Illustration of the angular orbital elements for a Keplerian orbit. The orbital inclination, i, represents the inclination of the orbital plane with respect to a reference plane (e.g., the plane of sky for a distant observer), and the longitude of the ascending node, Ω, corresponds to the angle between a reference direction (in our case the ϕ = 0 line on the equatorial plane) and the line of nodes, namely, the line in which the orbital plane intersects the reference plane. The argument of the pericenter, ω, is the angle along the orbital plane between the line of nodes and the orbit’s pericenter.