argument

The argument of periapsis of a satellite orbiting the Earth. Periapsis is the point at which the satellite is closest to Earth and the argument of periapsis is the angle formed between this point and the ascending node (point at which the satellite crosses the Equator from south to north).

In astronomy, angular distance of an orbiting spacecraft or other body from the object being orbited. For example, the argument of periapsis is the angle between the periapsis (closest point of the orbiting body to the object it orbits) and the ascending node (point where the orbit intersects the plane of the ecliptic, moving from south to north of the ecliptic). The argument of perihelion is the angle between the ascending node and the perihelion (closest point to the Sun of an orbiting body).