orbit

An orbit is the gravitationally bound path of one object around another in space. In celestial mechanics, the central body’s gravity provides the centripetal force that keeps the orbiting body in motion. In the idealized two-body problem with only gravitational interaction, the orbit is a conic section: a circle, ellipse, parabola, or hyperbola, with the central body at a focus. If the total mechanical energy is negative, the orbit is bound (circle or ellipse); zero energy yields a parabolic trajectory; positive energy yields a hyperbolic, unbound path.

Kepler's laws describe the motion. Orbits are conic sections with the central body at a focus; a line from the two bodies sweeps equal areas in equal times; the square of the orbital period is proportional to the cube of the semi-major axis. The dynamics rely on gravity; the specific orbital energy and angular momentum determine the size and shape. Orbits are classified by eccentricity e: e=0 for a circle, 0<e<1 for an ellipse, e=1 for a parabola, e>1 for a hyperbola. Orientation is described by orbital elements such as the semi-major axis a, eccentricity e, inclination i, and the angles Ω and ω.

Common examples include planets, moons, and artificial satellites. A bound, circular or near-circular orbit has small e and a defined period; an ellipse has periapsis and apoapsis. For Earth-related orbits, the terms perigee and apogee are used. Orbits can be perturbed by other bodies and may change over time.