92. One point which was of importance in later controversies deserves special mention here. The basis of the Coppernican system was that a motion of the earth carrying the observer with it produced an apparent motion of other bodies. The apparent motions of the sun and planets were thus shewn to be in great part explicable as the result of the motion of the earth round the sun. Similar reasoning ought apparently to lead to the conclusion that the fixed stars would also appear to have an annual motion. There would, in fact, be a displacement of the apparent position of a star due to the alteration of the earth’s position in its orbit, closely resembling the alteration in the apparent position of the moon due to the alteration of the observer’s position on the earth which had long been studied under the name of parallax (chapter II., [§ 43]). As such a displacement had never been observed, Coppernicus explained the apparent contradiction by supposing the fixed stars so far off that any motion due to this cause was too small to be noticed. If, for example, the earth moves in six months from E to E′, the change in direction of a star at S′ is the angle E′ S′ E, which is less than that of a nearer star at S; and by supposing the star S′ sufficiently remote, the angle E′ S′ E can be made as small as may be required. For instance, if the distance of the star were 300 times the distance E E′, i.e. 600 times as far from the earth as the sun is, the angle E S′ E′ would be less than 12′, a quantity which the instruments of the time were barely capable of detecting.[58] But more accurate observations of the fixed stars might be expected to throw further light on this problem.
Fig. 50.—Stellar parallax.
[CHAPTER V.]
THE RECEPTION OF THE COPPERNICAN THEORY AND THE PROGRESS OF OBSERVATION.
“Preposterous wits that cannot row at ease
On the smooth channel of our common seas;
And such are those, in my conceit at least,
Those clerks that think—think how absurd a jest!