Star
Planet (locked spin)
Body-fixed reference (spin, ∝ M)
True direction to star (∝ ν)
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A tidally locked exoplanet is often described as having one face permanently frozen toward its star — true only for a perfectly circular orbit. This simulator puts the planet on an adjustable-eccentricity ellipse and separates two angles that a real, physically locked world cannot keep in sync: the body's spin angle, which advances at the constant mean-motion rate a tidal torque actually locks onto, and the true direction to the star, which speeds up at periapsis and slows down at apoapsis under Kepler's second law. The gap between them, ψ = ν − M, is the planet's forced libration in longitude — the substellar point genuinely rocks back and forth across the surface once per orbit, with an amplitude that grows to roughly 2e radians as the orbit's eccentricity increases.