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An apsis, plural apsidesis a point of greatest or least distance of a body in an elliptic orbit about a larger body. For a body orbiting the Sun the greatest and least distance points are called respectively aphelion and perihelion, whereas for any satellite of Earth including the Moon the corresponding points are apogee and perigee.

In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are commonly used to describe the orbital altitude of the spacecraft from the surface of the central body (assuming a constant, standard reference radius).

r_{per} | periapsis distance (m) |

e | eccentricity (dimensionless) |

a | semi-major axis (m) |