In the strictest meaning of the words, the perigee and apogee are
**points **
in an elliptical
orbit at which the orbiting object has the smallest and largest radius vectors respectively.
They lie at opposite ends of the semi-major axis of the ellipse.

We often refer to the perigee and apogee
**heights; **
i.e. the height of the perigee and apogee
points above the planetary surface. Loosely, we drop the word `height' and refer to
these values simply as the perigee and apogee.

But complications arise when you start thinking about the fact that the Earth is not actually
a sphere, but (to a much better approximation) an oblate spheroid - its cross-section is also
an ellipse. Depending on the orientation of the orbit, the two ellipses are misaligned.
In the figure below, a satellite orbit in red is drawn around an exaggerated Earth in blue.
In this case,
the smallest height of the orbit around the spheroid Earth
is not necessarily the height of the perigee point P. In fact
in the example shown, the point of smallest height is about 40 degrees further around
the orbit.

Worse yet, the flattening of the Earth means that the orbit is not actually an
ellipse. The instantaneous (`osculating') orbital parameters
**change with time**
as you go round the orbit, so the true perigee and apogee are different from ones
obtained from time-averaged `mean elements'.

**It is conventional in space situational awareness contexts to quote perigee and apogee heights**
**relative to a fictitious perfectly spherical Earth with radius equal to the equatorial radius.**
There are a lot of reasons that this makes good sense, but the reader should remember
that the actual height above the Earth may be considerably higher if the perigee is over the
polar regions.

Central body | Terms | Plural |
---|---|---|

Still in use | ||

Earth | perigee, apogee | -gees |

Sun | perihelion, aphelion | -helia |

Moon | perilune, apolune | -lunes |

Moon | periselene, aposelene | -selenes? |

Mars | periares, apoares | -ares |

Jupiter | perijove, apojove | -joves |

Star | periastron, apoastron | -astrons |

Galaxy | perigalacticon, apogalacticon | -icones? (not used) |

Probably obsolescent | ||

Moon | pericynthion, apocynthion | -cynthia |

Venus | pericytherion, apocytherion | |

Saturn | perikrone, apokrone | |

Black hole | peribothron, apobothron | -bothroi |

In GCAT, angles are always expressed in degrees and distances are usually in km (but
see below for heliocentric orbits).

- Earth orbits are given with respect to the equator of date (strictly, TEME equator). Note that astronomical positions are often given with respect to the (inertially fixed) ICRF frame. The Earth wobbles due to precession, so the equator of date changes with time relative to the ICRF. Distances are always given in km.
- Heliocentric orbits are given with respect to the J2000 ecliptic. Distances (aphelion and perihelion) are given in astronomical units (AU). 1 AU = 149597870.700 km.
- Orbits around other central bodies (Moon, Mars, etc.) are given with respect to the IAU equator of that body. Where no IAU equator is yet defined in JPL Horizons, the ecliptic is used instead. Distances are given in km.