Terminology Natural satellite

size comparison of earth , moon

there no established lower limit on considered moon . every natural celestial body identified orbit around planet of solar system, small kilometer across, has been considered moon, though objects tenth size within saturn s rings, have not been directly observed, have been called moonlets. small asteroid moons (natural satellites of asteroids), such dactyl, have been called moonlets.

the upper limit vague. 2 orbiting bodies described double body rather primary , satellite. asteroids such 90 antiope considered double asteroids, have not forced clear definition of constitutes moon. authors consider pluto–charon system double (dwarf) planet. common dividing line on considered moon rests upon whether barycentre below surface of larger body, though arbitrary, because depends on distance relative mass.