The place of the telescope was to be exactly marked, that by replacing it the observations could be repeated at the necessary times for obtaining the desired information. The deviation of the star, if any, would thus evidently be observed, and if conformable to parallax, would establish the motion of the earth.
This method is perfect in theory, but would, had it been attempted, have been found attended with great practical difficulties, and, had they been overcome, it is obvious they would have been followed by a complete disappoint ment as to the object intended. It is clear Galileo had no knowledge whatever of the variation of refraction, from the variable weight and temperature of the atmosphere, or of the irregularities of refraction near the horizon, both of which causes would have completely defeated his inten tions. Circumstances like these are of great importance in the history of science. They afford us certain informa tion of the true state of its advancement. The discoverer of the uses of the telescope in astronomy, besides his theo retical, must have had great practical knowledge, and yet was ignorant of what now may be observed by the most inferior instruments. The other method suggested by Galileo, of observing at different seasons the angular dis tance between two fixed stars very near together, is among the most promising ; and it may even now be a matter of surprise that it has not hitherto succeeded. There will be an opportunity of mentioning this again.
It is well know n, that the astronomers of the times that followed the revival of learning indulged themselves pretty freely in forming hypotheses. One of these, adopt ed by and half approved by Galileo, tended to make them Iess anxious to deduce by actual observation the parallax of the fixed stars ; as, according to this hypothesis, it must be much less than their instruments could discern. They concluded, that as the fixed stars completed the inagnus annus in 25,000 years, the fixed stars were necessarily 25,000 times more remote than the sun from the earth. This would have made the parallax only about eight se conds.
Here is a tolerable specimen of an hypothesis. The distances of the fixed stars have just about as much con nection with the precession of the equinoxes as the mo tions of the planets have with the fortunes of !Len.
The first attempt of the discovery of the parallax of the fixed stars that seemed likely to be successful, was that of Dr. Hooke. The apparatus he constructed principally consisted of a telescope 36 feet long, which was erected at Gresham college in 1669. The result of his observa tions on y Draconis, which star was within 2' of his zenith, gave him a parallax exceeding 20". The powerful instru ment used by Hooke, and his known mechanical skill, seemed to have obtained for his result perfect confidence for many years. Flamstead refers to Hooke's observations for a confirmation of his own discoveries as to parallax made above twenty years after. And nearly sixty years after, the motive assigned by Bradley, for instituting those observations by which the aberration of light was dis covered, were the hopes of verifying and confirming those that Hooke had communicated to the public. We now know that Dr. Hooke's observations must have been quite incorrect, as the parallax of y Draconis certainly does not amount to half a second. His failure probably was occa sioned by his telescope not having been firmly fixed; but it is inconceivable how so acute and experienced a mecha nic could have been so much deceived.
Flamstead, who supposed he had ascertained the pa rallax of the Pole Star, by a continued series of observa tions made at Greenwich during seven years, also deceiv ed himself. But his error was of a very different nature from that of Hooke. The motion that Flamstead had ob served in the Pole Star, was not at all conformable to that which would have resulted from parallax. He found that the Pole Star appeared about 20" nearer to the pole in December, and about 20" farther from the pole in July, than in April and October. This it is easily understood cannot be the effect of parallax, because the effect of pa rallax is always towards the sun. Consequently, as the Pole Star is between the pole and sun in the beginning of April, the star must then appear most remote from the pole, and the contrary six months after. But the effects of the aberration of light afterwards discovered by Bradley, are conformable to the observations of Flamstead. Each star aberrates towards the point of the ecliptic, towards which the earth is moving ; that is, towards the point 90° behind the sun.