Illustration of the four geometrical elements: the semi-major axis a_*, the longitude of the ascending node Ω (also known as receding node), the inclination i, and the argument of periapsis ω. The light cyan polygon represents the sky plane in a perspective view; the reference direction is taken towards the north (N). The line of nodes, shown by the black arrow pointing towards the ascending node, is the intersection between the sky and the orbit planes. The orbit is shown as the red ellipse. The small red arrow is perpendicular to the true plane and shows the inclination.