Page:L. Silberstein - The Theory of Relativity.djvu/19

Rh to the coach on its particular journey. But, as a matter of fact, both of these reference-systems move relatively to one another, and the comparative insignificance of one of them would, by itself, be but a very feeble argument (as we shall see presently, from another example).

At any rate the earth, the 'firm ground,' allowance being made for occasional large shocks and for very small but incessant oscillations of every part of its surface, has proved to be an excellent system of reference for almost all motions, especially those on a small scale with regard to space and time, and practically without any reservation for all pieces of machinery and technical contrivance. In fact, the earth as a system of reference offered at once the advantage of a high degree of simplicity of description of states of equilibrium and motion, opening a wide field for the application of Newton's mechanics, at least as regards purely terrestrial observations and experiments. The earth is then a reference-system which is constantly used also by the most advanced modern student of mechanics.

But things become altogether different when we look up to the sky and desire to bring into our mechanical scheme also the motions of those luminous points, the celestial bodies, including, of course, our satellite, the moon, and our sun. Then the earth loses its privilege as a framework of reference. If it were only for the so-called 'fixed stars,' which form the enormous majority of those luminous points (and the moon too), we could still satisfy our vanity and continue to consider our globe as an universal mechanical system of reference, the system of reference, as it were. On our plane drawings, or in our three-dimensional models, we could then represent the earth by a fixed disc, or sphere, respectively, with a smaller sphere moving round it in a circular orbit, to imitate our moon, the whole surrounded by a large spherical shell of glass sown with millions of tiny stars, spinning gently and uniformly round the earth's axis,—very