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Hornsby, Thomas

Hornsby, Thomas

(b. Oxford, England, 28 August 1733; d. Oxford, 11 April 1810)


Hornsby is best remembered for his part in the foundation of the Radcliffe Observatory in Oxford. He made an accurate evaluation of the solar parallax.

The son of Thomas Hornsby of Durham, Hornsby matriculated at Corpus Christi College, Oxford, in December 1749. After taking his B.A. in 1753 and M.A. in 1757, he was elected a fellow of his college, where he built himself a small observatory. In 1763 he followed James Bradley as Savilian professor of astronomy at Oxford, where between 1766 and 1775 he gave a notable series of lectures on experimental philosophy. Their reputation is reported to have led James Watt’s partner, Matthew Boulton, to arrange for his son, who was not an undergraduate, to attend them. In 1763 Hornsby was made a fellow of the Royal Society.

At Corpus Christi, Hornsby observed with a fine mural quadrant with a radius of thirty-two inches; made by John Bird, it cost £80. As Savilian professor, he used the observatory in the tower of the Schools Quadrangle, from which he observed the transit of Venus on 3 June 1769, with twelve-foot and 7.5-foot refractors (Philosophical Transactions of the Royal Society, 59 [1769], 172–182). He was a friend of the earl of Macclesfield and had observed the transit of Venus of 6 June 1761 from Shirburn Castle, the earl’s home. From both he deduced a solar parallax of 8.78″ (ibid., 55 (1765), 326–344; 61 (1771), 574–576). (The fundamental constant adopted by the Conférence Internationale des Étolies Fondamentales at Paris in 1896, still accepted, is 8.80″.)

A printed document of 5 February 1771, signed by Hornsby as Savilian professor, recorded a petition made by him in 1768 to the earl of Litchfield and the Radcliffe trustees for the foundation of an observatory. (For a copy of S. P. Rigaud’s transcript of the document, the original of which R. T. Gunther thought to be lost, see Gunther’s Early Science in Oxford, II [Oxford, 1923], 88–89. For a copy of the original in the Bodleian Library, the shelf mark is Gough Oxf. 90) Hornsby asked for a transit instrument, two mural quadrants, a zenith sector, and an equatorial sector—to the tune of about £1, 300—to be made be the best instrument maker of the time, John Bird. He suggested that the professor of astronomy make regular observations, to be published annually, and that he give regular courses of lectures in practical astronomy.

The proposals were accepted, Hornsby was made first Radcliffe observer in 1772, and the buildings were completed by 1778. (For a plan, and list of rooms and instruments, see Gunther, op. cit., pp. 90–90, 318–324.) Hornsby had persuaded Bird to use achromatic object lenses in the telescopes on his sectors and quadrants, and the lenses were made by Peter Dollond (see Gunther, loc. cit., and p. 396). The outlay on buildings and instruments was £28,000—a considerable sum. Bird’s assessment of his own superbly well-divided eight-foot south mural quadrant as “by far the best instrument of the kind in the world” was undoubtedly then true. Like many other Radcliffe instruments, it is now in the Museum of the History of Science, Oxford.

Hornsby does not have any great astronomical discovery to his credit. He investigated the proper motion of Arcturus (Philosophical Transactions of the Royal Society, 63 [1773], 93–125); and in 1798 he pointed out that, despite the large proper motion of the double star Castor, the two components had remained at the same distance during the twenty years he had observed them. Even so, he did not suggest any physical connection between the components.

In 1783 Hornsby was made Radcliffe librarian; and in 1798—more than twenty years after having undertaken the project—he published the first volume of Bradley’s Astronomical Observations (see his preface).


I. Original Works. Apart from a few minor notes, such as those in connection with Oxford administration, Hornsby wrote only the five papers cited in text. He published nothing of any length, other than the Bradley ed. cited. The best collection of Hornsby manuscripts is in the Museum of the History of Science, Oxford (MSS Radcliffe 1–35, 54, 67, 71–73). For Hornsby’s notes for lectures on natural philosophy, in his own hand, see Bodleian Library MS Rigaud 54. The same library has a copy of Bradley’s (?) Propositiones mechanicae, with notes taken at Hornsby’s lectures, and also a syllabus of those lectures (shelf mark Vet. A 1 c.6 [51]), from about 1770.

II. Ssecondary Literature. R. T. Gunther, Early Science in Oxford, II (Oxford, 1923), is the most useful source. See also the pref. to S. P. Rigaud, Miscellaneous Works and Correspondence of the Rev. James Bradley (Oxford, 1832) on the question of Hornsby’s dilatory editing of Bradley. Many further but often outdated references to secondary literature are in Agnes Clerke’s biography of Hornsby in Dictionary of National Biography.

J. D. North

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