SUN (AS. auntie, Goth., 011G. sunno, Ger. Sonne; connected with AS. sol, Goth.
Lat. sol, Gk. Ows, halos, Ir. sal, Lith., Lett., OPruss. Slat. sear, aura, Av. hrm•a, sun). The central governing body of the solar system (q.v.), and the chief source of our light and heat. To us the sun is the most important of all the heavenly bodies, since upon it depends the possi bility of life upon the earth. The sun's apparent angular diameter is 32' 4" diameter in miles, 866,500; density, 1.41, water being 1: mean distance from the earth. 92,800,000 miles. The sun's distance from the earth is the fundamental unit of linear measures in cosmic astronomy. The method of the determination of this distance is given under PARALLAX.
Successive generations of astronomers have been able to gather much information as to the appearance, constitution, and structure of the sun. The theory at present accepted is as fol lows: The visible light-giving surface. as we see it in the telescope, is called the photosphere. We suppose it to be a collection of luminous clouds, holding in suspension certain particles condensed out of the extremely hot gaseous ma terial of which the innermost nucleus of the sun is probably composed. Just outside the photo sphere is a thin shell of gaseous matter called the rerersing layer. This is doubtless com posed of many substances such as we find among the known chemical elements of our earth. It is observed and its very existence is demonstrated by the spectroscope. According to Kirchhoff's theory, when light coming from incandescent sol ids or liquids is passed through vaporized matter, the vapors absorb o• cut of the light from certain definite parts of the spectrum. To each vaporized substance belongs its own series of such points in the spectrum; and these corre spond exactly to the positions which would be occupied by bright lines in a spectrum derived directly from such vaporized substances them selves, if rendered incandescent. Thus the ordi nary solar spectrum is crossed by certain well known dark lines corresponding to the absorption of certain parts of the photospheric light by the absorptive effect of the reversing layer. Now
if it were possible in sonic way to get rid of the light of the photosphere and examine directly the light of the reversing layer, we ought to see the spectrum of ordinary incandescent gases. This observation can actually be made at the time of a total solar eclipse. If we watch the solar spectrum very carefully just before the beginning of totality, when the moon's advancing edge is on the point of obscuring the sun com pletely, there will come a moment when nothing remains hi sight but the outer or reversing layer. At that instant the dark lines in the spectrum are suddenly reversed, becoming bright like the lines in the spectrum of a gas heated to incandescence artificially in the laboratory. This remarkable observation was made by Young (q.v.) at the Spanish eclipse of 1870, and repeated photographically in 1S96 by Shackleton, at the Nova Zembla eclipse, The so-called chromosphcrr is a great layer composed of very hot gases, principally hydro gen, and su•roundin the sun chiefly outside the reversing layer. It is supposed. however, that both these strata are mixed together, except that the hottest and densest gases are in the lower o• reversing layer. This chromosphe•ic layer can be seen for an instant at the begin ning or end of a total solar eclipse, and it is found to consist principally of a mass of red flames. The color is due to the preponderating presence of hydrogen, and it is this color that gives rise to the name chromosphere o• color sphere. The presence of flame does not, however, indicate a process of combustion in the usual sense of the word. For instance, coal burning in an ordinary stove is really being combined chemically with the oxygen of the air, and chem ical combination of that character is certainly not going on in the sun.