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and this fact is undoubtedly the cause of the va nation in the estimates made of the time of the sun's revolution on its axis. A spot near the equator
performs a synodic revolution in about twenty-five days, while one half-way to either pole requires twenty-eight days. One spot was noticed which had a motion three times greater than that of clouds driven along by the most violent hurricane. Again, immense cyclones occasionally pass over the surface with fearful rapidity, producing rotation and sudden changes in the spots. At other times, however, the spots seem " to set sail and move across the disk of the sun like gondolas over a silver sea."
The spots change their real form.—Spots break out and then disappear under the very eye of the astronomer. Wollaston saw one that seemed to be shattered like a fragment of ice when it is thrown on a frozen surface, breaking into pieces, and sliding off in every direction. Sometimes one divides itself into several nuclei, while again several nuclei combine into one. Occasionally a spot will remain for six or eight rotations, while often one will last only half an hour. In one case, Sir. W. Herschel relates that when examining a spot through his telescope, he turned away for a moment, and on looking back it was gone.
The appearance of the spots is periodical.—It is a remarkable fact that the nnmber of spots increases and diminishes through a regular interval of about 11.11 years. These variations seem also to be connected with periodical variations in the aurora, and magnetic earth-currents, which interfere with the telegraph. The regular increase and diminution in the spots was discovered by Schwabe of Prussia, who watched the siui so carefully that it is said, "for thirty years the sun never appeared above the horizon without being confronted by his imperturbable telescope." Besides this, it has now been found that the activity of the sun's spots goes through another regular period of about 56 years. Independently of this conclusion, it has also been discovered that the aurora has a similar period of 56 years.
The spots are influenced by the planets.—They appear to be especially sensitive to the approach of Venus, on account of its nearness, and of Jupiter, because of its size. The area of the spots exposed to view from the earth is uniformly greatest when Venus is on the opposite side of the sun from us, and least when on the same side. When both Venus and Jupiter are on the side of the sun opposite to us, the spots are much larger than when Venus alone is in that position. In part explanation of this influence of the planets, we may suppose that they, in some manner, modify reflection on the disk of the sun exposed to their action, and thus cause a condensation of gases.
The spots do not influence the fruitfulness of the season.—Sir W. Herschel first advanced the idea that years of abundant spots would be years also of plentiful harvest. This is not now generally received. What two years could be more dissimilar than 1859 and 1860? Both abounded in solar spots, yet one was a fruitful year and the other almost one of famine in Europe.
The spots are cooler than the surrounding surface.It seems that the breaking out of a spot sensibly diminishes the temperature of that portion of the sun's disk. The faculse, on the other hand, do not increase the temperature. (Secchi.)
The spots are depressions below the luminous surface. —This was thought probable before, but is conclusively proved by the photographs of the sun, which have been taken in large numbers of late at Kew Observatory.
Comparative brightness of spots and sun.—If we represent the ordinary brightness of the sun by 1,000, then that of the penumbra would be 469, and that of the nucleus 7. There may be much light and heat radiated by a spot, which seems totally black as compared with the sun: we remember that when we look through even a Drummond light at the sun, it appears as a black spot on the disk of that luminary.
Faculce, vnRoio-leaf, and mottled appearance.—Besides the variety of spots already described, there are other curious appearances worthy of note. Bright ridges or streaks appear, which constitute the most brilliant portions of the sun.— These are called facuke. They vary from barely discernible, softly-gleaming tracts 1,000 miles long, to lofty, piled-up, mountainous regions 40,000 miles long and 4,000 broad. Outside of the spots, the entire disk of the sun is covered with minute shady dots, giving it a mottled appearance not unlike that of the skin of an orange, though less coarse. Under a large telescope the surface seems to be entirely made up of luminous masses, imperfectly separated by dark dots called pores. These masses are said by Mr. NaBmyth to have a "willow-leaf" shape; many observers apply other descriptive terms, such as "rice grains," " untidy circular masses," " things twice as long as broad," "granules," etc. The accompanying cut represents the willow-leafed structure of the luminous surface, and also the "bridges"