The path described by a heavenly body in its revolution around a center of attraction. Since the attracting mass is also in motion, the orbit must necessarily be an ellipse. The position of the center of the attracting mass is the focus of the ellipse. The line from the focus to any point of the orbit is the radius vector. If the plane of the orbit intersects any other plane, the two points of intersection are the nodes. The nearest point to the center is the peri-center, or lower apsis (the smallest-distance); the most distant point, the apocenter, or higher apsis. As indicating the particular attracting center involved, the pericenter becomes perihelion (helio, the Sun) to a body revolving around the Sun; and perigee (geo, the Earth), around the Earth. Thus, according to Kepler's law that "the radius vector sweeps over equal areas (arcs) in equal times," as the body approaches the pericenter, its motion is accelerated; as it recedes, the motion is retarded. These points are collectively termed Apsides: the diameter running through the Line of Apses. Aphelion. The point at which any planet, including the Earth, is at its greatest distance from the Sun, the apo-center of its orbit. Perihelion. At the closest point to the Sun. Apogee. Said of the Moon, when at its greatest distance from the Earth. Perigee. At the closest point to the Earth.
The so-called six Elements of an orbit are: eccentricity; mean radius vector; inclination of its orbit plane to that of the Ecliptic; longitude of its ascending node; period of revolution; and time of passage across a given point, such as perihelion.
