A frame of reference in which any body initially at rest will remain at rest indefinitely, or in which a moving body moves in a straight line with constant speed indefinitely; in other words, it is free from any inertial forces. It can be defined as a frame of reference in which Newton’s laws of motion apply exactly. The frame of reference fixed with respect to the Earth’s surface is not inertial, because there are forces due to the Earth’s rotation. The concept of an inertial frame of reference is particularly useful in the special theory of relativity, in which two different inertial frames have a fixed relative velocity but no relative acceleration. Inertial frames do not exist in general relativity, however, since, although a freely falling frame will appear inertial locally, differential gravitational effects prevent it from being globally inertial. In astrometry, an inertial reference frame is a coordinate system defined by positions and proper motions of a set of stars or other distant objects. The ultimate inertial reference frame is that defined by the positions of extragalactic objects, which are so far away that they have negligible relative motions as seen from Earth.