PARALLAX OF THE FIXED STARS. When Copernicus pro posed his hypothesis of the earth's motion, one striking astronomical objection was, that the enormous displacement of the spectator's place which his system supposed was not supported by a corresponding :bongo in the positions of the fixed stare. Every improvement in inetrutnents, in the art of observing, or in the science of computation, seemed to increase the distance of the sun, and consequently the orbit of the earth, and still no sidereal change could be detected. Hence, until the discoveries of Newton brought forward physical arguments, of which, however, the conclusiveness is not immediatel7 visible, the Copernican hypothesis was embraced on the grounds of its symmetry and simplicity rather than on demonstration. Opponents might alwaya ask for the c.rperintentum cruel:, the effects of the earth's motion in the apparent displacement of the fixed stars, and this could not be supplied.
Many attempts were, however, made. Ilooke erected a zenith sector at his chambers in Gresham College, and made some incomplete obser vations in which he fancied ho could trace the effects of parallax. Flamsteed found variations in the north polar distances of fixed stars, which he attributed to parallax, although, as was shown by Cassini and Roemer, this would have produced results with a totally different law. Roemer himself, after many efforts to deduce parallax from observa tions in declination, renounced the attempt, on account, as he says, of certain variation in the declinations of stars which can neither be attributed to refractions nor parallaxes," and pursued his investigation by observations in right ascension, in which it seems he believed him self to bo successful. At length Bradley-commenced the research with
far better astronomical means, and by his discoveries of aberration and notation, fully explained the phenomena which had perplexed his pre decessors. It is curious that Roemer, who had discovered the gradual transmission and finite velocity of light, 'should have been perplexed with the necessary results of this very property in another shape ; and that Bradley, while pursuing the problem of parallax, which had been originally interesting as a proof of the earth's change of place, should have hit upon phenomena which satisfactorily proved the earth's motion, and so confirmed the Copernican hypothesis by evidence different from what ho sought. Bradley's observations further showed that the effect of parallax in any of the stars observed by him could not amount to 2" and probably was not 1".
The nature of the changes in the places in the fixed stars, which the change of position in the earth would produce, may be seen from the following figure. Let s be the place of the sun, E that of the earth, and a that of a fixed star. Then the star is seen from the earth in the direction and from the sun in the direction a s; the difference in these two directions is the angle E as, that is, the angle of parallax. If s 8', as', be drawn parallel to