Editing Selenographic coordinate system (section)

== Longitude ==

Longitude on the Moon is measured both east and west from its [[prime 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 [[International Astronomical Union|IAU]] recommends the "mean Earth/polar axis" system,<ref>[https://astrogeology.usgs.gov/Projects/WGCCRE/WGCCRE2003preprint.pdf IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements: 2003]</ref> in which the prime meridian is the average direction (from the Moon's center) of the Earth's center.<ref>[http://lunar.gsfc.nasa.gov/library/451-SCI-000958.pdf A Standardized Lunar Coordinate System for the Lunar Reconnaissance Orbiter]</ref>