latter maintaining that the irregularities of outline presented by the
"terminator," or line of demarcation between the illumined and
unillumined portion of her spherical superficies, are due to mountains
and their shadows; but more than fifteen centuries elapsed before the
truth of this sagacious conjecture was unquestionably demonstrated.
Selenography, as a branch of observational astronomy, dates from the
spring of 1609, when Galileo directed his "optic tube" to the moon, and
in the following year, in the _Sidereus Nuncius_, or "the Intelligencer
of the Stars," gave to an astonished and incredulous world an account of
the unsuspected marvels it revealed. In this remarkable little book we
have the first attempt to represent the telescopic aspect of the moon's
visible surface in the five rude woodcuts representing the curious
features he perceived thereon, whose form and arrangement, he tells us,
reminded him of the "ocelli" on the feathers of a peacock's tail,--a
quaint but not altogether inappropriate simile to describe the appearance
of groups of the larger ring-mountains partially illuminated by the sun,
when seen in a small telescope.

The bright and dusky areas, so obvious to the unaided sight, were found
by Galileo to be due to a very manifest difference in the character of
the lunar surface, a large portion of the northern hemisphere, and no
inconsiderable part of the south-eastern quadrant, being seen to consist
of large grey monotonous tracts, often bordered by lofty mountains, while
the remainder of the superficies was much more conspicuously brilliant,
and, moreover, included by far the greater number of those curious ring-
mountains and other extraordinary features whose remarkable aspect and
peculiar arrangement first attracted his attention. Struck by the analogy
which these contrasted regions present to the land and water surfaces of
our globe, he suspected that the former are represented on the moon by
the brighter and more rugged, and the latter by the smoother and more