The oblique ellipse represents the orbit of a companion A, for a given phase Φ₀ of the periastron. The grey-shaded ellipse is the projection of the light cylinder onto the orbital plane, and C is the point on the light cylinder projection at the same phase as A. The star F is both the focus of the orbit and the centre of the light cylinder projection. The polar angle of the periastron is φ₀, and the instantaneous polar angle of the companion A along its trajectory is φ.