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American Heritage Dictionary (8)

noun A star that is the basis of the solar system and that sustains life on Earth, being the source of heat and light. It has a mean distance from Earth of about 150 million kilometers (93 million miles) a diameter of approximately 1,390,000 kilometers (864,000 miles) and a mass about 330,000 times that of Earth.
noun A star that is the center of a planetary system.
noun The radiant energy, especially heat and visible light, emitted by the sun; sunshine.

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Century Dictionary (46)

The central body of the solar system, around which the earth and other planets revolve, retained in their orbits by its attraction, and supplied with energy by its radiance. Its mean distance from the earth is a little less than 93 millions of miles, its horizontal parallax being 8.″80 ± 0.″02. Its mean apparent diameter is 32′ 04″; its real diameter 806,500 miles, 109½ times that of the earth. Its volume, or bulk, is therefore a little more than 1,300,000 times that of the earth. Its mass—that is, the quantity of matter in it—is 330,000 times as great as that of the earth, and is about 900 times as great as the united masses of all the planets. The force of gravity at the sun's surface is nearly 28 times as great as at the earth's surface. The sun's mean density (mass ÷ volume) is only one fourth that of the earth, or less than one and a half times that of water. By means of the spots its rotation can be determined. It is found that the sun's equator is inclined 7¼° to the plane of the ecliptic, with its ascending node in (celestial) longitude 73° 40′. The period of rotation appears to vary systematically in different latitudes, being about 25 days at the equator, while in solar latitude 40° it is fully 27. Beyond 45° there are no spots by which the rate of rotation can be determined. The cause of this peculiar variation in the rate of the sun's surface motion is still unexplained, and presents one of the most important problems of solar research. The sun's visible surface is called the photosphere, and is made up of minute irregularly rounded “granules,” intensely brilliant, and apparently floating in a darker medium. These are usually 400 or 500 miles in diameter, and so distributed in streaks and groups as to make the surface, seen with a low-power telescope, look much like rough drawing-paper. Near sun-spots, and sometimes elsewhere, the granules are often drawn out into long filaments. (See sun-spot.) In the neighborhood of the sun-spots, and to some extent upon all parts of the sun, faculæ (bright streaks which are due to an unusual crowding together and upheaval of the granules of the photosphere) are found. They are especially conspicuous near the edge of the disk. At the time of a total eclipse certain scarlet cloud-like objects are usually observed projecting beyond the edge of the moon. These are the prominences or protuberances, which in 1868 were proved by the spectroscope to consist mainly of hydrogen, always, however, mixed with at least one other unidentified gaseous element (provisionally named helium), and often interpenetrated with the vapors of magnesium, iron, and other metals. It was also immediately discovered by Janssen and Lockyer that these beautiful and vivacious objects can be observed at any time with the spectroscope, and that they are only extensions from an envelop of incandescent gases which overlies the photosphere like a sheet of scarlet flame, and is known as the chromosphere. Its thickness is very irregular, but averages about 5,000 miles. The prominences are often from 50,000 to 100,000 miles in height, and occasionally exceed 200,000; they are less permanent than the spots, and their changes and motions are correspondingly swift. They are not conflned to limited zones of the sun's surface; those of the greatest brilliance and activity are, however, usually connected with spots, or with the faculæ which attend the spots. The corona—the most impressive feature of a total eclipse—is a great “glory” of irregular outline surrounding the sun, and composed of nebulous rays and streams which protrude from the solar surface, and extend sometimes to a distance of several millions of miles, especially in the plane of the sun's equator. The lower parts are intensely bright, but the other parts are faint and indefinite. Its real nature, as a true solar appendage and no mere optical or atmospheric phenomenon, has been abundantly demonstrated by both the spectroscope and the camera. Its visual spectrum is characterized by a vivid bright line in the green (the so-called 1474 line, first observed in 1869) and by the faintly visible lines of hydrogen. Since then many other lines have been brought out by photography in the violet and ultra-violet parts of the spectrum. This proves that the corona consists largely of some unidentified gaseous element (provisionally known as coronium), mingled to some extent with hydrogen and metallic vapors, and probably impregnated with meteoric dust. The fact that the corona is observable only during the few moments of a total solar eclipse makes its study slow and difficult. Huggins has attempted to overcome the difficulty by means of photography, and, though without an absolute success so far, the results are not wholly discouraging. The spectroscope enables us to determine the presence in the sun of certain well-known terrestrial elements in the state of vapor. The solar spectrum is marked by numerous dark lines (known as Fraunhofer's lines), and between 1850 and 1860 their explanation was worked out as depending upon the selective absorption due to the transmission of the light from the photosphere through the overlying atmosphere of cooler gases and vapors. Kirchhoff was the first (in 1859) to identify many of the familiar elements whose vapors thus impress their signature upon the sunlight. According to the recent investigations of Rowland (not yet entirely completed), thirty-six of the chemical elements are already identified in the solar atmosphere, all of them metals, hydrogen excepted. Among them barium, calcium, carbon, chromium, cobalt, hydrogen, iron, magnesium, manganese, nickel, silicon, sodium, titanium, and vanadium are either specially conspicuous or theoretically important. The fact that some of the most abundant and important of the terrestrial elements fail to show themselves is, of course, striking, and probably significant. Chlorin, oxygen (probably), nitrogen, phosphorus, and sulphur are none of them apparent; it would, however, be illogical and unsafe to infer from their failure to manifest themselves that they are necessarily absent. A difference of opinion prevails as to the precise region of the solar atmosphere in which Fraunhofer's lines originate. Some hold that the absorption which produces them takes place almost entirely in a comparatively thin stratum known as the reversing-layer, just above the surface of the photosphere. Lockyer holds, on the other hand, that many of them originate at a high elevation, and even above the chromosphere. Photometric observations show that the brilliance of the solar surface far exceeds that of any artificial light: it is about 150 times as great as that of the lime-cylinder of the calcium-light, and from two to four times as great as that of the “crater” of the electric arc. It is to be noted that the brightness of the sun's disk falls off greatly near the edge, owing to the general absorption by the solar atmosphere. The solar constant is defined as the quantity of heat (in calories) received in a unit of time by an area of a square meter perpendicularly exposed to the sun's rays at the upper surface of the earth's atmosphere, when the earth is at its mean distance from the sun. This quantity can be determined, with some approach to accuracy (say within 10 or 15 per cent.), by observations with pyrheliometers and actinometers. The earliest determinations (by J. Herschel and Pouillet, in 1838) gave about 19 calories a minute; later and more elaborate observations give larger results. Langley's observations make it very probable that its value is not under 30. Assuming it, however, as 25, it appears that the amount of energy incident upon the earth's atmosphere in the sun's rays is nearly 2⅓ continuous horse-power per square meter when the sun is vertical; at the sea-level this is reduced about one third by the atmospheric absorption. The total amount of energy radiated by the sun's surface defies conception; it is fully 100,000 continuous horse-power or more than 1.100,000 calories a minute for every square meter, and according to Ericsson more than 400 times as great as that radiated by a surface of molten iron. It would melt in one minute a shell of ice 50 feet thick incasing the photosphere : to supply an equal amount by combustion would require the hourly burning of a layer of the best anthracite more than 20 feet thick—more than a ton for every square foot of surface. As to the temperature of the sun, our knowledge is comparatively vague. We have no means of determining with accuracy from our present laboratory data the temperature the photosphere must have in order to enable it to emit heat at the known rate. Various (and high) authorities set it all the way from about 2,500° C. to several millions of degrees. Experiments with burning-glasses, however, and observations upon the penetrating power of the solar rays, demonstrate that the temperature of the photosphere is certainly higher than that of any known terrestrial source, even the electric arc itself. The only theory yet proposed concerning the maintenance of the sun's heat which meets the case at all is that of Helmholtz, who finds the explanation in a slow contraction of the solar globe. A yearly shrinkage of about 250 feet (or 300 feet, if we accept Langley's value of the solar constant) in the sun's diameter would make good the whole annual expenditure of radiant energy, and maintain the temperature unchanged. If this is the true explanation, it follows, of course, that in time—probably in about eight or ten millions of years—the solar heat will begin to wane, and will at last be exhausted. It should be noted also that certain other causes—such, for instance, as the fall of meteors on the sun—contribute something to its heat-supply; but all of them combined will account for not more than a small percentage of the whole. The view now generally accepted of the constitution of the sun accords with this theory of the solar heat. The sun is believed to be, in the main, a mass of intensely heated gas and vapor, powerfully compressed by its own gravity. The central part is entirely gaseous, because its temperature, being from physical necessity higher than that of the inclosing photosphere, is far above the so-called “critical point” for every known element; no solidification, no liquefaction even, can therefore occur in the solar depths. But near the outer surface radiation to space is nearly free, the temperature is lowered to a point below the “critical point” of certain substances, and under the powerful pressure due to solar gravity condensation of the vapors begins, and thus a sheet of incandescent cloud is formed, which constitutes the photosphere. The chromosphere consists of the permanent gases and uncondensed vapors which overlie the cloud-sheet, while the corona still remains in great degree a mystery, as regards both the substances which compose it and the forces which produce and arrange its streamers. See also cut under sun-spot. To fynde the degree in which the sonne is day by day after hir cours abowte. Chaucer, Astrolabe, ii. 1. I'll say this for him, There fights no braver soldier under sun, gentlemen. Fletcher, Humorous Lieutenant, i. 1. To him that sitting on a hill Sees the midsummer, midnight, Norway sun Set into sunrise. Tennyson, Princess, iv. Without solar fire we could have no atmospheric vapour, without vapour no clouds, without clouds no snow, and without snow no glaciers. Curious then as the conclusion may be, the cold ice of the Alps has its origin in the heat of the sun. Tyndall, Forms of Water, p. 7.
The sunshine; a sunny place; a place where the beams of the sun fall: as, to stand in the sun (that is, to stand where the direct rays of the sun fall).
Anything eminently splendid or luminous : that which is the chief source of light, honor, glory, or prosperity. The sun of Rome is set! Shak., J. C., v. 3. 63. I will never consent to put out the sun of sovereignty to posterity. Eikon Basilike.

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Just as the sun was about half hid beyond the Western horizon our car reached terra-firma in the state of New York. — The Bark Covered House
Tika blessed her stars that Otik had gone home early. — Dragons of Autumn Twilight
The rising of the sun does not constitute time; for the sun is always rising â” somewhere. — The Life and Genius of Nathaniel Hawthorne
Luckily we had packed some provisions in the carriage, for the sun was already declining, â” like the pace of the horses, â” and we were not yet at the end of the drive by a good distance. — Philip Gilbert Hamerton
Nansen stiffened. — Starfarers

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