perigeu

Perigeu, in English usually called perigee, is the point in an orbit around the Earth where the orbiting body is closest to the Earth. The term comes from Greek roots meaning “near” and “Earth.” The opposite point is the apogee, where the body is farthest from the Earth. In astronomy, perigee is a specific case of the general concept of periapsis, used when the central body is the Earth.

In orbital mechanics, the distance from Earth at perigee depends on the orbit’s size and shape. For an elliptical orbit with semi-major axis a and eccentricity e, the distance at perigee is r_p = a(1 − e). The orbital speed is highest at perigee due to conservation of energy and angular momentum; the velocity can be calculated with the vis-viva equation v^2 = μ(2/r − 1/a), where μ is Earth's standard gravitational parameter (~398,600 km^3/s^2).

Perigee is relevant for natural satellites such as the Moon and for artificial satellites. The Moon’s perigee varies due to gravitational perturbations and is typically around 363,000 kilometers, with apogee roughly 405,000 kilometers. For artificial satellites, perigee ranges from a few tens of kilometers in very low Earth orbits to around 35,786 kilometers for circular geostationary orbits (where perigee equals apogee since the orbit is circular).

Observational and operational significance includes planning satellite launches, predicting ground tracks, reentry risks for decommissioned objects, and assessing communication window timings. Perigee can be affected by perturbations from the Moon, the Sun, atmospheric drag (in low Earth orbit), and other gravitational influences, leading to slow changes in the orbital shape over time.