In celestial mechanics, the Hill sphere is a common model for the calculation of a gravitational sphere of influence. It is the most commonly used model to calculate the spatial extent of gravitational influence of an astronomical body in which it dominates over the gravitational influence of other bodies, particularly a primary. It is sometimes confused with other models of gravitational influence, such as the Laplace sphere or the Roche sphere, the latter of which causes confusion with the Roche limit. It was defined by the American astronomer George William Hill, based on the work of the French astronomer Édouard Roche.