perigees

Perigee is the point in the orbit of a body around Earth where the body is nearest to the planet. It is one of the orbital apsides, with the opposite point called apogee, the farthest distance. The general term for the closest approach to any primary body is periapsis; perigee is the Earth-specific form used in celestial mechanics and satellite contexts.

In an elliptical orbit, the distance at perigee is r_p = a(1 − e), where a is the orbit’s semi-major axis and e its eccentricity. The orbital speed is greatest at perigee; v_p = sqrt(μ(1+e) / [a(1−e)]), where μ is Earth’s standard gravitational parameter (approximately 398,600 km^3/s^2). These relationships reflect how gravity and geometry govern motion along an ellipse.

Examples and significance: The Moon’s perigee varies over time, typically around 356,000 to 363,000 km, with apogee around 405,000 to 406,000 km. Perigee distance influences tides, orbital energy, and visibility for satellites. Perigee passages are monitored in mission planning and launch window determinations. The term perigee is used specifically for Earth-centered orbits; for other primaries, analogous terms include perihelion for the Sun and perijove for Jupiter.

Perigee can interact with other orbital configurations to produce notable effects; for instance, when a perigee passage coincides with a full or new Moon, tides can be unusually pronounced, a phenomenon known as a perigean spring tide.