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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

Fig. 7.



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 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. During 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.)

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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 during this 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 straight lines.—Sometimes their path curves toward


Fig. 9.

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° 15').

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, and this fact is undoubtedly the cause of the variation in the estimates made of the time of the sun's revolution on its axis. A spot near the equator

Fig. 10.

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performs a synodic revolution in about twenty-five days, while one half way to either pole reqtures 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 shat

S tered 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 numberof 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 sun 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

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