Carr House, half a mile south of the church of St. Michael, in which he officiated, is pointed out as his lodging-place. Here, in the course of his studies, he became convinced that a transit of Venus across the sun, overlooked by Kepler, but predicted in a blundering fashion by Lansberg, would actually occur in the afternoon of 24 Nov. (O.S.) 1639. He announced the approaching phenomenon (one never previously recorded) to Crabtree, and prepared to observe it by throwing upon a screen in a darkened room the image of the sun formed by his little telescope. But 24 Nov. fell on a Sunday, and only the intervals between services were available for watching the heavens. They had just concluded, when at 3 h. 15 m. P.M. he saw with rapture in a perfectly clear sky the disc of Venus already entered upon the sun, over which he followed its advance until sunset at 3 h. 50 m. He and Crabtree were the sole observers of this unprecedented spectacle. His younger brother, Jonas Horrocks, whom he had warned at Liverpool of its advent, was hindered by clouds from seeing anything of it. Among the results secured by Horrocks's rough measurements were corrections to the orbital elements and apparent diameter of Venus.
Horrocks resigned his curacy, probably owing to ill-health, and returned to Toxteth in July 1640. His letters thence to Crabtree contain unexplained allusions to the precarious state of his affairs which obliged him to intermit astronomical occupations. He began, however, a continuous series of tidal observations (the first of the kind undertaken), hoping to derive from them proof of the earth's rotation, and finished, after re-writing it several times, his treatise ‘Venus in Sole visa.’ Desiring to confer with Crabtree about a publisher, he planned a visit to Broughton, fixed, by his last letter of 19 Dec. 1640, for 4 Jan. ‘if nothing unforeseen should occur.’ But on the morning of 3 Jan. 1641 he suddenly died at the age of not more than twenty-three years. He was buried without any monument in the ancient chapel at Toxteth Park; but in 1826 a commemorative tablet was set up in the adjacent church of St. Michael-in-the-Hamlet by Mr. Holden of Preston. A memorial chapel and window were in 1859 added to the church in which he had ministered at Hoole, and in 1875 a marble scroll, bearing an inscription composed by Dean Stanley, headed with his own words regarding his clerical duties on the day of the transit, ‘Ad majora avocatus quæ ob hæc parerga negligi non decuit,’ was placed in his honour in Westminster Abbey.
Horrocks's name barely escaped total oblivion. Evil fortune pursued his literary remains. Some were plundered and burnt by a party of soldiers during the civil war; others, taken to Ireland by Jonas Horrocks, were lost there after his death; a further portion, employed in the compilation of Shakerley's ‘British Tables,’ perished in the great fire of London. Only those rescued by Crabtree, and bought after his death by Dr. John Worthington, came eventually to light. Among these was the ‘Venus in Sole visa,’ of which a copy was in 1661 transmitted by Huygens to Hevelius, who published it as an appendix to his own ‘Mercurius in Sole’ at Danzig in 1662. The attention of the Royal Society being thus directed to Horrocks's writings, the papers in Dr. Worthington's possession were procured and entrusted for publication to Dr. Wallis [q. v.] After long delay through want of funds, a quarto volume, entitled ‘Jeremiæ Horroccii Angli Opera Posthuma,’ appeared at London in 1672. Some copies are dated 1673, and a reissue was attempted in 1678, with fresh matter added, ‘to revive the sale,’ at no time brisk. The book includes ‘Astronomia Kepleriana defensa et promota,’ digested by Wallis from various fragments of unfinished treatises, with ‘Prolegomena,’ giving Horrocks's autobiographical history of his studies, extracts from his letters to Crabtree translated into Latin, a catalogue of his observations, and his ‘Theory of the Moon,’ with Flamsteed's numbers added. The ‘Venus’ was omitted, as Flamsteed was understood to be preparing an edition of it (never printed) from a manuscript now in the library of the Royal Observatory at Greenwich. The papers used by Wallis are kept in the Bodleian Library at Oxford.
Horrocks had extraordinary intuitions of truth. He first ascribed to the moon an elliptic orbit of which the earth occupied one focus, adding a variation of the eccentricity, and a revolution of the line of apsides. Newton showed all these circumstances to result from gravity, and acknowledged in the ‘Principia’ (3rd edition, p. 461) his obligations to his young predecessor. The earliest hint of perturbative influences was, moreover, contained in Horrocks's explanation of the progression of the lunar apsides as due to the disturbing influence of the sun (Opera Posthuma, p. 311). Profound meditations on the physical cause of the planetary movements convinced him that they are compounded of a tangential impulse (supposed to depend upon the sun's rotation) and a central pull, and he illustrated his idea with the experiment of the ‘circular pendulum’ described in a letter to Crabtree of 25 July 1638 (ib. p. 312). It is even probable that he went
