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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

Fig. 50.—Stellar parallax.

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.^[1] But more accurate observations of the fixed stars might be expected to throw further light on this problem.

1. It may be noticed that the differential method of parallax (chapter vi., § 129), by which such a quantity as 12' could have been noticed, was put out of court by the general supposition, shared by Coppernicus, that the stars were all at the same distance from us.