Celestial mechanics is an old term for the application of physics, particularly Newtonian mechanics, to astronomical objects such as stars and planets.
Isaac Newton is credited with introducing the idea that the motion of objects in the heavens, such as planets, the Sun, and the Moon, and the motion of objects on the ground, like horses and falling apples, could be described by the same set of physical laws. In this sense he unified 'celestial' and 'terrestrial' dynamics.