Glossary of Orbital Mechanics Terms
Key terms used throughout this site. Links point to pages with more detailed treatment.
| Term | Definition |
|---|---|
| Aphelion | The point in an orbit farthest from the Sun. See First Law. |
| Perihelion | The point in an orbit closest to the Sun. |
| Apoapsis / Periapsis | General terms for farthest/closest point in an orbit around any body (not just the Sun). |
| Angular momentum | Measure of rotational motion; conserved in orbital motion. See Angular Momentum. |
| Astronomical Unit (AU) | The average Earth-Sun distance, about 149.6 million km. |
| Conic section | A curve formed by slicing a cone: circle, ellipse, parabola, or hyperbola. |
| Eccentricity (e) | A measure of how elongated an ellipse is. e=0 is a circle; 0<e<1 is an ellipse. |
| Ellipse | A closed oval curve with two foci. See Ellipse Geometry. |
| Epoch | A specific moment in time used as a reference point for orbital elements. |
| Focus (pl. foci) | One of two special points inside an ellipse. The Sun sits at one focus of each planet's orbit. |
| Gravitational constant (G) | The constant in Newton's gravity law, approximately 6.674 × 10−11 N·m²/kg². |
| Hohmann transfer | An efficient orbital maneuver using two engine burns. See Spacecraft Navigation. |
| Inclination | The angle between an orbit's plane and a reference plane (e.g., the ecliptic). |
| Kepler's Laws | Three laws of planetary motion. See Overview. |
| Orbital period (T) | The time for one complete orbit. |
| Perturbation | A small deviation from a Keplerian orbit caused by external forces. See Perturbations. |
| Semi-major axis (a) | Half the longest diameter of an ellipse; the "average orbital radius." |
| Semi-minor axis (b) | Half the shortest diameter of an ellipse. |
| Synodic period | The time between successive alignments of a planet and the Sun as seen from Earth. See Mars Mission Planning. |