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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.
Tlie 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 appearance. 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 I"
The spots consist of distinct parte.—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, umbra appear without a penumbra, and vice versa. The umbra itself has generally a dense black centre, called the
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 sido 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 puss across the disk.—A spot is seen on the eastern limb; day by day it progresses, with a grad-, nally increasing rapidity, until it reaches the cen
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 tho supposition that the sun revolves on its axis: indeed, they are the precise effects which the
laws of perspective demand in that case. About twenty-seven days (27 d., 7 h.) elapse from the appearance of a spot on the eastern limb before it reappears a second time. Daring this time the earth has gone forward in its orbit, so that the location of the observer is changed; allowing for this, the sun's time of rotation is about twentyfive days (25 d., 8 h., 10 m.: Langier).
Synodic and sidereal revolution of the spots.—We can easily understand why we make an allowance for the motion of the earth in its orbit. Suppose a solar spot at a, on a line passing from the centre of the earth to the centre of the sun. For the spot to pass around the sun and come into that same position again, requires about twenty-seven days. But duringfrthis time, the earth has passed on from T to T'. The spot has not only travelled around to a again, but also beyond that to a', or the distance from a to a' more than an entire revolution. To do this requires, as we have said, about two days. A revolution from a around to a' is called a synodic, and one from a around to a again is called a sidereal revolution.
The spots apparently do not always move in fttraight lines.—Sometimes their path curves toward
the north, and sometimes toward the south, as in the figure. This can be explained only on the supposition that the sun's axis is inclined to the ecliptic (7° W).
The spots have a motion of their own.—Besides the motion already named as assigned to the sun's rotation, the spots seem to have a motion of their own,