travels eastward, until it attains the figure of a half moon; still
growing larger as it pursues its course, it finally becomes a full
resplendent globe, rising about the time that the Sun sets and situated
directly opposite to him. Then, in a reverse manner, after full moon, it
goes through the same phases, until, as a slender crescent, it becomes
invisible in the solar rays; afterwards to re-appear in a few days, and,
in its monthly round, to undergo the same cycle of changes. The phases
of the Moon depend upon the changing position of the orb with regard to
the Sun. The Moon shines by reflected light derived from the Sun, and as
one half of its surface is always illumined and the other half totally
dark, the crescent increases or diminishes when, by the Moon's change of
position, we see more or less of the bright side. Visible at first as a
slender crescent near the setting Sun, the angular distance from the orb
and the width of the crescent increase daily, until, at the expiration
of seven days, the Moon is distant one quarter of the circumference of
the heavens from the Sun. The Moon is then a semi-circle, or in
quadrature. At the end of other seven days, the distance of the Moon
from the Sun is at its greatest--half the circumference of its orbit. It
is then visible as a circular disc and we behold the orb as full moon.
The waning Moon, as it gradually decreases, presents the same aspects
reversed, and, finally, its slender crescent disappears in the Sun's
rays. The convex edge of the crescent is always turned towards the Sun.
The rising of the Moon in the east and its setting in the west is an
effect due to the diurnal rotation of the Earth on her axis, but the orb
can be perceived to have two motions besides: one from west to east,
which carries it round the heavens in 29.53 days, and another from north
to south. The west to east motion is steady and continuous, but, owing
to the Sun's attractive force, the Moon is made to swerve from its path,
giving rise to irregularities of its motion called PERTURBATIONS. The
most important of these is the _annual equation_, discovered by Tycho
Brahe--a yearly effect produced by the Sun's disturbing influence as the
Earth approaches or recedes from him in her orbit; another irregularity,
called the _evection_, is a change in the eccentricity of the lunar
orbit, by which the mean longitude of the Moon is increased or
diminished. _Elliptic inequality_, _parallactic inequality_,