Periselene

Periselene is the point in a selenocentric orbit—the orbit around the Moon—where the orbiting body is nearest to the Moon’s center. It is the lunar counterpart to periapsis (and to perihelion for the Sun) in orbital mechanics. The distance from the Moon’s center at periselene is called the periselene distance, and the opposite point in the orbit (the farthest point from the Moon) is often referred to as the aposelene or apolune.

Etymology and usage: The term derives from peri-, meaning near, and Selene, the Greek goddess and personification of the Moon. In some sources, perilune is used synonymously with periselene, though periselene is the more formal term in modern selenography and astrodynamics.

Applications: Periselene is a key orbital parameter for missions that involve orbiting the Moon. It determines the lowest altitude a spacecraft achieves over the Moon’s surface during each orbit and influences mission planning, descent sequencing, and data collection strategies. Because the Moon’s gravity field is irregular, perturbations can shift the actual periselene from its nominal value, requiring tracking and occasional orbital maintenance.

Related concepts: Periselene is one example of an apsis in the Moon’s gravitational system. Its counterpart, the farthest orbital point, is commonly called the aposelene or apolune, depending on terminology used in a given mission or study.