Pinpointing the Moon's South Pole

In the system of lunar latitude and longitude adopted by the Lunar Reconnaissance Orbiter (LRO) mission, the Moon’s South Pole is located on the rim of Shackleton crater at a point marked by a red pin in this visualization. If you imagine Shackleton as the (very big!) face of a clock with noon pointing toward Earth, the South Pole is about halfway between 10 and 11 o'clock.



Before launch, the LRO team adopted the Mean Earth/Polar Axis (Moon ME) coordinate system for all of its data products, and this has become the standard for mapping all lunar data. In this system, the Z axis is the mean (average) rotation axis, and the X axis points in the mean direction of the Earth. Because of libration, both of these directions must be calculated as averages over long time spans. The specific calculation for LRO’s Moon ME coordinate system is embodied in the JPL ephemeris named DE421, released in 2008.



Internally, JPL’s ephemeris calculations use a different coordinate system called the Principal Axis (Moon PA) frame. Roughly speaking, the Moon PA system balances the mass along each axis, which simplifies the calculation of the Moon’s slightly wobbly rotations. Moon ME is then defined as a small rotation relative to Moon PA that amounts to a difference of about 875 meters (half a mile) between the two systems.



The definition and wide adoption of a standard coordinate system for the Moon is vital for mapping and exploring our nearest neighbor. With such a system, we can confidently pinpoint any feature on the Moon, including the exact location of its South Pole.

See A Standardized Lunar Coordinate System for the Lunar Reconnaissance Orbiter and Lunar Datasets (2008)