Photosphere.—The luminous surface of the sun directly visible in telescopes is called the photosphere (Greek phos, "bright"). It is probably a sheet of luminous clouds formed by the condensa tion of substances which exist as gases in the hotter central mass of the sun. Un der a moderate magnifying power it looks like rough drawing paper. With higher powers it looks something like snow flakes scattered over gray cloth. These flakes or grains are from 400 to 600 miles across, and are probably bright clouds floating in an atmosphere not so luminous. Near the edge of the sun the photosphere is much less brilliant than at the center, due to absorption of the solar atmosphere.
Section of the Sun.—There is an inner nucleus; around it is the photosphere, rising at some places into faculw and depressed in others in spots. Immedi ately above the photosphere is the "re versing stratum"; above this is the scarlet chromosphere, with prominences of various forms and dimensions; and over, and embracing all, is the coronal atmosphere, fading gradually away into darkness. Eclipses offer exceptional op portunities for studying the various phenomena of the sun that are outside the central nucleus.
Spots and Faculw.—The most promi nent feature of the sun's surface is the spots, some of which can almost always be seen except near the time of a sun spot minimum. They are dark depres sions in the photosphere, and consist of a central umbra with surrounding pen umbra which is not so dark. This pen umbra consists of radial filaments, and appears like the sloping sides of the sun spot cavity, as if the photosphere were drawn down into the spot by inward cur rents. In the central umbra there are also sometimes smaller, darker spots, called "Dawes's holes," from the name of their discoverer. The preceding is the description of what may be called a nor mal spot before it begins to break up; but they are seldom of so regular a character. Frequently there are sev eral umbra with a common penumbra, and there are sometimes streaks or "bridges" of the bright photosphere ex tending clear across the whole spot. The umbra is not always central, and the fila ments of the penumbra are frequently twisted and curled into the most fan tastic shapes, reminding one of swirling tongues of flame or smoke. The diameter of the umbra of a spot ranges all the way from 500 to 50,000 or 60,000 miles. The whole earth could be dropped into many of them without disturbing the edges. The penumbra surrounding some of the larger groups of spots has some times measured as much as 150,000 miles in diameter. The depth of the umbra
below the general surface of the photo sphere is difficult to determine, but ac cording to the best authorities may range from 500 to 2,500 miles. Sur rounding the spots there are generally streaks of the photosphere which are much brighter than its general surface, sometimes extending nearly radially from the spot, and these are called the facula (Latin fax, "a torch"). They are elevated ridges of the photosphere, and are much more prominent when near the edge of the sun, where they project up through a part of the atmosphere which dims the general photospheric. level. The facula; are not confined to the spot surroundings, but they are much more abundant there. They probably make surgings or upheavals, which are the surrounding accompaniment of sun spot action.
Life and Distribution of Spots.—They begin from insensible points, rapidly growing larger, but generally do not develop the penumbra till after the um bra is well formed. The projection of a bright streak, or facula, across the nucleus of a spot often precedes its seg mentation or breaking up into two or more. When a spot disappears it comes about by the encroachment of the photo sphere, which seems to fall into and fill the cavity, leaving its place covered by a group of bright facula . In the vicinity of a spot the motion of phospheric and other matter is generally inward, toward and down into the center, and occasional ly the latter is enough to be detected by the spectroscope. The duration of a spot ranges from a few days to occasionally more than a year, but is generally a month or two. It is a remarkable fact that their distribution is confined to two zones between 5° and 40° Theories of Sunspots.—The cause of spots is not yet satisfactorily made out, and among the many theories offered we can only note that of Faye, who con siders them to be cyclonic, like our ter restrial storms, caused by the forward drift of the equatorial photosphere, and the more probable suggestion of Young that they are depressions or sinks in the photosphere brought about by the di minution of pressure below, which would accompany eruptions in the surroundings of the spot, the cooler, darker gases flow ing down into the cavity thus formed. Certain it is that they are intimately as sociated with eruptions and explosions on the surface or from below, but which is cause and effect, or whether both spot and eruption are caused by some outside influence (like the fall of meteoric mat ter) is as yet undetermined.