The Discovery of the Aberration of Light

BRADLEY, James.

A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S. to Dr. Edmond Halley Astronom. Reg. &c. Giving an Account of a New Discovered Motion of the Fix'd Stars. In: Philosophical Transactions of the Royal Society of London for 1727-28, vol. 35, no. 406, pp. 637-661.

London: W. Innys, 1729.

1st Edition. Hardcover. Very Good. Item #003426

4to (215 x 168 mm). Entire volume offered: [6], 293-661 [1] pp., including general title, drop titles for individual numbers, content leaves outside pagination, and 14 engraved folding plates. Contemporary panelled calf (spine rebacked, extremities worn, corners heavily scuffed. Text and plates somewhat browned, dust-soiled (some pages stronger), occasional spotting, the general title creased, some creasing, dog-earing elsewhere, 2 leaves after title detached. Provenance: John Waterhouse Halifax, Halifax Literary & Philosophical Society (bookplates to front pastedown). ----

Sparrow 28; Evans 21. FIRST EDITION of the letter by James Bradley about the discovery of the aberration of light. "The explanation of the phenomenon placed Bradly among the great astronomers of the 18th century" (Evans). Bradly is best known for two fundamental discoveries in astronomy, the aberration of light, and the nutation of the Earth's axis. Delambre says: "It is to these two discoveries by Bradley that we owe the exactness of modern astronomy. ... This double service assures to their discoverer the most distinguished place (after Hipparchus and Kepler) above the greatest astronomers of all ages and all countries." (J.B.J. Delambre Histoire de l'astronomie au dix-huitième siècle, 1827, p. 413).
Bradley worked with Samuel Molyneux until Molyneux's death in 1728, trying to measure the parallax of Gamma Draconis. "If the Copernican theory was correct it ought to be possible to observe an annual parallax of the stars. In fact within the last three decades of the seventeenth century a number of striking observations had revived the interest in parallaxes. At the time of [Bradley's] voyage to Uraniborg (begun in 1671) Jean Picard noticed annual variations in the position of the polar star extending to nearly 40", but - and this is very remarkable - after having studied them he concluded that they could not be explained either by refraction or by parallax. A few years later, in 1674, Robert Hooke made similar observations and lacking Picard's prudence and method, he thought they were parallactic effects. Flamsteed made many observations between 1689 and 1697, and explained them in the same way as Hooke. However in 1699, J. Cassini proved that the parallax would produce very different effects. A similar demonstration was given by E. Manfredi, but neither of them suggested the true explanation. Bradley's success was due not only to his excellent instrumental means, to his own perfect experimental technique, but as well to his thoroughness and persistence. In that he was almost the opposite of Hooke [. . .] who took part in almost every scientific controversy of his time but hardly ever succeeded in achieving anything of great importance because he did not carry his investigations deeply enough and never reached the bedrock of any problem. Bradley is one of the best examples of the 'classical' type of scientists as opposed to the 'romantic' type. His thoughts were deep rather than brilliant and they matured but very slowly; he was anxious to improve his observations to the limit of his experimental possibilities and he succeeded in doing so; moreover he was all the time trying to improve the instruments themselves and to detect and measure their errors. Being inhibited by an extraordinary fear of error he published very little. [. . .] With regard to the aberration, [. . .] Bradley did not simply discover it but that his determination of it was, considering his instrumental means, extremely accurate. He concluded that the maximum aberration was included between 40" or 41" [. . .]; the value of the constant of aberration accepted to-day is 20" 47 (that is 40" 94 for the whole axis). He deduced from this value the speed of light, and found that the sunlight would reach us in 8 m. 13 sec. (our present estimate is 8 m. 19 sec.)" (Bradley, James, Edmond Halley, and George Sarton. Discovery of the Aberration of Light. In: Isis 16, no. 2 (1931), pp. 2.

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