# Selenographic coordinates

Geo- or selenographic coordinates on the sphere. On the moon, the latitude is usually β instead of φ.

Positions on the earth's moon are indicated by the coordinates selenographic latitude and selenographic longitude . In the way they are defined, they correspond to the geographical latitude and longitude on earth.

The term selenography corresponds to the word geography (description of the earth) and is composed of the Greek words for moon ( Σελήνη, Selene ) and to draw / describe (γραφειν, grafeïn) . It was introduced into astronomy primarily through the work of the lunar researchers Hieronymus Schröter (main work Selenotopographische Fragments , 1790) and Johann Heinrich von Mädler (1794–1874, first lunar atlas ).

## Selenographic latitude

The selenographic latitude (or more rarely ) refers to the lunar equator - i.e. H. the plane perpendicular to the axis of rotation that divides the moon into approximately two equal halves. The rotation poles have latitudes + 90 ° (north pole) and −90 ° (south pole). ${\ displaystyle \ beta}$${\ displaystyle \ varphi}$

## Selenographic longitude

The selenographic longitude refers to a prime meridian which , in contrast to the Greenwich meridian in geography, is not arbitrarily set, but relates to the mean position of the earth-moon system: ${\ displaystyle \ lambda}$

The selenographic prime meridian intersects the lunar equator in the middle of the moon , the point that points to the center of the earth on an average of 18½ years . The necessity of this definition arises from the libration , the apparent, monthly "wobbling" of the moon in relation to the earth, which is related to its elliptical orbit around the earth. The lunar orbit also shifts in space over a period of about 18½ years , so that only the exact averaging over this period can be the basis of the selenographic coordinate system .

## particularities

A second peculiarity of the selenographic compared to the geographical coordinates lies in the difference between the moon and the earth figure . The latter is approximately an ellipsoid , while the moon is almost exactly a sphere . For this reason, a distinction does not have to be made between ellipsoidal and (geo) centric latitudes for the moon , but a latitude related to the central moon sphere is sufficient as a coordinate specification.

Regardless of this, a small deviation of the lunar center from its center of gravity must be taken into account, which amounts to 1.8 km and is related to its orbit and the tides of the earth (" bound rotation "). While the selenographic coordinates refer to the center of the lunar sphere, the center of mass is relevant for the gravitational field , which is closer to the earth.