The most interesting features of the chromo sphere are the pa•oximmces. These are great eruptive jets of red hydrogen flame bursting out ward from the solar surface. sometimes to a dis tance of hundreds of thousands of miles into space. They were first seen during total solar eclipses. when the bright light of the central solar disk had been obscured by the interposed moon. Under ordinary circumstances the central solar light illuminates our terrestrial atmosphere very strongly, and enough of this light is re flected from the air itself into our telescopes to mask completely the fainter light of the promi nences. Lint it has been found possible to study the prominences speetroscopically during full sunlight. and without waiting* for a solar eclipse. This was first done visually in 1868 by Janssen, Loeleye• (q.v.) , and Huggins and photo graphically by Hale (q.v.) and Desland•es in 1890. The two latter astronomers working inde pendently of each other, the former in Chicago and the latter in Paris, devised certain instru ments called photographic speetro-heliographs, by means of which it is possible to take pictures of the chromosphere and prominences, over the entire solar surface at once and in full sun light. It has thus become known that the veloc ity of motion in these prominence eruptions may be as great as several hundred miles per second, real explosions of incalculable force.
Extending far out beyond the chromosphere and prominences, and usually recognized as the sun's outermost envelope, is the cormia. It is visible only at the time of total eclipse, when the central part of the solar• disk is completely obscured. The appearance of the corona at such times is very sudden; the advancing edge of the moon slowly and gradually covers more and more of the sun's; surface, until finally only a sickle shaped filament remains. Then suddenly this too disappears, and on the instant the superb corona bursts into view, truly one of the most magnificent objects in the whole range of nature's phenomena. The inner corona, close to the solar disk, is very bright, even dazzling; farther out it fades by insensible gradations into a beauti ful crown of filmy light. There are thread-like streamers. interlacing with complex involutions,• and extending often many millions of miles into space above the solar surface.
Although observed visually from the most an cient times. it was not until the application of photography to astronomical obse•vation that we have been able to fix with some approach to certainty the details of the coronal structure. See ASTRO-PIIOTOURAPIlY.
Spectroscopically the corona reveals the pres ence of luminous gases, possibly containing in suspension particles of solid o• liquid matter. No existing theory will explain quite satisfac torily its mechanical construction, so difficult is it to reconcile with gravitational law so enormous an extension outward from the sun. Possibly magnetic forces are wholly or partly responsible for its existence and equilibrium; certain it is that we still have in the corona one of the 'pending problems' of astronomy.
The chemical constitution of the sun has been studied .with great care spectroscopically. The spectroscope, when suitably arranged for the purpose, enables us to compare solar light with light obtained in the laboratory from vapors of terrestrial substances raised to incandescence by artificial heating. In this way many ter restrial chemical elements have been identified in the sun ; and we may consider extx•emely probable the hypothesis that sun and earth are composed of the same kinds of matte•. This, if true, is a fact of great importance in its bearing on the origin and evolution of the solar system. (See YEmu„m; COSMOGONY.) The most conspicuous phenomenon of the solar surface is the sun-spots. These are apparently great holes o• depressions in the pbotospheric surface. usu ally having a dark spot in the middle, stn.
ronnded by a sort of radial grating. They vary in diameter from 500 to 60.000 miles for the central black parts, and range up to Immo miles for the surrounding less dusky portions. At times they are large enough to be seen with the naked eye. Their depth is very uncertain, and has been variously estimated from a few miles up to 2500. The Menke are bright streaks seen in various parts of the photospherie surface, but usually near spots, and most conspicnons when approaching the edge of the solar disk. The spots usually begin as mere dots, and then grow gradually o• rapidly to the enormous di mensions already stated. At times they break into several pieces, giving rise to a group or collection of several spots. They are by no means immovable upon the solar surface, but have a motion of their own, those near the sun's equa tor traveling farther from the poles, and those in higher solar latitudes increasing their dis tance from the equator. But no sun-spots arc ever found farther from the solar equator than latitude 45°, and few are nearer the equator than latitude 5°. One of the most remarkable things about the spots is the fact that their frequency is periodic. Once in about eleven years they have a maximum frequency, and as tronomers are able to record their occurrence in greater numbers than usual. The cause of this periodicity is unknown.
Very interesting questions are raised by a study of the sun's light and heat. The quan tity of light given by the sun, as compared with a standard candle, can be measured, and the sun's 'candle power' thus made known. Experi ments show that this candle power is repre sented by the number 1575 with twenty-four zeros attached. The quantity of solar heat is no less stupendous: it has been estimated that the heat given out each hour• by the sun is equiva lent to the burning of a quantity of coal sufficient to cover the entire solar surface to a depth of more than twenty feet. The source of this vast amount of light and heat has been for a long time a mystery. It cannot be a question of com bustion, because in that case the sun would have been consumed long ago, even if made of solid coal. The theory at present accepted is due to Helm holtz. It assumes that the sun's bulk is slowly contracting, and that the energy thus produced is turned into heat. It has been computed that a diminution of 150 feet annually in the sun's radius would be sufficient to account for the heat developed. Such a contraction would remain invisible even in our most powerful telescopes until it had continued for at least 10.000 years, and the process would need to continue for ten million years before the sun would lose the power to give us heat enough to keep life on the earth.
The rotation of the sun on its axis occupies about 25 days, S hours. This period is deter mined by watching the spots go around with the sun : and it is somewhat uncertain, because, as we have seen, the spots are not themselves quies cent on the solar surface. The rotation axis is inclined to the ecliptic plane, like that of the earth ; if continued to the celestial sphere. it would pierce it. midway between the bright star Lyra- and the Pole star. And there is one very peculiar feature in the rotation due perhaps to the fact that the sun's vast hulk has not yet been completely transformed from the gaseous to the liquid or solid stage. The equatorial regions rotate faster than those in higher latitudes, showing the existence of strong rotary surface currents of different velocities. No thoroughly satisfactory explanation of this phenomenon has yet been obtained by astronomers. See ECLIPSE; PLANETS; PERTURBATIONS; ORBIT: SOLAR Sys TEM. Consult: .tstrophysieal Journal (Chicago) ; Bayer, Handbuch, der Spectroscopic (Leipzig, 1902) ; Young, The Sun (New York, 1900) : Lockyer, Physics (London. 16'74) ; Proctor, S'It(1 (London, 1671).