 As the basic programme of Ptolemy and Copernicus gave way to that of early classical mechanics as developed by Galileo, this equivalence of points of view was made more precise and explicit. Galileo was unable to present a decisive argument for the motion of the earth around the sun. He demonstrated, however, that the Copernican view does not contradict our experience of a seemingly stable earth. He accomplished this through a principle that, in the precise form that it takes in Newtonian mechanics, has become known as the “principle of Galilean relativity”: mechanical experiments will have the same results in a system in uniform motion that they have in a system at rest. Arguments against the motion of the earth had typically appealed to experimental evidence—e.g., that a stone dropped from a tower falls to the base of the tower, instead of being left behind as the earth rotates during its fall. But Galileo argued persuasively that such experiments would happen just as they do whether the earth were moving or not, provided that the motion is sufficiently uniform. (See Figure 1.) Galileo’s account of this was not precisely the principle that we call “Galilean relativity”; he seems to have thought that a system in uniform circular motion, such as a frame at rest on the rotating earth, would be indistinguishable from a frame truly at rest. The principle was named in his honor because he had grasped the underlying idea of dynamical equivalence: he understood the composition of motion, and understood how individual motions of bodies within a system—such as the fall of a stone from a tower—are composed with the motion of the system as a whole. This principle of composition, combined with the idea that bodies maintain their uniform motion, formed the basis for the idea of dynamically indistinguishable frames of reference. 