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HENDERSON, THOMAS (1798–1844), astronomer, born at Dundee in Scotland on 28 Dec. 1798, was the youngest of five children of a respectable tradesman, who died early. He was educated at the local schools, and learnt mathematics from Mr. Duncan, principal of the Dundee Academy, who described him as ‘remarkable for everything that was good.’ At the age of fifteen he entered the office of Mr. Small, a writer in Dundee, with whom his brother was in partnership. He was employed partly in classifying the burgh records, and after six years placed himself under a writer to the signet in Edinburgh. His business capabilities there attracted the notice of Sir James Gibson Craig [q. v.], through whose influence he was appointed advocate's clerk to John Clerk, lord Eldin [q. v.], and he acted from 1819 to 1831 as secretary to the Earl of Lauderdale and Lord Jeffrey.

Henderson was of a weak constitution, and at times nearly blind, but seemed to acquire scientific knowledge by intuition (Grant, University of Edinburgh, ii. 362). His familiarity with astronomical methods, acquired during his leisure at Dundee, introduced him to Professors Leslie and Wallace, and to Captain Basil Hall [q. v.] He joined the Astronomical Institution of Edinburgh, and was allowed the use of the instruments in their observatory on the Calton Hill. He showed special dexterity in the computing processes of practical astronomy, and he forwarded to Dr. Thomas Young [q. v.] in 1824 an amended method of calculating occultations, inserted in the ‘Nautical Almanac’ for 1827 and four subsequent years. He received the thanks of the board of longitude for this improvement, which was published in the ‘Quarterly Journal of Science’ (xviii. 344, 1825), and was followed by similar communications.

In a paper ‘On the Difference of Meridians of the Royal Observatories of Greenwich and Paris’ (Phil. Trans. cxvii. 286), sent by him to the Royal Society of London in 1827, he greatly added to the value of Sir John Herschel's result by rectifying an error in the data furnished to him; and his discussion of transit observations made on the Calton Hill in 1827 (Memoirs Royal Astr. Soc. iv. 189) showed his early adoption of the German method of deducing the probable errors of results. The thanks of the Royal Astronomical Society were voted to him in 1830 for various computations, including a list of moon-culminating stars for Sir James Ross's Arctic expedition. He declined all remuneration, although much of his small income was at this time devoted to the support of his mother and sisters.

Henderson's connection with the Earl of Lauderdale involved an annual visit to London, where he made many astronomical acquaintances, and was allowed to use Sir James South's fine instruments. He failed to succeed Dr. Robert Blair (d. 1828) [q. v.] as professor of practical astronomy at Edinburgh in December 1828, although Dr. Young had supported his claims, besides leaving a posthumous recommendation of him as his successor in the superintendence of the ‘Nautical Almanac.’ Pond was nominated; and Henderson, though invited to co-operate on advantageous terms, chose to continue his legal career. On the death of Fearon Fallows [q. v.] in 1831, he was persuaded to become royal astronomer at the Cape of Good Hope, where he arrived in April 1832. The instruments at his disposal were a ten-foot transit by Dollond, and a defective mural circle by Jones (ib[. viii. 141). With Lieutenant Meadows as his sole assistant he made five or six thousand observations of declination to ascertain the places of southern stars, observed Encke's and Biela's comets (Phil. Trans. cxxiii. 549; Memoirs Royal Astr. Soc. vi. 159), the transit of Mercury of 5 May 1832 (ib. p. 195), occultations of stars, and eclipses of Jupiter's satellites, besides making special series of observations for parallax on Mars and the moon. He was nevertheless suffering from incipient heart disease, was depressed by many difficulties, and resigned his post in May 1833.

On his return he settled at Edinburgh, and devoted himself to the arduous task of reducing his Cape observations. His sole maintenance was a pension of 100l. a year, to which he had become entitled on the resignation of Lord Eldin. A discussion of the observations of Mars, made during the opposition of November 1832, at Greenwich, the Cape, Cambridge, and Altona, gave him for the solar parallax the improved value of 9″.028 (ib. viii. 103); and he deduced, from simultaneous observations at the three first-named observatories, a lunar parallax of 57° 1′ 8″ (Monthly Notices, iv. 92). His reduction of Captain Foster's observations of the comet of March 1830, and a catalogue of the declinations of 172 southern stars, were communicated by him to the Royal Astronomical Society in June 1834 and April 1837 respectively (Memoirs, viii. 191, x. 49). The right ascensions of the same stars were published later (ib. xv. 129). His most striking result was the discovery of the first authentic case of annual parallax in a fixed star, the brilliant double star α Centauri. On 3 Jan. 1839, the discovery having been partially confirmed by Meadows's observations, he announced to the Royal Astronomical Society his conclusion of a parallax of about 1″ (lately diminished to 0″.75), implying a real distance of nearly twenty billions of miles (ib. xi. 61). Its ratification by Maclear's subsequent observations was communicated by him on 8 April 1842 (ib. xii. 329). Symptoms of orbital movement in the components of α Centauri were first adverted to by Henderson in 1839. A parallax of 0″.25 for Sirius (ib. xi. 239) and a mean parallax of 0″.29 for twenty southern stars (Monthly Notices, v. 223) were most likely illusory.

Henderson was elected a fellow of the Royal Astronomical Society in 1832, of the Royal Societies of Edinburgh and London respectively in 1834 and 1840. He became the first astronomer-royal for Scotland on 1 Oct. 1834, when he was appointed to the professorship of practical astronomy in the university of Edinburgh, combined with the charge of the Calton Hill observatory, then resigned to the university by the Astronomical Institution. (For Carlyle's curious application for the post see Froude, Thomas Carlyle, ii. 391.) His salary was 300l. a year. Although closely occupied with the Cape reductions, he made with his assistant during ten years upwards of sixty thousand observations, chiefly of planets and zodiacal stars, in themselves of high excellence, but vitiated (as was reported by the commission of 1876) by large errors, due to the expansiveness under heat of the sandstone piers of the transit instrument.

Henderson married in 1836 the eldest daughter of Alexander Adie, a well-known optician in Edinburgh; her death in 1842, shortly after the birth of their only child, was a shock from which he never fully recovered. He enjoyed, nevertheless, intensely a trip to the highlands with Bessel and Jacobi in the ensuing summer. He died at Edinburgh, of hypertrophy of the heart, on 23 Nov. 1844, having worked until a month before his death, when illness made it impossible for him to mount the Calton Hill. Five volumes of his Edinburgh observations were published by himself 1838–43, and five more 1843–52, under the editorship of his successor, Professor Piazzi Smyth. The mass of his Cape observations remains unpublished; their reduction wanted only a few months of completion when he died. His preface, too, to Lacaille's ‘Catalogue of Southern Stars,’ the reduction of which he had superintended for the British Association, had to be supplied by Sir John Herschel.

Henderson possessed considerable mathematical attainments, and unfailing discretion in the application of his powers. His memory was remarkable, and his acquaintance with modern astronomical history unusually extensive. He gave no lectures in his own official capacity, but read a course on mathematics for Professor Wallace in 1835–6, and one on natural philosophy for Professor Forbes in 1844. He computed the orbits of several comets, publishing his results in the ‘Astronomische Nachrichten.’ He was upright, benevolent, and enthusiastic; his disinterestedness left his orphan daughter with little provision, save the product of the sale of his fine library. Her uncle, Mr. John Adie, however, left her a fortune.

[Memoirs of Royal Astronomical Society, xv. 368; Proceedings Royal Society, v. 530; Proceedings Royal Society of Edinburgh, ii. 35 (Kelland); Philosophical Magazine, xxvii. 60, 3rd ser.; Ann. Reg. 1845, p. 226; Athenæum, 1845, p. 365; Sir A. Grant's Story of the University of Edinburgh, i. 381, ii. 362; Grant's Hist. Physical Astronomy, pp. 212, 228, 551; Clerke's Popular History of Astronomy, 2nd ed. p. 46; Encycl. Brit. 8th ed. i. 863 (Forbes); Mémoires couronnés par l'Acad. des Sciences, t. xxiii. p. 66, Brussels, 1873, 8vo; Chambers's Biog. Dict. of Eminent Scotsmen; André et Rayet's L'Astronomie Pratique, ii. 8.]

A. M. C.