e, but at all other times it is gibbous, and approaches
nearest to that of a half-moon when at the quadratures.

In the year 1610, on directing his telescope to the Sun, Galileo
detected dark spots on the solar disc. Similar spots, sufficiently large
to be distinguished by the naked eye, had been observed from time to
time for centuries prior to the invention of the telescope, but nothing
was known of their nature. In 1609 Kepler observed a spot on the Sun,
which he thought was the planet Mercury in conjunction with the orb; the
short time during which it was visible, in consequence of clouds having
obscured the face of the luminary, prevented him from being able to
determine the accuracy of his surmise, but since then it has been
ascertained that no transit of Mercury took place at that time, and
Kepler afterwards acknowledged that he had arrived at an erroneous
conclusion. Galileo was much puzzled in trying to find out the true
nature of the spots. At first he was led to imagine that planets like
Mercury and Venus revolved round the Sun at a short distance from the
orb, and that their dark bodies, travelling across the solar disc, gave
rise to the phenomenon of the spots. After further observation, he
ascertained that the spots were in actual contact with the Sun; that
they were irregular in shape and size, and continued to appear and
disappear. Sometimes a large spot would break up into several smaller
ones, and at other times three or four small spots would unite to form a
large one. They all had a common motion, and appeared to rotate with
the Sun, from which Galileo concluded that the orb rotated on his axis
in about twenty-eight days. Galileo believed that the spots were clouds
floating in the solar atmosphere, and that they intercepted a portion of
the light of the Sun.

The Milky Way, that wondrous zone of light which encircles the heavens,
remained for many ages a source of perplexity to ancient astronomers and
philosophers, who, in their endeavours to ascertain its nature, had
arrived at various absurd and erroneous conclusions. On directing his
telescope to this luminous tract, Galileo discovered, to his
inexpressible admiration, that it consists of a vast multitude of stars,
too minute to be visible to the naked eye. He also discerned that its
milky luminosity is created by the blended light of myriads of stars, so
remote as to be incapable of definition by his telescope. In his
'Nuncius Sidereus' he gives