The following celestial mechanics terms are commonly used in deriving the free flight of ballistic missiles.

Anomaly

An angle; for example, eccentrica nomaly, mean a nomaly, true a nomaly.

• Eccentric Anomaly An angle at the center of an ellipse between the line of apsides and the radius of the auxiliary circle through a point having the same apsidal distance as a given point on the ellipse.

• Mean Anomaly The angle through which an object would move at the uniform average angular speed n measured from the principal focus. Commonly, the angle n(t ? t ^o) is called the mean anomaly, where n is the mean motion.

• True Anomaly The angle at the focus between the line of apsides and the radius vector measured in the direction of orbital motion; the angle measured in the direction in which the orbit is described, starting from perihelion.

Aphelion

The point on an elliptical orbit about the sun that is farthest from the sun.

Apoapsis

The point farthest from the principal focus of an orbit in a central force field.

Apogee

The highest point on an Earth-centered elliptical orbit. The point of intersection of the trajectory and its semimajor axis that lies farthest from the principal focus.

Apsides (or Line of Apsides)

In an elliptical orbit, the major axis.

Apsis

The point on a conic where the radius vector is a maximum or a minimum.

Celestial Equator