AIM

4/3/14

10−2−9

Special Operations

c. Information on Moon Phases and Changes in

the Percentage of the Moon Illuminated
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. 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. There are eight distinct, tradition-

ally 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 in FIG 10−2−3.

FIG 10

−2−3

Phases of the Moon

New Moon − The Moon’s unilluminated side is facing the Earth. The Moon is not visible

(except during a solar eclipse).

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.