Apsis (astronomy): periapsis, apoapsis, and the line of apsides
An apsis is the extreme point in an orbit — nearest (periapsis) or farthest (apoapsis) — with named variants for different central bodies and importance in orbital mechanics and celestial dynamics.
Overview
In celestial mechanics, an apsis (plural: apsides) is either of the two extreme points on an orbit where a moving body is at its minimum or maximum distance from the body it is orbiting. The closest point is called the periapsis and the farthest point the apoapsis. Together these two points define the line of apsides, which coincides with the major axis of an elliptical orbit and indicates the orientation of the orbit in its plane. For a general astronomical object, the terms periapsis and apoapsis are commonly used; a pronunciation guide is often provided for technical terms (pronunciation).
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Names for different central bodies
When the central attractor is specified, English uses special names derived from Greek or Latin roots. Common pairs include:
- Perigee / Apogee — for orbits about Earth.
- Perihelion / Aphelion — for orbits about the Sun.
- Periastron / Apastron — for orbits around another star in a binary system.
- Perijove / Apojove — for Jupiter; similar forms exist for other planets and moons (e.g., periselene for the Moon) and historic mission-specific terms such as pericynthion / apocynthion used during the Apollo program.
Orbital mechanics and formulas
For an elliptical orbit, the distances at periapsis (rp) and apoapsis (ra) are directly related to the orbit's semi-major axis (a) and eccentricity (e): rp = a(1 − e) and ra = a(1 + e). These values appear among the standard orbital elements; the argument of periapsis locates periapsis within the orbital plane, while the line of apsides gives the orbit's long axis. Circular orbits (e = 0) have no distinct apsides because every point is equidistant from the center. In open trajectories such as parabolic or hyperbolic paths, a distinct apoapsis does not exist, though a periapsis may still be defined.
History, examples and importance
Tracking apsides is crucial in astronomy and spaceflight. The slow rotation of the line of apsides — called apsidal precession — can result from perturbations by other bodies, tidal interactions, or relativistic effects. The anomalous advance of Mercury's perihelion was a key observational test of general relativity. Mission planners use periapsis and apoapsis to design transfer orbits, rendezvous maneuvers and atmospheric passes; reaching a low periapsis can increase science return for planetary probes while avoiding atmospheric drag or reentry.
Distinctions and notable facts
Different communities favor different terms: spacecraft engineers often say pericenter/apoapsis for neutrality, while astronomers use specialised names tied to the central body. The etymology reflects Greek and Latin roots for "near" (peri-) and "away/apo-" combined with the name of the attractor. Understanding apsides links simple geometric properties of ellipses to practical mission design and deep physical effects in celestial dynamics.
Further reading and resources can be found through general astronomy references and orbital mechanics texts (see references and external resources).
Questions and answers
Q: What is an apsis in astronomy?
A: An apsis in astronomy is the point of greatest or least distance of the elliptical orbit of an astronomical object from its center of attraction.
Q: What is the point of closest approach in an elliptical orbit called?
A: The point of closest approach in an elliptical orbit is called the periapsis or pericentre.
Q: What is the point of farthest excursion in an elliptical orbit called?
A: The point of farthest excursion in an elliptical orbit is called the apoapsis, apocentre or apapsis.
Q: What is the major axis of an ellipse?
A: The major axis of an ellipse is the line through the longest part of the ellipse, which is drawn through the periapsis and apoapsis and is also known as the line of apsides.
Q: What are the similar words used to identify the body being orbited?
A: The similar words used to identify the body being orbited are perigee and apogee for orbits around the Earth, and perihelion and aphelion for orbits around the Sun.
Q: What are the terms used for referring to the moon during the Apollo program?
A: The terms used for referring to the moon during the Apollo program were pericynthion and apocynthion.
Q: What does the term apoapsis mean etymologically?
A: Etymologically, the term apoapsis comes from the Greek word από (apo) which means "from," and refers to the point farthest away from the center of attraction in an elliptical orbit.
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AlegsaOnline.com Apsis (astronomy): periapsis, apoapsis, and the line of apsides Leandro Alegsa
URL: https://en.alegsaonline.com/art/5064