One of Galileo's most striking discoveries, when he pointed his
telescope to the heavenly bodies, was that of the irregularly shaped
spots on the sun, with the dark central _umbra_ and the less
dark, but more extensive, _penumbra_ surrounding it, sometimes
with several umbrae in one penumbra. He has left us many drawings of
these spots, and he fixed their period of rotation as a lunar month.

[Illustration: SOLAR SURFACE, As Photographed at the Royal
Observatory, Greenwich, showing sun-spots with umbrae, penumbrae, and
faculae.]

It is not certain whether Galileo, Fabricius, or Schemer was the first
to see the spots. They all did good work. The spots were found to be
ever varying in size and shape. Sometimes, when a spot disappears at
the western limb of the sun, it is never seen again. In other cases,
after a fortnight, it reappears at the eastern limb. The faculae, or
bright areas, which are seen all over the sun's surface, but specially
in the neighbourhood of spots, and most distinctly near the sun's
edge, were discovered by Galileo. A high telescopic power resolves
their structure into an appearance like willow-leaves, or rice-grains,
fairly uniform in size, and more marked than on other parts of the
sun's surface.

Speculations as to the cause of sun-spots have never ceased from
Galileo's time to ours. He supposed them to be clouds. Scheiner[1]
said they were the indications of tumultuous movements occasionally
agitating the ocean of liquid fire of which he supposed the sun to be
composed.