The area swept since perihelion, | z s p | = b a ⋅ ⋅ | z s x | = b a ⋅ ⋅ | z c y | = b a ⋅ ⋅ a 2 M 2 = a b M 2 , is by Kepler's second law proportional to time since perihelion. So the mean anomaly, M, is proportional to time since perihelion, t. M = n t , where n is the mean motion.