Hornsby became a Fellow of
Corpus Christi College, Oxford in 1760. He occupied the
Savilian Chair of Astronomy at
Oxford University from 1763. In 1782, he became
Sedleian Professor of Natural Philosophy. Hornsby was especially concerned with the observation of the
transit of Venus. In 1761, he observed the transit of Venus from
Shirburn Castle, in
Oxfordshire, the
seat of the
Earl of Macclesfield.
George Parker, 2nd Earl of Macclesfield (ca. 1695–1764), celebrated as an
astronomer, had spent most of his time conducting astronomical observations at Shirburn Castle; here he had built an
observatory and a chemical laboratory. On the 1st April 1764 Hornsby observed the partial phases of an annular
solar eclipse. Despite the international efforts made to observe the 1761 transit, poor weather conditions hampered observations. In 1766 Hornsby informed the Royal Society that preparations needed to begin for the 1769 transit, his publication in the
Philosophical Transactions of the Royal Society focusing attention on the "cone of visibility" indicating some of the better places to observe the transit. Hornsby himself viewed the 1769 transit at the
Tower of the Five Orders, the entrance to the
Bodleian Library in Oxford. In the periodical
Philosophical Transactions, Hornsby published a comparative analysis of 1761 transit (1763); a plan for suitable viewing stations for 1769, including possible locations in the
Pacific (1765); a description of organising and reporting observing groups in Oxford (1769); and a comparative analysis of the 1769 transit (1771). Using the solar parallax values obtained from the 1769 transit, he wrote in
Philosophical Transitions December 1771 that "the mean distance from the Earth to the Sun (is) 93,726,900 English miles." The
radar-based value used today for the astronomical unit is . This is only a difference of eight-tenths of one percent. The work was within the bounds of the
aphelion and
perihelion distances to the sun, ~95 million miles and ~91 million miles respectively. These results have been described as "absolutely remarkable" considering what the astronomers had to work with. Hornsby was instrumental in the establishment of the
Radcliffe Observatory at Oxford in 1772, and was made Radcliffe Observer in the same year. In 1782, he was appointed
Sedleian Professor of Natural Philosophy. In 1783, he became
Radcliffe Librarian. He was elected a Foreign Honorary Member of the
American Academy of Arts and Sciences in 1788. Hornsby made tens of thousands of astronomical observations. These were not published, however, until 1932, and were donated to
Corpus Christi College, Oxford in 1935. They include a determination of the rate of change of the
axial inclination of the Earth and the
proper motion of
Arcturus, both close to the modern values, whose combination of visual magnitude and large proper motion led Hornsby to argue (incorrectly) that "We may, I think, fairly conclude that Arcturus is the nearest Star to our System visible in this Hemisphere". The crater
Hornsby on the
Moon is named after him.
N.B. His death announcement in the Kentish Gazette 24 April 1810 quotes his death as "Wednesday (11th Apr. 1810) at the Observatory Oxford, aged 76, the Rev. Thomas Hornsby D.D. & F.R.S. Savilian Professor of Astronomy, Professor of Natural Philosophy, & Librarian of the Radcliffe Library." The
Oxford Dictionary of National Biography gives this information: "Hornsby, Thomas (1733–1810), astronomer, the son of Thomas Hornsby (bap. 1704, d. 1771), an apothecary and later alderman, and his wife, Thomasine Forster, née Coulson (bap. 1705, d. 1775), was baptized in the parish of St Nicholas, Durham, on 27 August 1733. Hornsby died in Oxford on 11 April 1810, and was buried at St Giles' there on 19 April." ==Sources==