Thomas Henderson, astronomer, received his schooling in Dundee and was apprenticed to a solicitor in that city at the age of fifteen. He moved to Edinburgh in 1819, where he completed his legal training and was appointed advocate's clerk to John Clerk (Lord Eldin), and later secretary to the Earl of Lauderdale and Lord Jeffrey. Although his health was rather poor and he suffered periodically from an eye ailment, his spare time was devoted to astronomy. Encouraged by Captain Basil Hall* and others he used the small observatory at Carlton Hill, Edinburgh. His outstanding traits as an astronomer were a natural flair for computational work, a superb memory, and later an extensive knowledge of recent astronomical history. From 1824 he published a number of important papers. The first of these was "Improvement of Dr Young's rules for computing an observed occultation" (Quarterly Journal of Science, Literature and Arts, 1825). His method was applied in the Nautical Almanac for 1827 to 1831. Other papers dealt with the difference in longitude between the observatories at Greenwich and Paris (1827), and with transit observations at Carlton Hill (1827). As a result he was appointed His Majesty's Astronomer at the Royal Observatory, Cape of Good Hope, in October 1831, following the death of Reverend Fearon Fellows*.
Leaving Scotland with some reluctance Henderson assumed duty at the observatory in April 1832. During the next thirteen months, assisted by Lieutenant William Meadows*, he made a large number of observation of high quality, relating to a wide variety of phenomena. Early in 1833 he started a new time service, firing a pistol at a specific hour each night so that ships in Table Bay could regulate their chronometers. However, depressed by the practical problems associated with life and work at the observatory he resigned, leaving the Cape towards the end of May 1833. Upon his return to Scotland he settled in Edinburgh to reduce his Cape observations.
Henderson had made numerous observations of the two components of Alpha Centauri at the Cape, in order to establish their positions accurately, but also for the purpose of determining his instrumental errors. On learning that the doublet had recently been found to have a large proper motion (and might therefore be relatively close), he decided to analyse his observations with a view to determining whether it had a detectable annual parallactic motion, relative to the background of distant stars. In this he succeeded, estimating the parallax to be 1,16 seconds of arc, a value that later proved to be too high. This work represented the first authentic measurement of the distance of a star by trigonometric methods. However, by the time the results were made public in 1838, and published in the Memoires of the Royal Astronomical Society the next year, other successful parallax measurements had already been announced by F.W. Struve in Russia and W. Bessel in Germany. The parallax of Alpha Centauri was confirmed by a series of measurements made at the Cape by Thomas Maclear* in 1839-1840. Henderson analysed the observations, finding a parallax of 0,91 seconds of arc, and published the result in 1842.
He also made thousands of meridian observations, which were used to determine the accurate positions of the southern stars. His paper "On the declinations of the principal fixed stars, deduced from observations made at the Observatory, Cape of Good Hope, in the years 1832 and 1833" was read before the Royal Astronomical Society in April 1837 and published as one of its Memoirs. The work was completed seven years later with the publication of a complimentary Memoir, "On the right ascensions of the principal fixed stars..." (1844). On 5 May 1832 he observed the transit of Mercury. The next month he observed Enke's comet, reporting the results in the Philosophical Transactions of the Royal Society (1833). He also discussed the positions of both Encke's comet and Biela's comet in two papers in the Memoirs of the Royal Astronomical Society. Other observations led to papers on the parallax of Mars, the latitude and longitude of the Cape observatory, and the refraction of stars near the horizon. His observations of the moon were used in conjunction with observations made at Greenwich and Cambridge to calculate an improved value of the lunar horizontal parallax.
In October 1834 Henderson was appointed as the first Astronomer Royal of Scotland, stationed at Carlton Hill Observatory, and professor of astronomy at the University of Edinburgh. His observations at Edinburgh were published in five volumes between 1838 and 1843. In line with his interest in computational work he supervised the reduction of the stellar observations made at the Cape by Abbé N.L. De la Caille* in the mid-eighteenth century. The work was published after his death as A catalogue of 9766 stars in the southern hemisphere... from the observations of the Abbé de Lacaille, made at the Cape of Good Hope in the years 1751 and 1752. Reduced... under the immediate superintendence of the late Professor Henderson... (1847). The author of about 70 scientific papers, he was elected a Fellow of the Royal Astronomical Society in 1832, of the Royal Society of Edinburgh in 1834, and of the Royal Society of London in 1840. He died from heart disease a month before his 46th birthday. His successor as Astronomer Royal of Scotland was another Cape astronomer, Charles P. Smyth*.