The selenographic colongitude is the longitude of the morning terminator on the Moon, as measured in degrees westward from the prime meridian. The morning terminator forms a half-circle across the Moon where the Sun is just starting to rise. As the Moon continues in its orbit, this line advances in longitude. The value of the selenographic colongitude increases from 0° to 359° in the direction of the advancing terminator.
Sunrise occurs at the prime meridian when the Lunar phase reaches First Quarter, after one fourth of a lunar day. At this location the selenographic colongitude at sunrise is defined as 0°. Thus, by the time of the Full Moon the colongitude increases to 90°; at Last Quarter it is 180°, and at the New Moon the colongitude reaches 270°. Note that the Moon is nearly invisible from the Earth at New Moon phase except during a solar eclipse.
The low angle of incidence of arriving sunlight tends to pick out features by the sharp shadows they cast, thus the area near the terminator is usually the most favorable for viewing or photographing lunar features through a telescope. The observer will need to know the location of the terminator to plan observations of selected features. The selenographic colongitude is useful for this purpose.
The selenographic longitude of the evening terminator is equal to the colongitude plus 180°.
Read more about this topic: Selenographic Coordinates