selenocentric

Selenocentric is an adjective used in astronomy and spaceflight to describe quantities, frames, or measurements that use the Moon as their origin. In a selenocentric reference frame, the center of the Moon (its center of mass) is the origin, and coordinates describe positions with respect to the Moon rather than to Earth or the Sun.

A selenocentric frame is contrasted with geocentric (Earth-centered) and heliocentric (Sun-centered) systems. The orientation of the selenocentric axes is defined by convention and may vary by mission, but commonly relates to the Moon’s equator and rotation, and to the Earth–Moon line. Some realizations align one axis with the mean Earth direction and another with the lunar north pole, forming a right-handed coordinate system.

Applications of selenocentric frames include describing spacecraft in lunar orbit, planning trajectories and landings, and calculating distances to features on the Moon from the Moon’s center. Selenocentric coordinates are often used together with selenographic coordinates, which specify positions on the lunar surface in a Moon-centered frame anchored to the Moon’s rotation and a prime meridian defined relative to the Earth-facing point.

The term derives from Selene, the Greek goddess of the Moon, and is part of the broader vocabulary distinguishing lunar-centered references from Earth-centered or Sun-centered descriptions.