1. (also Sun) the star around which the earth orbits. ∎  any similar star in the universe, with or without planets. 2. (usu. the sun) the light or warmth received from the earth's sun: we sat outside in the sun. ∎ poetic/lit. a person or thing regarded as a source of glory or inspiration or understanding: the rhetoric faded before the sun of reality. ∎ poetic/lit. used with reference to someone's success or prosperity: the sun of the Plantagenets went down in clouds. 3. poetic/lit. a day or a year: after going so many suns without food, I was sleeping. • v. (sunned , sun·ning ) (sun oneself) sit or lie in the sun: Buzz could see Clare sunning herself on the terrace below. ∎  [tr.] expose (something) to the sun, esp. to warm or dry it: the birds are sunning their wings. PHRASES: against the sun Naut. against the direction of the sun's apparent movement in the northern hemisphere; from right to left or counterclockwise. catch the sunsee catch. make hay while the sun shinessee hay¹ . on which the sun never sets (of an empire) worldwide. place in the sunsee place. shoot the sun Naut. ascertain the altitude of the sun with a sextant in order to determine one's latitude. under the sun on earth; in existence (used in expressions emphasizing the large number of something): they exchanged views on every subject under the sun. with the sun Naut. in the direction of the sun's apparent movement in the northern hemisphere; from left to right or clockwise.DERIVATIVES: sun·less adj. sun·less·ness n. sun·like / -ˌlīk/ adj. sun·ward / -wərd/ adj. & adv. sun·wards / -wərdz/ adv.

621. Sun (See also Light.)

1. Apollo sun god; his chariot ride spanned morning to night. [Gk. Myth.: Benét, 42]
2. Aton (Aten) solar deity worshiped as the one god by Amenophis IV. [Egypt. Myth.: Parrinder, 33]
3. Bast cat-headed goddess representing sun and moon. [Egypt. Myth.: Parrinder, 41]
4. Belenus sun god. [Celtic Myth.: Parrinder, 42]
5. Buto goddess and mother of the sun and moon. [Egypt. Myth. Kravitz, 48]
6. cock Helios’s sacred bird; sacrificed to the sun in Mexico. [Rom. and Mex. Myth.: Leach, 239]
7. Cuchulain sun-figure and powerful fighter. [Irish Myth.: Parrinder, 68]
8. double ax symbol of the sun. [Hindu and Western Folklore: Cirlot, 22]
9. eagle symbol represents the sun. [Gk. Myth.: Brewer Dictionary, 358]
10. fire representation of the sun. [Western Symbolism: Cirlot, 105–106]
11. gold color of the sun’s rays. [Color Symbolism: Jobes, 357]
12. Helios sun in its astronomic aspects; aspect of Apollo. [Gk. Myth: Espy, 28]
13. Horns solar deity, portrayed as a hawk-headed man. [Egypt. Myth.: Benét, 478]
14. Hyperion Titan and father of the sun. [Gk. Myth.: Zimmer-man, 132]
15. lion symbol of the sun gods; corresponds to the sun. [Western Symbolism: Cirlot, 189–190]
16. Mithra (Mithras) god of sunlight. [Persian Myth.: EB, VI: 944–945]
17. Phaëthon Apollo’s son; foolishly attempted to drive sun chariot. [Gk. Myth.: Zimmerman, 202]
18. Phoebus epithet of Apollo as the sun god. [Gk. Myth.: Benét, 42]
19. Ra personification of the sun. [Egypt. Myth.: Parrinder, 235]
20. Sol the sun god. [Rom. Myth.: Zimmerman, 245]

sun OE. sunne (fem.) = OS., OHG., ON. (poet.) sunna (Du. zon, G. sonne), Goth. sunnō, beside OE. sunna (m.), OHG., OS. sunno :- Gmc. *sunnōn, -an-; f. IE. *su- with n-formative, beside *sāu- with l-formative in Homeric Gr. ēélios :- sāwelijos (Attic hḗlios), L. sōl, ON. sól, Goth. sauil, W. haul.
Hence vb. XVI; sunny (-Y1) XIII. Comps. sunbeam OE. sunn(e)bēam. sunburn sb. XVII; f. the vb. (XVI), back-formation from sunburning (XVI), sunburnt (sunne ybrent XIV). sundew plant of the genus Drosera. XVI. tr. Du. son-, sundauw = G sonnentau, tr. L. rōs sōlis. sundial XVI. sundown setting of the sun XVII. perh. shortening of †sunne gate downe (XV), †sun go downe (XVI). sunflower †heliotrope; plant of the genus Helianthus, with showy golden-rayed flowers. XVI. tr. modL flōs sōlis XVI. sunrise XV. perh. evolved, through syntactical ambiguity, from a clause such as before the sun rise (pres. subjunctive of the vb.); cf. ME. sonne rist (XIII). sunset OE. (late Nhb.) sunset; perh. partly from a clause like ere the sun set. sunshade parasol. XIX. sunstroke XIX. For earlier stroke of the sun, tr. F. coup de soleil. sun-up (U.S.) sunrise XIX. After sun-down.

Sun Star at the centre of our Solar System, around which all other Solar System bodies revolve in their orbits. The Sun is a typical, average star. It consists of c.70% hydrogen (by weight) and 28% helium, with the remainder mostly oxygen and carbon. Its temperature, pressure, and density increase towards the centre. Like all stars, the Sun's energy generates by nuclear fusion reactions taking place under the extreme conditions in the core. This core is c.400,000km (250,000mi) across. Energy released from the core passes up through the radiative zone, which is c.300,000km (200,000mi) thick, then passes through the 200,000km (125,000mi) thick convective zone to the surface, the photosphere, from where it radiates into space. Most of the Sun's visible activity takes place in this 500km (300mi) -thick photosphere. Above the photosphere lies the chromosphere, which consists of hot gases and extends for thousands of kilometres. Extending outwards from the chromosphere for millions of kilometres is the corona, which emits the solar wind. The solar wind and the Sun's magnetic field dominate a region of space called the heliosphere, which extends to the boundaries of the Solar System.

http://lpl.arizona.edu/nineplanets/nineplanets/sol.html

Sun

The largest object in the sky, the sun is the source of light, heat, and life. It can also be a symbol of destructive power. Since earliest times, people in all parts of the world have observed the position of the sun and its rising and setting throughout the year. Many cultures have created solar calendars to govern such things as the planting of crops and the timing of religious festivals. They have also given the sun a major place in their mythologies, often as a deity.

Solar Deities. The pantheons of many cultures have included a sun deity, usually a god but occasionally a goddess. Some myths reflect the sun's vital role in supporting life: Solar deities are often creators who bring people into existence. Native Americans from the Pacific Coast, for example, tell how the sun god Kodoyanpe and the trickster Coyote together created the world and set about making people to live in it.

Solar deities have also been associated with fertility of people and the earth. The Hittites of ancient Turkey worshiped Arinna, an important goddess of both the sun and fertility. In traditional myths from Uganda in Central Africa, the creator god Ruhanga, the sun god Kazooba, and the giver of life Rugaba are all the same deity.

solar relating to the sun

deity god or goddess

pantheon all the gods of a particular culture

trickster mischievous figure appearing in various forms in the folktales and mythology of many different peoples

In some mythologies, sun gods have healing powers. Shamash, the solar god of the Babylonian* people of the ancient Near East, was known as "the sun with healing in his wings." Ancient Celtic* peoples had Belenus, the god of sunlight. Besides driving away the predawn mists and fogs each day, Belenus could melt away disease from the sick. When the Romans conquered the Celts, they identified Belenus with their own sun god, Apollo, who was also a god of healing.

As the most important and splendid deities of their pantheons, some solar deities have been associated with earthly rulers, the most powerful people in society. The Incas of Peru in South America regarded the sun god Inti, their chief deity, as the ancestor of the Inca royal family. According to Japanese tradition, the country's imperial family is descended from Amaterasu, the sun goddess.

Myths About the Sun. Some solar myths explain the sun's daily movement across the sky from east to west and its disappearance at night. Such stories often take the form of a journey, with the sun deity traveling across the heavens in a chariot or boat. Helios, a Greek solar deity later identified with Apollo, was a charioteer who drove his fiery vehicle through heaven by day. At night he floated back across the ocean in a golden bowl, only to mount his chariot again the next morning. The Navajo people of the American Southwest portray their sun god as a worker named Jóhonaa'éí, or sun bearer. Every day Jóhonaa'éí laboriously hauls the sun across the sky on his back. At night, he hangs the sun from a peg in the wall and rests.

The Egyptian sun god Ra made a similar circuit. Each day he traveled across the sky in his sun boat, and at night he passed through the underworld, greeting the dead and facing many dangers. Ra's daily cycle was more than a journey, though—it was a daily rebirth. Dawn saw the newborn sun god rise in the sky. During the morning he was a child, at noon he was mature, and by sunset he was an old man ready for death. Each sunrise was a celebration of the god's return, a victory of life over the forces of death and darkness.

The Celts also viewed the sun's journey as a cycle of death and rebirth but on a yearly rather than a daily cycle, with midwinter as death and spring as rebirth. The Celtic celebration called Beltane, held in spring, honored their sun god Belenus.

underworld land of the dead

In some solar myths the sun is paired with the moon. The two may be husband and wife, brother and sister, or two brothers. In the mythology of many Native Americans, the sun god and moon god are sister and brother who also become forbidden lovers. The moon god's face is smeared with ash from the sun's fires, which accounts for the dark patches on the moon's surface. In some accounts, the moon flees in shame when he learns that his lover is also his sister. This is why the moon leaves the sky when the sun comes near.

Many cultures have myths of monsters or evil spirits that steal or devour the sun or stories of the sun falling from the heavens or withdrawing its light for a time. Some of these myths may explain eclipses, times when the earth's shadow temporarily blots out the sun or moon. A solar eclipse creates a period of eerie near-darkness in the middle of the day—an event that surely cried out for a reassuring explanation.

A well-known myth about the Japanese sun goddess Amaterasu tells how she became so angry with her brother, who was misbehaving, that she retreated into a cave. The goddess's withdrawal deprived the world of light and warmth. Finally, the other gods tricked her into emerging.

Too Many Suns

If one sun is good, are ten suns ten times better? Not according to the Chinese myth of Yi and the ten suns. Yi, a famous soldier, was an archer of great skill. At that time, ten suns lived in the Fu Sang tree beyond the eastern edge of the world. Normally the suns took turns lighting the earth, one sun at a time. The suns grew rebellious, and one day all ten of them rose into the sky at the same time. The extra light and heat pleased the people below—until their crops shriveled and their rivers began to dry up. The Lord of Heaven sent Yi, the divine archer, to handle the problem. Yi shot nine of the suns out of the sky.

According to a traditional myth from the Hindu Kush mountains of Afghanistan, the giant Espereg-era once stole the sun and the moon. The hero god Mandi disguised himself as a child and tricked the giants into adopting him. After a time with the giants, Mandi rescued the sun and moon and rode off with them on a magical horse. The supreme god then hurled them into the sky to shine on the world.

See also Amaterasu; Apollo; Aten; Inti; Lug; Mithras; Moon; Ra (Re); Shamash; Stars.

Sun The central star (G spectral type) in the solar system, 696 000 km in diameter, 333 000 × Earth mass, 1 300 000 × Earth volume, and with a mean density of 1410 kg/m³. The equator is inclined at 7.25° to the plane of the ecliptic. It is principally composed of hydrogen and helium. The visible surface is the ‘photosphere’ (temperature 6000K). The Si-normalized solar abundances for the terrestrially non-gaseous elements match those of the C1 carbonaceous chondrites.

sun References to the sun in the Bible include its beneficial effects in agriculture (Deut. 33: 14) and also the unpleasant injuries it causes (Ps. 121: 6). It symbolizes the glory of God (Rev. 1: 16) but on the day of the Lord the sun will be eclipsed (Joel 2: 10; Rev. 6: 12).

Worship of the sun was common in the ancient world and was strictly forbidden to Israelites.

Sun river, c.130 mi (210 km) long, rising in the Rocky Mts., NW Mont., and flowing generally E to the Missouri River at Great Falls. The Sun River project of the U.S. Bureau of Reclamation utilizes the Sun and its tributaries to irrigate c.92,000 acres (37,230 hectares) of land. Of the system of dams and reservoirs, Gibson Dam is one of the project's largest.

sun intensely hot, self-luminous body of gases at the center of the solar system . Its gravitational attraction maintains the planets, comets, and other bodies of the solar system in their orbits.

General Characteristics of the Sun

The sun is actually a star of about medium size; it appears larger than the other stars because of its relative nearness to the earth. The earth's distance from the sun varies from 91,377,000 mi (147,053,000 km) at perihelion to 94,537,000 mi (152,138,000 km) at aphelion (see apsis ). The mean distance is c.92,960,000 mi (149,591,000 km); this is taken as the astronomical unit (AU) of distance used for measuring distances within the solar system. The sun is approximately 865,400 mi (1,392,000 km) in diameter, and its volume is about 1,300,000 times that of the earth. Its mass is almost 700 times the total mass of all the bodies in the solar system and 332,000 times that of the earth. The sun's surface gravity is almost 28 times that of the earth; i.e., a body on the surface of the sun would weigh about 28 times its weight on earth. The density of the material composing the sun is about one fourth that of the earth; compared with water, the sun's average density is 1.41. At its center, the sun has a density of over 100 times that of water, a temperature of 10 to 20 million degrees Celsius, and a pressure of over 1 billion atmospheres.

Observations of sunspots and studies of the solar spectrum indicate that the sun rotates on its axis from east to west; because of its gaseous nature its rate of rotation varies somewhat with latitude, the speed being greatest (a period of almost 25 days) in the equatorial region and least at the poles (a period of about 35 days). The axis of the sun is inclined at an angle of about 7° to the plane of the ecliptic .

The bright surface of the sun is called the photosphere . Its temperature is about 6,000°C. The photosphere appears darker near the edge (limb) of the sun's disk because of greater absorption of light by the sun's atmosphere in this area; this phenomenon is called limb darkening. During an eclipse of the sun the chromosphere and the corona (the outer layers of the sun's atmosphere) are observed. Also of interest is the high-speed, tenuous extension of the corona known as the solar wind .

Production of Solar Energy

The vast and continual production of solar energy cannot be attributed merely to combustion, to the gradual cooling of a hot body, to the fall of meteorites into the sun, or to gradual shrinkage with transformation of potential energy into heat (a theory proposed by Helmholtz). The theory of relativity with its implication of the equivalence of mass and energy led to the assumption that energy stored in the atoms constituting the sun's gases is constantly being released by conversion of some of the masses of the atom's nuclei during nuclear transmutations (see nuclear energy ). H. A. Bethe proposed a cycle of nuclear reactions known as the carbon cycle, or CNO bi-cycle, to account for the nuclear changes. In this cycle carbon acts much as a catalyst, while hydrogen is transformed by a series of reactions into helium and large amounts of high-energy gamma radiation are released. It is now thought that the so-called proton-proton process is a more important energy source; this process begins with the collision of two protons and ends with the production of helium, while gamma radiation is released throughout.

See nucleosynthesis ; stellar evolution .

The Study of the Sun

By means of the spectroscope much has been learned about the composition of the sun. There are numerous dark lines of varying widths in the solar spectrum . These were first intensively studied by Joseph Fraunhofer and are commonly known by his name. From a study of the lines the chemical composition of the sun is determined on the basis of the discovery by Kirchhoff that the dark lines correspond in position to the bright lines characteristic of the spectra produced by elements in the laboratory. The darkness of the lines in the sun's spectrum is attributed to the presence of a slightly cooler layer of gases above the photosphere, known as the reversing layer, which absorbs selectively the light of the photosphere and thus causes dark lines instead of bright ones to be observed through the spectroscope. By comparison of the sun's spectrum with laboratory spectra of incandescent elements, most of the elements known on earth have been identified in the sun's atmosphere.

Beyond the red portion of the visible solar spectrum is the infrared spectrum; for the study of these heat rays S. P. Langley invented the bolometer, a highly sensitive electrical device for measuring temperature. Solar heat and energy are measured by an instrument called the pyrheliometer. Other instruments devised especially for the study of the sun are the coronagraph and the spectroheliograph . These instruments have revealed a number of interesting phenomena occurring during the periods of solar activity associated with sunspots , e.g., faculae, plages (flocculi), prominences, and flares.

Importance to Terrestrial Life

Without the heat and light of the sun, life as we know it could not exist on the earth. Since solar energy is used by green plants in the process of photosynthesis, the sun is the ultimate source of the energy stored both in food and fossil fuels. Solar heating sets up convection currents, and thus is the source of the energy of moving air. Falling rain also owes its energy to the sun because of the relation of solar radiation to the water cycle.

Bibliography

See K. Hufbauer, Exploring the Sun: Solar Science since Galileo (1993); R. Krippenhahn, Discovering the Secrets of the Sun (1994); K. J. H. Phillips, Guide to the Sun (1995); P. O. Taylor, Beginners Guide to the Sun (1996); S. T. Suess and B. T. Tsurutani, ed., From the Sun: Auroras, Magnetic Storms, Solar Flares, Cosmic Rays (1998).

sun •begun, bun, done, Donne, dun, fine-spun, forerun, fun, gun, Gunn, hon, Hun, none, nun, one, one-to-one, outdone, outgun, outrun, pun, run, shun, son, spun, stun, sun, ton, tonne, tun, underdone, Verdun, won •honeybun • handgun • flashgun •air gun • sixgun • popgun • shotgun •blowgun, shogun •speargun • scattergun • homespun •endrun • sheep run • grandson •stepson • godson • kiloton • megaton •anyone • everyone • someone