apsis

An apsis of an orbit with some eccentricity is either its closest point to the object being orbited (periapsis or periapse) or the furthest point (apoapsis or apapsis or apoapse). The terms pericenter and apocenter similarly mean the closest and furthest points in the orbit from the orbit's center of mass. Specific terms are used for bodies orbiting some specific hosts, such as the Earth or Sun, which are familiar and occasionally used for other hosts:

• perigee - closest point in an Earth orbit.
• apogee - furthest point in an Earth orbit.
• perihelion - closest point in a solar orbit.
• aphelion - furthest point in a solar orbit.
• periastron - closest point of one star from its binary-star companion.
• apastron - furthest point of one star from its binary-star companion.
• perigalacticon - an orbiting star's closest point to the galactic center.
• apogalacticon - an orbiting star's furthest point from the galactic center.

The argument of periapsis (or argument of perifocus or argument of pericenter, or incorporating terms for orbits of a specific body, such as argument of perihelion for an orbit around the Sun) is an angle describing where the periapsis is in relation to a reference plane. For example, if a comet's orbit is inclined to the ecliptic (the plane of Earth's orbit, a common reference plane for the solar system), then the argument of periapsis is the angle at the Sun (within the plane of the comet's orbit) from the direction toward the orbit's intersection with the ecliptic to the direction toward the comet's perihelion. If the comet's argument of periapsis is zero degrees, then its periapsis is on the ecliptic. If it is ninety degrees, then the periapsis is as far away from the ecliptic is it could be given the comet's eccentricity and orbital plane.