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.
|Still in use|
|Galaxy||perigalacticon, apogalacticon||-icones? (not used)|
|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).