Page:Popular Science Monthly Volume 1.djvu/157

Rh about 460 miles. These facts give us some idea of the mighty scale of solar phenomena.

The solar spots have a general movement with the surface of the revolving sun, and they have also minor movements of displacement. They always make their first appearance on the same side of it; they travel across it in from twelve to fourteen days, and then disappear on the other side. The form of a spot in its first appearance is that of a small, dark streak, the length of which is much greater than the breadth. Its motion appears slow at first; it afterward increases, then slowly diminishes, until again assuming the form of a narrow streak. Fig. 1, from Dr. Schellen's finely-illustrated work on Spectrum Analysis (as are all the illustrations of the present article), represents this change of appearance of a spot as it emerges, passes across the field of view, and disappears. Of course, its relative size, as here shown, is enormously exaggerated.

The solar spot consists principally of a dark, almost black, central portion, generally irregular in form, called the umbra. Mr. Dawes has shown that, within this part of a spot, one or more still blacker spots may generally be observed, to which he gives the name of nucleus. The term black, however, as applied to an object on the sun, is to be taken with caution, and means merely a diminished intensity of solar light, which, by contrast, appears black. Zollner states that the black umbra of a spot emits four thousand times as much light as that derived from an equal area of the full moon; and Sir John Herschel says that the Drummond light, which is so bright that the eye can hardly endure it, when projected on the sun, appears as a black spot!

Surrounding the umbra is a tract less dark, usually more regular in form, and of a fringe-like aspect, called the penumbra. Its appearance is illustrated in Fig. 2, and is represented by the half-tints around the darker umbra in all the accompanying delineations. But, before we can understand the structure of the penumbra, it will be necessary to refer to what is known of the surrounding bright solar surface.

Viewed by a telescope of moderate power, the photosphere loses its uniform aspect, and exhibits a coarsely mottled appearance. But, if an instrument of the highest grade is used, the photosphere is seen to have a far more definite structure, and exhibits appearances which are variously described by different observers. Sir William Herschel calls these appearances "corrugations." They are small points of unequal light, imperfectly separated from each other by rows of minute dark spots called pores; the intervals between them being extremely small, and occupied by a substance decidedly less luminous than the general surface. Some call them "rice-grains;" Mr. Huggins names them "granules," and his representation of them is given in Fig. 3. The photosphere is again described as resembling a net of bright meshes interwoven with dark threads.

Certain portions of the photosphere are much brighter than others;