Baanafstand

Baanafstand is a Dutch term used in astronomy and astrodynamics to denote the distance of an object in orbit around a central body. In most contexts it corresponds to the semi-major axis (symbol a) of the orbit. For circular orbits the baanafstand equals the orbital radius; for elliptical orbits it represents the average distance over a full orbit and is often the most convenient single parameter to describe the size of the orbit.

Measurement and calculation of baanafstand rely on orbital elements and observations. It can be determined from tracking data and timing measurements, or derived from the central body's mass and the orbital period using Kepler's laws. The relation a^3 ∝ μ T^2 holds, where μ is the standard gravitational parameter of the central body and T is the orbital period.

Units are typically kilometers or astronomical units (AU) in the solar system, with AU being common for planetary orbits. Examples include Earth's baanafstand of about 1 AU (approximately 149.6 million kilometers) and Mars at about 1.52 AU. For artificial satellites around Earth, the baanafstand is often given as the distance from Earth's center; the corresponding altitude is approximately a minus Earth's radius (e.g., geostationary orbit has a ≈ 42,164 km).

Usage and relevance: the baanafstand is a fundamental descriptor of orbital size and appears in discussions of planetary systems, satellite trajectories, and exoplanet studies. It is a central element in orbital mechanics alongside eccentricity, inclination, and orbital period.

See also: semi-major axis, orbital elements, Kepler's laws, periapsis and apoapsis.