Objects in space, whether natural satellites or artificial spacecraft, do not maintain a uniform distance from the body they orbit. Instead of tracing a perfect circle, their path is slightly elongated, causing their separation from the central body to constantly change. Every orbit thus includes a point of maximum distance and a point of minimum distance. These two extreme positions are mathematically defined and have implications for orbital mechanics and space missions.
Defining the Extremes of Orbit
The general term for either of these two extreme points in an orbit is an apsis (apsides in the plural). When an object orbits the Earth, the specific terms used are apogee and perigee. The suffix “-gee” denotes an Earth-centered orbit, while the prefixes “apo-” and “peri-” distinguish the farthest and closest points.
Apogee is the point where the orbiting body is farthest from the center of the Earth. Conversely, perigee is the location of the closest approach. The difference between apogee and perigee measures how much the orbit deviates from a perfect circle. For example, the Moon’s distance varies from approximately 225,600 miles at perigee to about 252,000 miles at apogee.
This distinction is important for Earth-orbiting satellites because their speed, altitude, and the power required for communication change dramatically between these two extremes. Apogee determines the maximum altitude of a satellite, while perigee determines the minimum altitude. The relationship between these two points defines the orbit’s overall shape.
The Cause of Orbital Variation
The existence of apogee and perigee is a direct consequence of the physics governing gravitational two-body systems. Johannes Kepler established that orbits are ellipses, not circles, with the central body located at one of the two foci of the ellipse. The degree of stretching in this elliptical path is quantified by a value called eccentricity.
Eccentricity is a value between zero and one; zero indicates a perfect circle, while values closer to one indicate a highly elongated ellipse. Since no natural orbit is perfectly circular, every object in space has a measurable eccentricity. This eccentricity guarantees the existence of a distinct closest point (perigee) and a farthest point (apogee). The initial velocity and position of the orbiting body dictate the resulting eccentricity and the distance differential between the two apsides.
This variation in distance also causes a corresponding change in orbital speed, as described by Kepler’s Second Law of Planetary Motion. The law states that a line connecting the orbiting body to the central body sweeps out equal areas in equal amounts of time. To achieve this, the orbiting object must accelerate as it approaches perigee, moving fastest at the point of closest approach. It then gradually slows down, reaching its lowest speed at apogee.
Specialized Terminology Based on the Central Body
While apogee and perigee are used for orbits around the Earth, a specialized nomenclature system applies to orbits around other celestial bodies. The generic terms for the farthest and closest points in any orbit are apoapsis and periapsis. The suffix “-apsis” is replaced by a specific root word related to the central body.
The most common examples use the root “helion” for orbits around the Sun: aphelion (farthest) and perihelion (closest). Similarly, an object orbiting Mars would reach apoareion and periareion, derived from the Greek god of war, Ares. For Jupiter, the terms are apojove and perijove.
Even the Moon, when acting as the central body for an orbiting satellite, uses apolune and perilune. This system ensures precise communication by immediately identifying the specific body being orbited. The prefixes “apo-” and “peri-” always denote the farthest and closest distances.