spect of a Sun-Spot.]

These spots, which appear of insignificant dimensions to the observers
on the Earth, are in reality absolutely gigantic. Some that have been
measured are ten times as large as the Earth's diameter, _i.e._, 120,000
kilometers (74,500 miles).

Sometimes the spots are so large that they can be seen with the unaided
eye (protected with black or dark-blue glasses). They are not formed
instantaneously, but are heralded by a vast commotion on the solar
surface, exhibiting, as it were, luminous waves or _faculae_. Out of this
agitation arises a little spot, that is usually round, and enlarges
progressively to reach a maximum, after which it diminishes, with
frequent segmentation and shrinkage. Some are visible only for a few
days; others last for months. Some appear, only to be instantly
swallowed in the boiling turmoil of the flaming orb. Sometimes, again,
white incandescent waves emerge, and seem to throw luminous bridges
across the central umbra. As a rule the spots are not very profound.
They are funnel-shaped depressions, inferior in depth to the diameter of
the Earth, which, as we have seen, is 108 times smaller than that of the
Sun.

* * * * *

The Sun-Spots are not devoid of motion, and from their movements we
learn that the radiant orb revolves upon itself in about twenty-five
days. This rotation was determined in 1611, by Galileo, who, while
observing the spots, saw that they traversed the solar disk from east
to west, following lines that are oblique to the plane of the ecliptic,
and that they disappear at the western border fourteen days after their
arrival at the eastern edge. Sometimes the same spot, after being
invisible for fourteen days, reappears upon the eastern edge, where it
was observed twenty-eight days previously. It progresses toward the
center of the Sun, which is reached in seven days, disappears anew in
the west, and continues its journey on the hemisphere opposed to us, to
reappear under observation two weeks later, if it has not meantime been
extinguished. This observation proves that the Sun revolves upon itself.
The reappearance of the spots occurs in about twenty-seven days, because
the Earth is not stationary, and in its movement round the burning
focus, a motion effected in the same direction as the solar rotation,
the spots are still visible two and a half days after they disappeared
from the point at which they had been twenty-five da