The cause of these fluctuations in the solar energy is at present wholly unknown. Some astronomers, however, have attributed it to the influence of the planets Venus and Jupiter, the period of revolution of the latter planet being not much longer than the Sun-spot period; but this supposition lacks confirmation from direct observations, which, so far, do not seem to be in favor of the hypothesis. At the present time the solar activity is on the increase, and the Sun-spots will probably reach their maximum in 1883. The last minimum occurred in 1879, when only sixteen small groups of spots were observed during the whole year.
Sun-spots vary in size and appearance; but, unless they are very small, in which case they appear as simple black dots, they generally consist of two distinct and well-characterized parts, nearly always present. There is first, a central part, much darker than the other, and sharply divided from it, called the "Umbra;" second, a broad, irregular radiated fringe of lighter shade, completely surrounding the first, and called the "Penumbra."
Reduced to its simplest expression, a Sun-spot is a funnel-shaped opening through the chromosphere and the photosphere. The inner end of the funnel, or opening, gives the form to the umbra, while its sloping sides form the penumbra.
The umbra of Sun-spots, whose outlines approximately follow the irregularities of the penumbral fringe, has a diameter which generally exceeds the width of the penumbral ring. Sometimes it appears uniformly black throughout; but it is only so by contrast, as is proved when either Mercury or Venus passes near a spot during a transit over the Sun's disk. The umbra then appears grayish, when compared with the jet-black disk of the planet.
The umbra of spots is rarely so simple as just described; but it is frequently occupied, either partly or wholly, by grayish and rosy forms, somewhat resembling loosely-entangled muscular fibres. These forms have been called the Gray and Rosy Veils. Frequently these veils appear as if perforated by roundish black holes, improperly called Nuclei, which permit the sight to penetrate deeper into the interior. To all appearance the gray and rosy veils are of the same nature as the chromosphere and the faculæ, and are therefore mainly composed of hydrogen gas.
Whatever can be known about the interior of the Sun, must be learned from the observations of these openings, which are comparatively small. But whatever this interior may be, we certainly know that it is not homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very thin shell. Below this shell exists a large open space filled with invisible gases, in which, through the openings constituting the Sun-spots, the gray and rosy veils described above are occasionally seen floating.
The fringe forming the penumbra of spots is much more complicated than the umbra. In its simpler form, it is composed of a multitude of bright, independent filaments of different forms and sizes, partly projecting one above the other, on the sloping wall of the penumbra, from which they seem to proceed. Seen from the Earth, these filaments have somewhat the appearance of thatched straw, converging towards the centre of the umbra. It is very rare, however, that the convergence of the penumbral filaments is regular, and great confusion sometimes arises from the entanglement of these filaments. Some of these elements appear straight, others are curved or loop-shaped; while still others, much larger and brighter than the rest, give a final touch to this chaos of filaments, from which results the general thatched and radiating appearance of the penumbra.
The extremities of the penumbral filaments, especially of those forming the border of the umbra, are usually club-shaped and appear very brilliant, as if these elements had been superheated by some forces escaping through the opening of the spots.
Besides these characteristics, the Sun-spots have others, which, although not always present, properly belong to them. Comparatively few spots are so simple as the form just described. Very frequently a spot is accompanied by brilliant faculæ, covering part of its umbra and penumbra, and appearing to form a part of the spot itself.
When seen projected over Sun-spots, the faculæ appear intensely bright, and from these peculiarities they have been called Luminous Bridges. They are, in fact, bridges, but in most cases they are at considerable heights above the spots, kept there by invisible forces. When such spots with luminous bridges approach the Sun's limb, it is easy to see, by the rapid apparent displacement which they undergo, that they are above the general level.