precession

Precession is a systematic change in the orientation of a rotation axis. For example, the orbits of planets in our solar system precess. Each planet follows an elliptical path around the Sun, with the Sun at one of the focal points of the ellipse. The long axis of the ellipse slowly rotates in the plane of the orbit with the Sun as a pivot point, so the planet never follows exactly the same path through space as it continues to orbit in its elliptical path. The precession measured in Mercury's orbit was found to be different from the prediction of Newtonian gravity but matched the prediction of general relativity, providing some of the first concrete evidence that Einstein's version of gravity is correct.