periapsis

Periapsis is the point in an object's orbit around a primary body where the orbiting object is closest to that body. It is one of the two apsides, the other being apoapsis, the farthest point. The term is used generically; specific forms include perihelion (around the Sun) and perigee (around Earth). The corresponding far point forms are aphelion and apogee.

In an elliptical orbit, the distance at periapsis, q, equals the semi-major axis a times (1 minus eccentricity e): q = a(1 − e). If the orbit is circular, e = 0 and q equals the orbital radius. The orbital speed at periapsis is given by v_p = sqrt( μ (1 + e) / [a (1 − e)] ), where μ is the Sun’s or primary body's standard gravitational parameter. For hyperbolic trajectories, the periapsis distance can be expressed as r_p = |a|(e − 1) (with a negative for hyperbolas).

Examples help illustrate: Earth’s perihelion occurs each year in early January, at about 147 million kilometers (roughly 0.98 astronomical units) from the Sun. Its aphelion occurs in early July, near 152 million kilometers. The Moon’s perigee is approximately 363,000 kilometers from Earth, though this distance varies over each orbit.

Periapsis is important in mission design and orbital analysis because it sets the minimum distance to the primary body, influences orbital speed at that point, and affects gravitational interactions, tides, and observational geometry. In astronomy and astrodynamics, it is a key parameter in describing eccentric orbits and in translating orbital elements into the shape and timing of a body's motion.