Glossary

This glossary is meant to provide definitions of common astrodynamics terms. The documentation attempts to provide introductions to potentially new terminology as encountered, but there is no required reading order. So if you're hoping around this page attempts to capture the introduction of significant terms and concepts here that may have otherwise been missed.

Term	Definition
Aphelion	Furthest point away for a Sun-orbiting object from the Sun.
Apoapsis	Furthest point away for an orbiting object from its orbital focus.
Apogee	Furthest point away for a Earth-orbiting object from the Earth.
Earth-Centered Earth-Fixed (ECEF)	A rotating reference frame with its coordinate origin at the center of the Earth. Reference frame rotates with Earth's crustal (surface) motion.
Earth-Centered Inertial (ECI)	An inertial reference frame or system with its coordinate origin at the Earth's center of mass. In an inertial reference frame Newtwon's Laws are applicable, meaning that no corrections for fictitious, observer-based forces like the Coriolis Force have to be performed.
Osculating Orbital Elements	Instantaneous orbital elements describing the satellite trajectory for a specific time if no perturbations beyond point-mass gravity were present
Mean Motion	Average angular rate a satellite progresses through its orbit.
Periapsis	Point of closest approach for an orbiting object with respect to its orbital focus.
Perihelion	Point of closest approach for an Sun-orbiting object with respect to the Sun.
Perigee	Point of closest approach for an Earth-orbiting object with respect to the Earth.