Astrodynamics combines Newtonian physics of celestial mechanics, orbital mechanics, and ballistics into a science which can be applied to satellites, rockets, and spacecraft.
required for a body to complete one orbit.
In it's simplest form mean motion can be expressed as `"mean_motion" = n = "rev"/ P`,where P is the period and rev is a representation of the time to complete one orbit.
The Orbit time can be expressed in units of `"revolutions"/"time"`, `"radians"/"time"`, `"degrees"/"time"`
In the two-line element sets used by the both the government sector and commercial industry to specify a satellite's orbit, mean motion is captured in units of `"revolutions"/"day"`
Mean Motion can be expressed as a function of orbital period as:
`n = "1 rev" / P` = `"360 deg" / P` = `(2pi " radians")/ P`
`P = 1/n`, where `n` is the mean motion.
Kepler's 3rd law of planetary motion states, the square of the periodic time is proportional to the cube of the mean distance, or
where a is the semi-major axis or mean distance, P is the orbital period as above, and ? is a constant for any particular gravitational system.