sun-spot

Century Dictionary and Cyclopedia

1. n. One of the dark patches, from 1,000 to 100,000 miles in diameter, which are often visible upon the photosphere. The central part, or umbra, appears nearly black, though the darkness is really only relative to the intense surrounding brightness. With proper appliances the umbra itself is seen to contain still darker circular holes, and to be overlaid by films of transparent cloud. It is ordinarily surrounded by a nearly concentric penumbra composed of converging filaments. Often, however, the penumbra is unsymmetrical with respect to the umbra, and sometimes it is entirely wanting. The spots often appear in groups, and frequently a large one breaks up into smaller ones. They are continually changing in form and dimensions, and sometimes have a distinct drift upon the sun's surface. They last from a few hours to many months. They are known to be shallow cavities in the photosphere, depressed several hundred miles below the general level, and owe their darkness mainly to the absorption of light due to the cooler vapors which fill them. Their cause and the precise theory of their formation are still uncertain, though it is more than probable that they are in some way connected with descending currents from the upper regions of the solar atmosphere. The spots are limited to the region within 45° of the sun's equator, and are most numerous in latitudes from 15° to 20°, being rather scarce on the equator itself. They exhibit a marked periodicity in number: at intervals of about eleven years they are abundant, while at intermediate times they almost vanish. The explanation of this periodicity is still unknown. Numerous attempts have been made to correlate it with various periodic phenomena upon the earth—with doubtful success, however, except that there is an unmistakable (though unexplained) connection between the spottedness of the sun's surface and the number and violence of our so-called magnetic storms and auroras.
2. n. A freckle.
3. n. A spot on leaves produced by excessive heat of the sun. Compare heliosis, 1.