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THE SUN, EARTH, AND MOON

or the thatched lines which form it, frequently encroaches on the nucleus or may, in places, extend quite across it.

A most remarkable law connected with the spots, which has been established by nearly three centuries of observation, is that their frequency varies in a regular period of eleven years and about forty days. During a certain year no spots will be visible for about half of the time.

Fig. 17.—Appearance of a Sun-spot with High Magnifying Power, showing also the Mottling of the Photosphere.

This was the case in 1889 and again in 1900. The year following a slightly greater number will show themselves; and they will increase year after year for about five years. Then the frequency will begin to diminish, year after year, until the cycle is completed, when it will again begin to increase. These mutations have been traced back to the time of Galileo, although it was not till about 1825 that they were found by Schwabe to take place in a regular period.

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