The surface gravity, g, of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. Surface gravity may be understood as the acceleration due to gravity experienced by a hypothetical test particle located very close to the object's surface, which has negligible mass so as not to disturb the system. For objects where the surface lies deep within an atmosphere and the radius is not well defined, the surface gravity is given at the 1-bar pressure level in the atmosphere.