Individual sunspots appear spasmodically, remain visible for periods varying from a few days to several months, and then disappear. This apparently capricious behaviour, however, con tributes to a striking regularity which is revealed only when large numbers of spots and a great length of time are considered. Samuel Heinrich Schwabe, in 1843, found that if the number of spots appearing per year (or the total area covered by them) were plotted against time, as in the diagram, a markedly periodic relation was shown, the number reaching a maximum approxi mately every i i 3 years. It was afterwards discovered that a similar regularity characterized the location of the spots. At a time of minimum those of a new cycle began to appear in the higher latitudes, both north and south, of their appointed belts, and as the cycle progressed the place of outbreak gradually moved towards the equator. The dotted lines in the figure illustrate this.
Sunspots are often accompanied by exceptionally bright areas on the photosphere, known as faculae. They are most easily seen near the limb, where the brightness of the photospheric back ground is diminished.
On the comparatively rare occasions on which the sun is ob served in eclipse, red flames are seen apparently rising from various points on the circumference of the dark moon. They be long in reality to the sun, and are known as prominences. They are not seen through the telescope alone in full daylight, because the intense photospheric light, diffused by our atmosphere as by a screen of ground glass, acts as a veil through which prominences and stars alike are invisible as individual objects although their radiation contributes to the sum total of the light of day. Prom
inences assume various shapes and sizes, sometimes reaching heights of hundreds of thousands of miles. On such occasions they can easily be seen by the naked eye during eclipses.
The most striking solar eclipse phenomenon, however, is the corona, a pearly white halo enveloping the sun and extending in more or less definite rays or streamers to a distance of several of its radii. The brilliance of the corona diminishes fairly rapidly with distance from the sun's limb, and although its total brightness is not far short of that of full moon, it is still less able than the prominences to maintain the individuality of its appearance in full daylight. No two aspects of the corona seen at different eclipses are identical, but here again the apparent arbitrariness is subject to conformity with a rather vague but unquestionable relation with the sunspot period. At sunspot maximum the corona appears to extend from the sun's limb to roughly the same distance all around. At sunspot minimum, on the other hand, the poles of the sun are marked by comparatively small tufts of light, while from the equatorial regions long streamers shoot out to great distances. There is also a not fully determined relation between the corona and prominences of certain types.