Thomas Hornsby FRS (1733 in Durham – 11 April 1810 in Oxford) was a British astronomer and mathematician.
In 1761, he observed the transit of Venus from Shirburn Castle, in Oxfordshire, the seat of the Earl of Macclesfield.
On the 1st April 1764 Hornsby observed the partial phases of an annular solar eclipse.
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).
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".