Selenographic coordinates

Positions on the Earth's moon are given by the coordinates selenografische width and length selenografische. In the nature of their definition they correspond to the geographical latitude and longitude on Earth.

The term Selenografie corresponds to the word geography and is composed of the Greek words for moon ( Σελήνη, Selene ) and draw / describe ( γραφειν, grafeïn ) together. It was introduced into astronomy, especially through the work of the famous Moon researchers Hieronymus Schröter ( main work " Selenotopographische fragments " 1790) and Johann Heinrich von Mädler ( 1794-1874, first special Mondatlas ).

The selenografische width refers to the lunar equator - that is, the plane perpendicular to the axis of rotation plane that divides the moon into approximately two equal halves. The rotation poles have the width 90 ° (north pole ) and -90 ° ( South Pole ). The selenografische length refers to a zero meridian, which is not fixed arbitrarily, in contrast to the Greenwich Meridian of geography, but refers to the average position of the Earth-Moon system:

The selenografische prime meridian intersects the equator moon in the lunar center, the point has the center of the earth in the middle of a 18 ½ - year period. The need for this definition arises from the so-called libration: the apparent monthly " wobbling " of the moon with respect to Earth, associated with its elliptical orbit around the Earth. The moon's orbit also moves in a period of about 18 ½ years in space, so that only the exact averaging over this period can be the basis of selenographic coordinate system.

A second peculiarity of the selenographic to the geographic coordinates is located in the difference between the moon and the earth's shape. The latter is approximately an ellipsoid, while the Moon is almost exactly one ball. Therefore, do not need to distinguish between ellipsoidal latitude and ( geo) centric width, but a related to the mean moon ball width ranges as specified coordinates from.

Nevertheless, a small deviation of the lunar center is taken into account by its center of gravity, which accounts for 1.8 km and with its orbit and the earth tides ( " tidally locked " ) related. 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 toward Earth.