Argument of Periapsis

The semi-major axis and eccentricity determine the size and shape of an orbit relative to its orbital plane. But multiple orbits, even on the same plane, may differ in their rotation relative to some reference direction.

The angle of this rotation is referred to as the argument of periapsis, designated ω. The periapsis ( in the animation below) is the point in the orbit closest to the point being orbited around. It is always on the major axis. For planets orbiting the Sun, the periapsis is often referred to as the perihelion. A perfectly circular orbit (e = 0) technically would not have a specific periapsis as all points would be equally close.

Here is a series of exaggerated orbits, varying in each of the three elements e, a, and ω one at a time to give you a sense of the impact of changing each.