340
HISTORY OF PHYSICAL ASTRONOMY.
proceeded to compare its variations throughout the year, with the cor-responding numbers assigned by his hypothesis. The result of thiscomparison was in the highest degree satisfactory. Of seventy declina-tions of y Draconis determined in the course of a year, one only differedmore than 2" from the hypothesis, and even in this case the observationwas marked doubtful on account of clouds. The comparison of the ob-served and computed declinations of n TJrsce Majoris exhibited an equallygratifying accordance *.
The phenomenon referred to in the foregoing account received fromBradley the name of Aberration. Its discovery is universally regarded asone of the most important accessions which astronomical science has everreceived. The sagacity which Bradley evinced as well in the establish-ment of its laws, as in the detection of its physical cause, would entitlehim to a place among the greatest philosophers of ancient or moderntimes, even if no other triumphs had adorned his career.
It has been mentioned that Bradley estimated the maximum ofaberration at 20".25. Upon further consideration he was induced to fixit at 20" f. The delicacy of modern researches on astronomy has ren-dered a closer approximation to the true value indispensable, even althoughthe corrections assigned by the various enquirers who have devoted theirattention to the subject, do not in any case exceed a few tenths of asecond. Delambre, by comparing together observations of a thousandeclipses of Jupiter ’s first satellite, deduced 20".255 as the most probablevalue of the coefficient of aberration. This result, as we have seen, agreesvery nearly with that originally obtained by Bradley. Various deter-minations of this element, have been assigned by modern astronomers,which generally fluctuate between 20".7080 and 20".2110. In 1844,Baily, having compared together all those that were known to him, de-duced 20".4192 as the most probable value. This result has received astrong confirmation from the recent researches of MM. Peters and Struve.M. Peters, by means of a great number of observations made at Pulkowa,with a vertical circle constructed by Ertel, has determined the maximumvalue of aberration to be 20".481. M. Struve makes it 20".445 by a dis-cussion of observations made also at Pulkowa, with a transit fixed in theprime vertical J.
* Bradley’s account of this brilliant discovery is contained in the volume of the Philoso-phical Transactions for 1729. It will be seen that his researches are wholly founded onobservations of the declinations of the stars. It does not appear that he attempted toverify his theory by means of right ascensions. At any rate he makes no especial allu-sion in his paper to the effect which the displacement would produce upon those co-ordinates, although the fact announced by him, that every star described an apparentellipse necessarily implied variations in right ascension, corresponding to those in decli-nation. It is evident that he deemed observations of right ascension not to be sufficientlytrustworthy for so delicate an investigation. In 1730 Manf'redi asserted that his obser-vations, although subject to occasional irregularities, exhibited a motion in right ascen-sion, analagous to that which Bradley had detected in declination. (Comm. Acad.Bonon, vol. i. p. 634.)
+ Phil. Trans. 1748, p. 23.
( Struve’s Etudes d’Astronomie Stellaire, p. 96. In the same work it is statedthat M. Peters obtained 20' .676 for the maximum value of aberration with a probableerror of l".ll, by a discussion of certain observations of Flamstead’s which are containedin a letter from that astronomer to Wallis, published in the third volume of the OperaMathematica of the latter. Flamstead was under the impression that the variations ofthe star observed by him ( Polaris ) were occasioned by parallax.