It is in the brilliant photosphere that the dark areas known as
sun-spots appear. Some of these dark spots--they are dark only by
contrast with the photosphere surrounding them--are of enormous size,
covering many thousands of square miles of surface. What they are we
cannot positively say. They look like great cavities in the sun's
surface. Some think they are giant whirlpools. Certainly they seem to be
great whirling streams of glowing gases with vapours above them and
immense upward and downward currents within them. Round the edges of the
sun-spots rise great tongues of flame.
Perhaps the most popularly known fact about sun-spots is that they are
somehow connected with what we call magnetic storms on earth. These
magnetic storms manifest themselves in interruptions of our telegraphic
and telephonic communications, in violent disturbances of the mariner's
compass, and in exceptional auroral displays. The connection between the
two sets of phenomena cannot be doubted, even although at times there
may be a great spot on the sun without any corresponding "magnetic
storm" effects on the earth.
A surprising fact about sun-spots is that they show definite periodic
variations in number. The best-defined period is one of about eleven
years. During this period the spots increase to a maximum in number and
then diminish to a minimum, the variation being more or less regular.
Now this can only mean one thing. To be periodic the spots must have
some deep-seated connection with the fundamental facts of the sun's
structure and activities. Looked at from this point of view their
importance becomes great.
[Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.
Macmillan_)