"During medieval times, men accepted Ptolemy's view that the earth was the natural center of the universe. ...[A]dapting earth as a universal ' (standard to which all motions are referred) was justified... Once the Ptolemaic point of view was abandoned, the choice... was reopened. Copernicus substituted the sun... as the "natural" frame of reference, and his choice was indeed excellent for describing the motions within the solar system. Today, however, it is understood that the sun is but one of millions of fixed stars in the galaxy... one of innumerable galaxies... Newton was well aware of the profundities involved in the choice of a proper frame of reference. All of his fundamental laws of mechanics involved statements concerning accelerations, changes to velocities... rather than the velocities themselves. The accelerations were tied to distances between the bodies... The choice of the frame of reference had no effect on the determination of distances... but the accelerations which resulted from the mutual attractions and repulsions of bodies were to be reckoned in relation to a universal norm... intimately bound up with the choice of a frame of reference. ...[T]here was no such thing as absolute rest, or absolute motion, for that matter, but only absolute acceleration... governed by the forces resulting from the proximity of other bodies."