 The term “reference frame” was coined in the 19th century, but it has a long prehistory, beginning, perhaps, with the emergence of the Copernican theory. The significant point was not the replacement of the earth by the sun as the center of all motion in the universe, but the recognition of both the earth and the sun as merely possible points of view from which the motions of the celestial bodies may be described. This implied that the basic task of Ptolemaic astronomy—to represent the planetary motions by combinations of circular motions—could take any point to be fixed, without sacrificing predictive power. Therefore, as Copernicus suggested in the opening arguments of On the Revolutions of the Heavenly Spheres, the choice of any particular point required some justification on grounds other than mere successful astronomical prediction. The most persuasive grounds, seemingly, were physical: we don’t perceive the physical effects that we would expect the earth’s motion to produce. Copernicus himself noted, however, in reply, that we can indeed undergo motions that are physically imperceptible, as on a smoothly moving ship (1543, p.6). At least in some circumstances, we can easily treat our moving point of view as if it were at rest. 