Thomas Hornsby FRS (1733, London – 1810, London) was a British astronomer and mathematician.
Hornsby became a Fellow of Corpus Christi College, Oxford in 1760.
He occupied the Savilian Chair of Astronomy at Oxford University from 1763. In the same year, he became professor of experimental 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 possession 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.
In 1769, Hornsby viewed Venus' transit at the Tower of the Five Orders, where the Bodleian Library is situated.
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).
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.
Hornsby made tens of thousands of astronomical observations. These were not published, however, until 1932.
The crater Hornsby on the Moon is named after him.