The elements of astronomy; a textbook . Fig. 46.—The Great Sun Spot of September, 1870, and the Structure of the a Drawing by Professor Langley. From the New Astronomy, by permission ofthe Publishers. the photosphere, but elevated above the general level, and in-tensified in brightness. 124 SUN SPOTS. [§184 Fig. 47 shows faculse around a spot near the suns photosphere is probably a sheet of clouds floating in aless luminous atmosphere, just as a cloud formed by the con-densation of water-vapor floats in the air. It is intenselybrilliant, for the same reason that the m
The elements of astronomy; a textbook . Fig. 46.—The Great Sun Spot of September, 1870, and the Structure of the a Drawing by Professor Langley. From the New Astronomy, by permission ofthe Publishers. the photosphere, but elevated above the general level, and in-tensified in brightness. 124 SUN SPOTS. [§184 Fig. 47 shows faculse around a spot near the suns photosphere is probably a sheet of clouds floating in aless luminous atmosphere, just as a cloud formed by the con-densation of water-vapor floats in the air. It is intenselybrilliant, for the same reason that the mantle of a Wels-bach burner outshines the gas-flame which heats it: the radiat-ing power of the solid and liquid particles which compose theclouds is extremely Fig. 47. — Faculse at Edge of the Sun. (De La Rue.) 185. Sun Spots. — Sun spots, whenever visible, are the mostconspicuous and interesting objects upon the solar appearance of a normal sun spot, Fig. 48, fully formed,and not yet beginning to break up, is that of a dark centralumbra, more or less nearly circular, with a fringing pe-numbra, composed of converging filaments. The umbra itselfis not uniformly dark throughout, but is overlaid with filmyclouds, which usually require a good telescope and helioscopeto make them visible, but sometimes, though rather infre- §185] SUN SPOTS. 125 quently, are conspicuous, — as in the figure. Usually, also,within the umbra there are a number of round and very blackspots, sometimes called nucleoli, but often referred to as Dawes holes after the name of their first discoverer. The darkest portions of the umbra, however, are dark onlyby contrast. Photometric observations show that even the
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