Selenographic colongitude

Selenographic coordinates are coordinates that refer to locations on the surface of the Moon. Any position on the lunar surface is completely determined by two numerical values, which are comparable to the latitude and longitude on the Earth. The longitude gives the position east or west of the prime meridian of the Moon, which is the point on the lunar surface directly facing Earth. This can be thought of as the mid-point of the visible Moon as seen from the Earth. The latitude gives the position north or south of the lunar equator. Both of these coordinates are given in terms of degrees.

The selenographic co-longitude is the longitude of the morning terminator on the Moon, as measured in degrees westwards from the central 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 co-longitude increases from 0° to 360° in the direction of the advancing terminator.

Sunrise occurs at the prime meridian when the Moon reaches First Quarter, after one fourth of a lunar day. At this location the selenographic co-longitude at sunrise is defined as 0°. Thus by the time of the Full Moon the co-longitude increases to 90°; at Last Quarter it is 180°, and at New Moon the coordinate reaches 270°.

Due to the low angle of the arriving sunlight, the area near the terminator is usually the most favorable for viewing or photographing lunar features through a telescope. Thus the observer will need to know the location of the terminator to plan when to observe selected features on the Moon. The selenographic co-longitude at sunrise is used for this purpose.

The selenographic longitude of the evening terminator is equal to the co-longitude plus 180°.