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The density of the sun is only about one-fourth that of the earth, or 1.43 that of water, so that the weight of a body transferred from the earth to tho sun would not be increased in proportion to the comparative size of the two. On account also of the vast size of the sun, its surface is so far from its centre that the attraction is largely diminished, since that decreases, we remember, as the square of the distance. However, a man weighing at the earth's equator 150 lbs., at the sun's equator would weigh about 4,080 lbs.,—a force of attraction that would inevitably and instantly crush him. At the earth's equator a stone falls 16 feet the first second; at the sun's equator it would fall 437 feet.
Telescopic Appearance Of The Sun: Sun-spots.— We may sometimes examine the sun at early morning or late in the afternoon with the naked eye, and at midday by using a smoked glass. The disk will appear to us perfectly distinct and circular, and with no spot to dim its brightness. If we use, however, a telescope of moderate power, taking the precaution to properly shield the eye with a colored eye-piece, we shallfinditssurface sprinkled with irregular spots, somewhat as shown in the accompanying figure.
Curious opinions concerning solar spots.—The natural purity of the sun seems to have been formerly an article of faith among astronomers, and therefore on no account to be called in question. Scheiner, it is said, having reported to his superior that he had seen spots on the sun's face, was abruptly dismissed with these remarks: "I have read Aristotle's writings from end to end many times, and I assure you I do not find anything in them similar to that which you mention. Go, my son, tranquillize yourself; be assured that what you take for spots are the faults of your glasses or your own eyes."
Discovery of the solar spots.—They seem to have been noticed as early as 807 A. D., although the telescope was not invented until 1610, and Galileo discovered the solar spots in the following year. We read in the log-book of the good ship Richard of Arundell, on a voyage, in 1590, to the coast of Guinea, that "on the 7, at the going, downe of the sunne, we saw a great black spot in the sunne; and the 8 day, both at rising and setting, we saw the like,—which spot to me seeming was about the bignesse of a shilling, being in 5 degrees of latitude, and still there came a great billow out of the souther board."
Number and location of spots. — Sometimes, but rarely, the disk is clear. During a period of ten years, observations were made on 1982 days, on 372 of which there were no spots seen. As many as two hundred spots have been noticed at one time. They are found in two belts, one on each side of the equator, within not less than 8° nor more than 35° of latitude. They seem to herd together—the length of the straggling group being generally parallel to the equator.
The size of the spots.—It is not uncommon to find a spot with a surface larger than that of the earth. Schroter measured one more than 29,000 miles in diameter. Sir J. W. Herschel calculated that one which he saw was 50,000 miles in diameter. In 1843 one was seen which was 14,816 miles across, and was visible to the naked eye for an entire week. On the day of the eclipse in 1858, a spot over 107,000 miles broad was distinctly seen, and attracted general attention in this country. Some who read this paragraph will doubtless recall its ap
pearance. In 1839, Captain Davis saw one which he computed was not less than 186,000 miles long, and had an area of twenty-five billion square miles. If these are deep openings in the luminous atmosphere of the sun, what an abyss must that be at "the bottom of which our earth could lie like a boulder in the crater of a volcano!"
The spots consist of distinct parU.—From the accompanying representation it will be seen that the spots generally consist of one or more dark portions called the umbra, and around that a grayish portion styled the penumbra (pene, almost, and umbra, black).— Sometimes, however, umbrae appear without a penumbra, and vice versa. The umbra itself has generally a dense black centre, called the nucleus. Besides this, the umbra is sometimes divided by luminous bridges.
The spots are in motion.—They change from day to day; but they all have a common movement. About fourteen days are required for a spot to pass
across the disk of the sun from the eastern side or limb to the western; in fourteen days it reappears, changed in form perhaps, but generally recognizable. The spots change their rapidity and apparent form as they pass across the disk.—A spot is seen on the eastern limb; day by day it progresses, with a gradually increasing rapidity, until it reaches the cen
CHANGE IN SPOTS AS THET CROSS THE DISK.
tre; it now gradually loses its rapidity, and finally disappears on the western limb. The diagram illustrates the apparent change which takes place in the form. Suppose at first it is of an oval shape; as it approaches the centre it apparently widens and becomes circular. Having passed that point, it becomes more and more oval until it disappears.
This change in the spots proves the sun's rotation on its axis.—These changes can be accounted for only on the supposition that the sun revolves on its axis: indeed, they are the precise effects which the