ges, called _faculae_, and
dark 'pores' become conspicuous, between which greyish patches appear,
that seem to lie underneath a thin layer of the photosphere; this is
rapidly dispelled and a fully formed spot comes into view. When a
sun-spot has completed its period of existence, the photospheric matter
overwhelms the penumbra, and rushes into the umbra, which it
obliterates, causing the spot to disappear. The duration of sun-spots is
subject to considerable variation; some last for weeks or months, and
others for a few days or hours. A spot when once fully formed maintains
its shape, which is usually rounded, until the period of its breaking
up. Spots of long duration rotate with the Sun. Those which become
visible at the edge of the Sun's limb have been observed to travel
across his disc in less than a fortnight, disappearing at the margin of
the opposite limb; afterwards, if sufficiently long-lived, they have
reappeared in twelve or thirteen days on the surface of the orb where
first observed. It was by observation of the spots that the period of
the axial rotation of the Sun became known.

Sun-spots vary very much in size--some are only a few hundred miles in
width, whilst others have a diameter of 40,000 or 50,000 miles or
upwards. In some instances the umbra alone has a breadth of 20,000 or
30,000 miles--three times the extent of the diameter of the Earth. Spots
of this size are visible to the naked eye when the Sun is partially
obscured by fog, or when his brilliancy is diminished by vapours near
the horizon. A year seldom passes without the occurrence of several of
such spots being recorded. The largest sun-spot ever observed had a
diameter of about 150,000 miles. A group of spots, including their
penumbrae, will occupy an area of many millions of square miles.

By long observation it has been ascertained that sun-spots increase and
diminish in number with periodical regularity, and that a maximum
sun-spot period occurs at the end of each eleven years. When spots are
numerous on the Sun's disc there is great disturbance of the solar
surface, accompanied by fierce rushes of intensely heated gases. This
solar activity is known to influence terrestrial magnetism by causing a
marked oscillation of the magnetic needle, and giving rise to so-called
'magnetic storms,' accompanied by magnificent displays of aurorae, with
variations in electrical earth-currents. It would therefore appear that
sun-spots have a pronounced