The field was founded on Newton's laws of motion and the law of universal gravitation, which describe how bodies attract each other based on their mass and distance.

: These three laws describe planetary motion, stating that orbits are elliptical, sweep out equal areas in equal time, and have periods related to their semi-major axes.

In reality, orbits are rarely perfect ellipses due to perturbing forces that constantly alter a body's path. Celestial mechanics - Scholarpedia