Celestial mechanics predicts the motion of planets, Moons, stars and spacecraft from gravity and other relevant forces. Its simplest useful approximation is the two-body problem, whose Kepler orbits explain the connection between orbital shape, energy and angular momentum.
Johannes Kepler's orbital description and Isaac Newton's inverse-square law meet in this model. Additional bodies introduce orbital perturbations; general relativity changes the prediction when relativistic corrections matter. Comparing a predicted trajectory with observations tests both the inferred masses and the adopted dynamical model.[ref]
Introduction to Celestial Mechanics by Richard Fitzpatrick. Author's overview and online sections on Keplerian orbits, perturbations and perihelion precession
Celestial mechanics is a branch of astronomy and physics that deals with the motions and gravitational interactions of celestial bodies, such as planets, moons, asteroids, comets, and stars. It involves the application of classical mechanics, particularly Newton's laws of motion and the law of universal gravitation, to understand and predict the behavior of these bodies in space.
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