Page:Collier's New Encyclopedia v. 09.djvu/198

LEFT SUN 164 SUN has attempted to photograph it in full sunshine, but thus far it is somewhat doubtful if he has succeeded in this. The spectrum of the corona consists principally of a bright line in the green which has not been identified with that of any terrestrial element, and for which the name coronium has been proposed. The lines of hydrogen are also visible, but not so bright by far as the line in the green. There is also a faint con- tinuous spectrum, and some observers have claimed to see on this the faint absorption lines of the solar spectrum. The nature of the corona is one of the most puzzling things to explain. It can- not be an atmosphere in any sense of the word, as the gaseous pressure there must be less than that of the most perfect vacuum we can make. Comets sweep through it without hindrance. It is a product of some sort of the enormous forces at play in the vicinity of the sun. Meteoric matter, cometic matter, matter ejected from the sun, are probably all concerned in it, and possibly electricity may play some part in the display. It is possible, and indeed perhaps probable, that the zodiacal light which reaches far out toward and perhaps beyond the earth's orbit, is a faint extension of the equatorial coronal streamers. Radiation. — Compared with other familiar sources of light we find that the amount of it received from the sun is about 600,000 times that from the full moon, 7,000,000,000 times that from Sirius, 40,000,000,000 times that from Vega or Arcturus, and 1,575,000,000,000,- 000,000,000,000,000 times as much as a standard candle would give at the dis- tance of the sun. The intrinsic bright- ness of its disk is about 90,000 times that of a candle flame, 150 times that of the lime in a calcium light, and from two to four times as bright as the brightest spot in the crater of an electric arc light. The darkest part of a sunspot is brighter than the lime light. The brightness of the sun's surface near the edge of the disk is only about one-third that at its center and is of a brownish-red color. This makes it appear still fainter at the edge of a photograph of the sun. If the sun were stripped of its atmosphere it would probably shine from two to five times as brightly as at present, and would be of a decidedly blue color. Con- sidering solar radiation in its heating effect and measuring it in terms of its power to melt ice, we find that the total amount of heat received annually would melt a sheet of ice 174 feet thick at the equator, or 136 feet thick over the whole surface of the earth if the radiation were equally distributed in all latitudes. Con- verted by means of the mechanical equiv- alent of heat and expressed as energy, we find that, neglecting the absorption of our atmosphere, each square meter of the earth's surface would receive from an overhead sun about two and one- third horse power continuously. Atmos- pheric absorption cuts this down to about one and a half horse power. Transfer- ring ourselves from the surface of the earth to that of the sun, and considering the radiation per unit of surface there, the figures are enormous, and the energy in question is almost incomprehensible. Every square meter of the sun's surface is continuously radiating more than 100,- 000 horse power. A shell of ice 50 feet thick would there be melted in one min- ute. To keep up such a development of heat by combustion would require that a layer of the best anthracite coal over the whole surface from 16 to 20 feet thick should be burned each hour, a ton an hour for every square foot of surface, at least nine times as much as the con- sumption of the most powerful blast fur- nace per foot of grate surface. At this rate the sun, if made of solid coal, would not last 6,000 years. Of this enormous amount of energy, so far as we know, only about .000,000,001 is intercepted and utilized by all the bodies of the solar system, and to the best of our knowl- edge, and according to human ideas, the rest of it goes to waste. Temperature. — While we can measure with a fair degree of accuracy the amount of solar radiation, the determina- tion of the actual temperature of its surface is a very different matter. All that can with certainty be said is that it is much higher than any temperature that can be produced by terrestrial means. The various estimates have taken the widest possible range, depend- ing on the assumed law connecting radi- ation with temperature. The most re- liable estimates place the probable ef- fective temperature as something like 10,000° C, or 18,000° F. There must also be a considerable range of tempera- ture at different depths below its surface. Sources of Energy. — We have noted the tremendous expenditure or energy by the sun in the form of radiation. A natural question is. How does it keep it up? We have mentioned the insufficiency of any combustion hypothesis. The only others worth mentioning are the meteoric and the contraction theories, as it can easily be shown that the theory of a, cooling sun vnll not suffice, since, if this were the source of its radiant energy, it must have cooled enough within historic times to have affected very decidedly its