(a) The planet–moon barycenter orbit around the star (b) The moon orbit around the barycenter. The reference frames and angles used to describe the Keplerian orbits of the planet–moon system barycenter around the parent star (panel a) and the Keplerian orbit of the moon around the planet-moon system barycenter (panel b). Plane p₁ (panel a) is the plane of the sky as seen by the observer on the positive z₁ -axis. Plane p₂ (panels a and b) is the barycenter’s orbital plane, and plane p₃ (panel b) is the orbital plane of the moon around the barycenter. Angle i is the orbitalinclination angle, ω the argument of periastron, Ω the right ascension of the ascending node, and ν the true anomaly. Subscript bs refers to the barycenter of the planet–moon system around the star, and mb to the moon around the barycenter. Vectors r_bs and r_mb are the position vectors of the barycenter and the moon, respectively. The barycenter and the moon are indicated by b and m, respectively.