Lunar orbit Lunar orbit In astronomy, lunar orbit (also known as a selenocentric orbit) is the orbit of an object around the Moon. As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon. The altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune, apocynthion, or aposelene, while the periapsis (point closest to the surface) is known as perilune, pericynthion, or periselene, from names or epithets of the moon goddess. Low lunar orbit (LLO)—orbits below altitude—are
Earth-centered inertial specifying the direction toward celestial objects. The extent to which an ECI frame is actually inertial is limited by the non-uniformity of the surrounding gravitational field. For example, the Moon's gravitational influence on a high-Earth orbiting satellite is significantly different than its influence on Earth, so observers in an ECI frame would have to account for this acceleration difference in their laws of motion. The closer the observed object is to the ECI-origin, the less significant the effect of the gravitational disparity is. It is convenient to define the orientation of an ECI frame using the Earth's orbit plane and