ABERRATION, in Astronomy, is a change in the position of the fixed stars, arising- from the progres sive motion ()I light, combined with the annual motion of the earth, by inuans of w hie h they sometimes ap pear twenty seconds distant from their true position.
This apparent motion of the heavenly bodies was de tected in 1725, by our celebrated countryman Dr Brad ley, and is one of the most brilliant discoveries which has enriched the science of astronomy.
During the seventeenth century, the supporters of the Copernican system laboured to prove the annual motion of the earth, by detecting a change in the posi tion of the fixed stars. They supposed, that if ASBD was the earth's orbit, A its position in December, B its position in June, and S any fixed star, the observer on the earth at A would see the star S, in the point 71 of the heavens; while the observer, when the earth came to B, would see it in the part ni, the star having appear ed to move through the arch at n, equal to the angle ASB, or the angle subtended at the star, by the diame ter of the earth's orbit. They saw, that when the star S was in the pole of the ecliptic, or equidistant from A and B, this angle was a maximum; for the angle AsB, subtended by AB, at any other star s, is evidently less than ASB, and therefore they selected a star nearest to the pole of the ecliptic, with the hope of detecting the earth's annual parallax, and thus putting to silence the abettors of the Ptolemaic and Tychonic systems.
With this view, Dr Hooke made a number of obser vations in 1669, on v Draconis; and he informs us in his ?ittempt to prove the Motion of the Earth from Ob servations, that he found this star 25" more northerly in July than in October. These observations, how ever, were not accurate, as afterwards appeared from those which were made by Dr Bradley upon the same star.
About the same time, M. Picard, when going to Uraniburg to determine the difference of longitude be tween it and Paris, had observed the motions of the pole star, and found a variation amounting to nearly 40" in a year. As this variation was in a direction opposite to that which should have been produced by the paral lax of the earth's orbit, Picard confessed himself unable to account for it; but insisted that the error in his ob servations at Uraniburg could not amount to 10". Col lect. Observat. Paris, 1691.
In 1680, Mr Flamstead made a number of observa tions on the pole star. He found that its declination was 40" less in July than in December ; and falling into the same error as Dr Hooke, he endeavoured to show that this variation was the effect of the annual parallax.
Cassini and Manfredi maintained, that the motion of the earth could not account for the observed variation ; but none of them were able to give an adequate explana tion of the annual changes which every astronomer ob served in the position of the stars.
In order to confirm the observations of Dr. Hooke, Mr. Samuel Molyneux erected an instrument, about the end of November, 1725, constructed by the cele brated Mr Graham. He made his first observation on Draconis on the 3d December, "i): ntcd it on the 10th, 11th, and 12th of the same month ; but this was done rather v, uli Uue Intelit1011 1,1 U 3 ing the instru ment, than detecting any parallax in the stars, as no sensible alteration of the parallax could have been ex pected at that season of the year. Dr Bradley was at this time on a 1. ISit to Mr ;Moly neux at KA., (Brad ley's Letter to Dailey, Phil. Trans. 1726,) and being anxious to try the instl he ()List rvsm Dra conis on the 17th December, and thund that it was mere southerly than it had been vi hull Mr Molyncux observed it about the beginning of the month. This t..ey naturally ascribed to the inaccuracy of their observations; hut upon, repeating them on the 20th of December, they found that the star was still continuing to move southward. The direction of this motion, howel er, being opposite to that w hich would have been produced by parallax, they w ere more per plexed than before ; and, as they were satisfied of the accuracy or their observations, they began to suspect the correctness of the instrument. But being convinced, from ninny trials, that their suspicions were groundless, they continued to observe the star, and found that on the beginning of March, 1726, it had mot eel 20" south since the commencement of their observations ;--that it remained stationary till the middle of April, when its motion was northerly ;--that it continued to move northward till the month of September, when it again became stationary, having advanced 20" farther north than it was in June ; and that it again resumed its mo tion towards the south. The law of the variation in the motion of the fixed stars being thus determined, Dr Brad ley became very anxious to discover its cause. Iu his first speculations on this subject, he ascribed these changes to the nutation of the earth's axis ; but this hypothesis was soon abandoned ; for he afterw ands saw, that stars which, from the equality of their polar distances, ought to have had the same nutation, sustained very different changes of declination.