The selenographic coordinate system is used to refer to locations on the surface of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

. Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

and

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

of Earth. The longitude gives the position east or west of the Moon's prime meridian, which is the line passing from the

lunar north pole The lunar north pole is the point in the Northern Hemisphere of the Moon where the lunar axis of rotation meets its surface. The lunar North Pole is the northernmost point on the Moon, lying diametrically opposite the lunar south pole. It defines ...

through the point on the lunar surface directly facing Earth to the

lunar south pole The lunar south pole is the southernmost point on the Moon. It is of interest to scientists because of the lunar water, occurrence of water ice in Crater of eternal darkness, permanently shadowed areas around it. The lunar south pole region fea ...

. (See also Earth's prime meridian.) This can be thought of as the midpoint of the visible Moon as seen from the Earth. The latitude gives the position north or south of the lunar

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

. Both of these coordinates are given in degrees. Astronomers defined the fundamental location in the selenographic coordinate system by the small, bowl-shaped satellite crater ' Mösting A'. The coordinates of this crater are defined as: : Later, the coordinate system has become more precisely defined due to the

Lunar Laser Ranging Experiment Lunar Laser Ranging (LLR) is the practice of measuring Lunar distance (astronomy), the distance between the surfaces of the Earth and the Moon using Lidar, laser ranging. The distance can be calculated from the Round-trip delay, round-trip time ...

. Anything past 90°E or 90°W would not be seen from Earth, except for

libration In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...

, which makes 59% of the Moon visible.

Longitude

Longitude on the Moon is measured both east and west from its

prime meridian A prime meridian is an arbitrarily chosen meridian (geography), meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. On a spheroid, a prime meridian and its anti-meridian (the 180th meridian ...

. When no direction is specified, east is positive and west is negative. Roughly speaking, the Moon's prime meridian lies near the center of the Moon's disc as seen from Earth. For precise applications, many coordinate systems have been defined for the Moon, each with a slightly different prime meridian. The

IAU The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

recommends the "mean Earth/polar axis" system, in which the prime meridian is the average direction (from the Moon's center) of the Earth's center.A Standardized Lunar Coordinate System for the Lunar Reconnaissance Orbiter
/ref>

Selenographic colongitude

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 A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...

reaches First Quarter, after one fourth of a

lunar day A lunar day is the time it takes for Earth's Moon to complete on its axis one synodic rotation, meaning with respect to the Sun. Informally, a lunar day and a lunar night is each approximately 14 Earth days. The formal lunar day is therefore t ...

. 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 In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse. ...

the colongitude reaches 270°. Note that the Moon is nearly invisible from the Earth at New Moon phase except during a

solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...

. 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

photograph A photograph (also known as a photo, or more generically referred to as an ''image'' or ''picture'') is an image created by light falling on a photosensitivity, photosensitive surface, usually photographic film or an electronic image sensor. Th ...

ing lunar features through a

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

. 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°.

Antonín Rükl Antonín Rükl (22 September 1932 – 12 July 2016) was a Czech astronomer, cartographer, and author. He was born in Čáslav, Czechoslovakia. As a student, he developed what was to be a lifelong interest in astronomy. He graduated from the Cz ...

: ''Atlas Měsíce'', Aventinum (Praha 1991), chapter Tabulky pro výpočet hodnoty colongitudo, page 212,

References

*"A Unified Lunar Control Network — The Near Side", Merton E. Davies, Tim R. Colvin, & Donald L. Mayer, RAND Corporation, Santa Monica, 1987. {{The Moon Lunar science Angle Coordinate systems