Mean Anomaly

Mean anomaly (M) is a uniformly increasing angle from 0° at perigee to 360° over one orbit period. Unlike the true anomaly (actual geometric position), mean anomaly increases at a constant rate — it is the fraction of the orbital period elapsed since perigee, expressed as an angle. This mathematical convenience makes orbit computation tractable. TLEs include mean anomaly at epoch, which SGP4 uses as the starting point for propagation. Related quantities: true anomaly (actual angle), eccentric anomaly (geometric intermediate).