The Moon
From any location on the Earth, the Moon appears to be a
circular disk which, at any specific time, is illuminated to
some degree by direct sunlight. Like the Earth, the Moon is a
sphere which is always half illuminated by the Sun, but as
the Moon orbits the Earth we get to see more or less of the
illuminated half. During each lunar orbit (a lunar month), we
see the Moon's appearance change from not visibly illuminated
through partially illuminated to fully illuminated, then back
through partially illuminated to not illuminated again.
Although this cycle is a continuous process, there are eight
distinct, traditionally recognized stages, called phases. The
phases designate both the degree to which the Moon is
illuminated and the geometric appearance of the illuminated
part. These phases of the Moon, in the sequence of their
occurrence (starting from New Moon), are listed below.
Waxing Crescent - The Moon appears to be partly but less than
one-half illuminated by direct sunlight. The fraction of the
Moon's disk that is illuminated is increasing.
First Quarter - One-half of the Moon appears to be
illuminated by direct sunlight. The fraction of the Moon's
disk that is illuminated is increasing.
Waxing Gibbous - The Moon appears to be more than one-half
but not fully illuminated by direct sunlight. The fraction of
the Moon's disk that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth.
The Moon appears to be completely illuminated by direct
sunlight.
Waning Gibbous - The Moon appears to be more than one-half
but not fully illuminated by direct sunlight. The fraction of
the Moon's disk that is illuminated is decreasing.
Last Quarter - One-half of the Moon appears to be illuminated
by direct sunlight. The fraction of the Moon's disk that is
illuminated is decreasing.
Waning Crescent - The Moon appears to be partly but less than
one-half illuminated by direct sunlight. The fraction of the
Moon's disk that is illuminated is decreasing.
Following waning crescent is New Moon, beginning a repetition
of the complete phase cycle of 29.5 days average duration.
The time in days counted from the time of New Moon is called
the Moon's "age". Each complete cycle of phases is called a
"lunation". Because the cycle of the phases is shorter than
most calendar months, the phase of the Moon at the very
beginning of the month usually repeats at the very end of the
month. When there are two Full Moons in a month (which
occurs, on average, every 2.7 years), the second one is
called a "Blue Moon".
New Moon, First Quarter, Full Moon, and Last Quarter phases
are considered to be primary phases. The two crescent and two
gibbous phases are intermediate phases, each of which lasts
for about a week between the primary phases, during which
time the exact fraction of the Moon's disk that is
illuminated gradually changes.