sentences:  In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body.However, in the case of a spacecraft, the terms are commonly used to refer to the orbital altitude of the spacecraft above the surface of the central body (assuming a constant, standard reference radius).The words "pericenter" and "apocenter" are often seen, although periapsis/apoapsis are preferred in technical usage.