perigeum

Perigeum is a term occasionally encountered to describe the point in an orbit that is closest to the central body. In modern astronomical usage, the preferred term is periapsis, with the Earth-centered case commonly called perigee. Perigeum can appear in historical or Latinized texts and is not widely used in contemporary English astronomy.

Definition and usage: For a bound orbit around a primary body, the orbiting object reaches its minimum distance at the periapsis. When the central body is Earth, the term perigee is used; otherwise the general term periapsis applies to any central body. Some older or Latinized sources may spell the concept as perigeum.

Orbital characteristics: In an elliptical orbit with semi-major axis a and eccentricity e, the distance at periapsis is r_p = a(1 − e). The orbital speed at periapsis is v_p = sqrt( μ (1 + e) / ( a (1 − e) ) ), where μ is the standard gravitational parameter of the central body. For hyperbolic trajectories, a similar framework applies with the appropriate sign conventions.

Significance: The periapsis distance influences surface or atmospheric exposure for satellites and spacecraft and is a key factor in mission design, ranging from satellite launches to flyby planning. It also affects the dynamics of natural satellites, close approaches, and tidal interactions.

See also: Periapsis, Perigee, Perihelion, Apoapsis, Apogee.