Perigee is the point in an orbiting object’s path where it achieves its closest distance to the Earth. Derived from Greek, perigee exists because orbits are not perfect circles. This nearest approach is a recurring astronomical event.
Defining the Closest Approach
Orbits are not circular paths, but follow an elliptical shape. Because the orbital path is an ellipse, the distance between the orbiting body and the Earth constantly changes, necessitating both a closest and farthest point.
The perigee identifies the location of minimum separation. This distance is measured from the center of the orbiting object to the center of the Earth, which acts as the gravitational focus. The orbit’s eccentricity, or degree of elongation, influences the difference between the closest and farthest points.
The Complementary Distance: Apogee
Every elliptical orbit around the Earth has a counterpart, the apogee, which marks the point of maximum distance. Perigee and apogee lie on opposite ends of the ellipse’s major axis.
These terms are specific to Earth-centered orbits, indicated by the suffix -gee. The general term for the closest point in any orbit is periapsis. The name changes based on the central body (e.g., perihelion for the Sun, periareion for Mars).
Where Perigee Occurs
The Moon passes through its closest point to Earth approximately once every 27.5 days. The perigee distance is around 363,300 kilometers from Earth’s center, though it is variable due to gravitational influences from the Sun.
Artificial satellites, such as those in Low-Earth Orbit (LEO), also have a perigee. Satellite operators manage this distance to maintain the orbit, as a low perigee increases atmospheric drag. Maintaining a precise perigee ensures the satellite remains above the denser layers of the atmosphere.
Observational Consequences
An object’s passage through perigee has three main effects. The increased proximity makes the orbiting object appear slightly larger in the sky. When the Moon’s perigee coincides with a full moon, the phenomenon is known as a “Supermoon,” appearing up to 14 percent larger and 30 percent brighter than at apogee.
Second, the object’s orbital velocity is at its maximum when it passes through perigee. This is due to Kepler’s Second Law of Planetary Motion, which requires the orbiting body to move faster when it is closer to the gravitational source.
Third, the closer distance enhances the object’s gravitational pull on Earth, leading to stronger tidal effects. When the Moon is at perigee, its gravitational force is magnified, resulting in a greater range between high and low tides, known as Perigean tides.