True Anomaly

True anomaly (ν or θ) is the actual angle between the direction of perigee and the satellite's current position, measured at the focus of the ellipse (Earth's centre). It varies non-uniformly: the satellite sweeps through angles faster near perigee and slower near apogee, following Kepler's second law of equal areas. True anomaly is the most physically meaningful position parameter but is harder to compute directly than mean anomaly, which is why TLEs use mean anomaly and the SGP4 propagator converts internally.