The bright yellow surface of the sun which every one sees, is called the photosphere or light-sphere. If strongly magnified through a telescope, this surface is seen to have a decided "mottled" appearance, like a layer of little clouds with dark spaces between, spread over the whole face of the sun. These are now known to be masses of calcium gas.
Practically all that is known of the physical constitution of the sun has been discovered through the study of the fine, dark lines in the solar spectrum. Through the aid of the spectroscope, many terrestrial elements have been discovered, among these being calcium, sodium, magnesium, potassium, aluminum, silver, iron, nickel, copper, zinc, tin, lead, hydrogen, oxygen and carbon. All these elements are either in the form of permanent gases or metallic vapors.
The photosphere or light-sphere lies below the chromosphere or color-sphere. Beyond the crimson color-sphere lies another envelope of very delicate light called the corona. This may only be seen at the time of total eclipse. It was once thought to belong to the moon but it is now known that the moon has very little atmosphere. Besides, the moon's black disk may be seen floating across this beautiful halo of light.
The corona has been described by those who have seen it, as having a "soft, pearly radiance" "tinged with green"; its form is quite irregular and varies at different times, extending from 1,000,000 to 3,000,000 and sometimes even 9,000,000 miles beyond the surface of the sun. In the eclipse of 1879 the streamers were quite extensive in the equatorial plane of the sun where they united in a formation like the petals of a narcissus while at the solar poles they were shorter and extended in well-defined rays. Yet no matter what its form, it is a magnificent background during the sun's eclipse for the narrow rim of deep crimson projects beyond the inky edges of the moon.
The corona seems to be caused by electrical discharges in a medium of fine dust and rarefied gases, chiefly coronium and protofluorine. Various electrical phenomena take place in the strong magnetic fields of the sun as a whole and in the sun-spots, and the changing fields of magnetic force cause the changing appearance of the corona.
The sun-spots, however, are of the greatest interest to amateurs for these may often be seen with very little optical aid. These dark, movable spots were first noted by Galileo, although the idea that the sun could have spots upon its surface was ridiculed by the scholars of the medieval schools who argued in the fervid language of scholastic philosophy and medieval dogmatism, that the sun was a sphere of pure fire undefiled by any spots.
Sun-spots are, as a rule, thousands of miles across and usually appear in groups which cover an immense area on the surface of the sun, but, since the sun lies 93,000,000 miles away, a spot large enough to contain the earth would appear to us about the size of a pinhead on a pincushion. Right here one should be warned again not to attempt to look at the sun without adequate protection (such as smoked glasses) for the eyes. A photograph taken through a large telescope is the most satisfactory. Although these dark blotches—which are only comparatively dark—were called "spots" and were later drawn as saucer-like depressions, modern research and increased power in the telescopes has revealed them as gigantic solar tornadoes. These tornadoes usually last from a few days to several months, and are especially numerous in cycles of 11 years. During sun-spot activity eruptive prominences rise to heights of many thousands of miles and streams of electrons are shot forth with such force that they may possibly even penetrate deeply into our own atmosphere causing disturbances such as magnetic storms and auroras.
SUN-SPOTS.