Argument of perihelion

The argument of perihelion is one of three orbital elements that makes up the orientation of an elliptical orbit. The other two orbital elements are the inclination, and the ascending node. In an elliptical orbit with eccentricity, e and semi-major axis, a — the perihelion is the point of closest approach between the orbiting body (e.g. a planet) and the focus (e.g. the Sun lies at one focus of a planetary orbit, the other focus is empty).