Librational geometry, where θ and M are the true and mean anomalies of Phobos in its orbit, ψ is the tidal libration angle, γ is the physical libration angle, and F′ represents the empty focus of the ellipse. The angle between the moon-to-empty-focus and the apse lines is equal to M up to O(e²) (Murray & Dermott 1999). The eccentricity of this figure has been greatly exaggerated for visualization purposes.