In the study of orbital dynamics the mean anomaly is a measure of time, specific to the orbiting body p, which is a multiple of 2π radians at and only at periapsis. It is the fraction of the orbital period that has elapsed since the last passage at periapsis z, expressed as an angle. In the diagram below, it is M (the angle z-c-y).
In astrodynamics mean anomaly <math>M\,\!<math> can be calculated as follows:
<math>M – M_0=n(t-t_0)\,\!<math>
- <math>M_0\,\!<math> is the mean anomaly at time <math>t_0\,\!<math>,
- <math>t_0\,\!<math> is the start time,
- <math>t\,\!<math> is the time of interest,
- <math>n\,\!<math> is the mean motion.
<math>M=E – e \cdot \sin E\,\!<math>