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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

. 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 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 the south pole to 90° at the north pol ...

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 letter l ...

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 define ...

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, at 90°S. It is of special interest to scientists because of the occurrence of water ice in permanently shadowed areas around it. The lunar south pole region features craters that ar ...

. (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 a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

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

libration In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different hemispheres of the surface at different tim ...

, 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 arbitrary meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great c ...

. 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; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

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
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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 Concerning the lunar month of ~29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon's directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the t ...

reaches First Quarter, after one fourth of a

lunar day A lunar day is the period of time for Earth's Moon to complete one rotation on its axis with respect to the Sun. Due to tidal locking, it is the time the Moon takes to complete one orbit of the Moon, orbit around Earth Earth is the thi ...

. At this location the selenographic colongitude at sunrise is defined as 0°. Thus, by the time of the

Full Moon The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic coordinate system, ecliptic longitudes of the Sun and Moon opp ...

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 the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six month ...

. 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, image, or picture) is an image created by light falling on a photosensitive surface, usually photographic film or an electronic image sensor, such as a CCD or a CMOS chip. Most photographs are now create ...

ing lunar features through a

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

. 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 (September 22, 1932 – July 12, 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 Cze ...

: ''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