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A Short History of Astronomy
[Ch. IV.
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.[1] But more accurate observations of the fixed stars might be expected to throw further light on this problem.