Thomas Maclear, astronomer at the Royal Observatory, Cape of Good Hope, was destined to be educated as an Anglican cleric. His objections to this fate led to a permanent breach with his father. He was sent to Biggleswade, England, where he was apprenticed to his maternal uncle, the eminent surgeon Dr T. Magrath, in May 1808. In 1814 he went to London to study medicine at Guy's and St Thomas's Hospitals and the next year, after achieving great success in his examinations, was admitted as a Member of the Royal College of Surgeons of London (MRCS). After a number of years as house surgeon to the Bedford Infirmary he entered into a partnership with his uncle in 1823. Two years later he married Mary Pearse of Bedford and they eventually had four sons and seven daughters. She died in 1861.
Maclear's increasing interest in astronomy led him to study mathematics in his spare time and make observations with a small telescope. Encouraged by Captain (later Admiral) Smyth (father of Charles P. Smyth*) and others he became active in astronomical circles and met Sir John Herschel*, with whom he formed a life-long friendship. He built his own observatory at Biggleswade and equipped it with a small transit instrument and a telescope borrowed from the Royal Astronomical Society. His observations and calculations relating to occultations were published in seven papers between 1828 and 1833, some of them written in collaboration with Thomas Henderson*. The work was of such quality that he was elected a Fellow of the Royal Astronomical Society in 1828, a Fellow of the Royal Society of London in December 1831, and a member of the Academy of Sciences, Palermo, in 1835.
In 1833, when Henderson resigned as His Majesty's Astronomer at the Cape, the post was offered to Maclear and he accepted it in July that year, despite his uncle's opposition. After a difficult voyage of three months duration the family (then already including several daughters) arrived at the Cape on 7 January 1834. During the first four years of his work at the Royal Observatory Maclear enjoyed the friendship and collaboration of Herschel, with whom he was in regular contact by means of notes and letters carried by servants. Shortly after his arrival he suffered an attack of Bell's palsy (paralysis of one side of his face), a condition that had first been described only a few years earlier. It cleared up after about a month.
During his long term of office the observatory was expanded by the erection of several small buildings and, with the help and influence of Herschel, additional instruments. These included a 175 mm refractor in 1847. A time ball was installed at the observatory in 1836 and the time service extended in later years. After the departure of the observatory's unsatisfactory first assistant, Lieutenant William Meadows*, the post was filled in 1835 by the young Charles P. Smyth*, who proved a great success. Another successful appointment was that of William Mann* to the new post of second assistant in 1839. Though Maclear was a dimunitive person, his authority is illustrated by the fact that his assistants referred to him as "The Emperor".
Astronomical work at the observatory consisted largely of observations with a transit instrument and mural circle to determine accurately the positions of numerous southern stars. In 1854 a transit circle was installed which combined the functions of these two instruments. Maclear concentrated on accumulating observations, but his many and varied activities prevented him from reducing and publishing most of them. The first year's observations were published in Astronomical observations made at the Royal Observatory, Cape of Good Hope, in the year 1834 (Cape Town, 1840), to be followed by catalogues for 1835, 1836 and 1837 later, but the comprehensive catalogues of southern stars which he had been instructed to produce did not appear. After his retirement in 1870 he worked on the reduction of his observations. The resulting catalogues of southern stars and other publications were completed and published under the supervision of his successors, E.J. Stone* and D. Gill*.
Maclear did, however, publish other astronomical work, particularly positional determinations of many comets (17 papers between 1835 and 1866), Uranus and Neptune (1848), Mars at its opposition of 1849/50 (1850), and the moon (1864, 1866). He also determined the difference in longitute between the Cape and Madras (1840), repeated Henderson's pioneer determination of the parallax of Alpha Centauri (1840), and determined the parallax of Beta Centauri (1852). Other contributions by him of interest to astronomy included accounts in several journals of a fall of meteorites in the Cold Bokkeveld on 13 October 1838 (1839, 1840), some pendulum observations (1840), and observations of the tides.
One of Maclear's tasks was to re-measure and extend the arc of meridian measured some 80 years earlier by Abbe N.L. De la Caille* and at the same time determine the geographical positions of points for later use in mapping the colony. From 1837 to 1847 Maclear and his assistants spent a large proportion of their time on the field work of this endeavour. Despite intensive efforts to identify De la Caille's survey points (Maclear, 1838) he could not find the exact site of the northern end point of the earlier arc and therefore conducted a new survey. Six months were spent in accurately measuring a baseline some 13 km long in the Zwartland; Maclear estimated the probable error in its length to be about 8 mm - less than one part per million. The angular measurements were made with the best available theodolites and were often repeated 100 times or more. The arc stretched from the Royal Observatory in Cape Town to a point beyond the Kamiesberg in Bushmanland (a north-south distance of some 500 km), while the triangulation was extended eastwards from Cape Town to Cape Agulhas. The latitudes at the north and south points of the arc, and at several other stations, were accurately measured with an unwieldy zenith sector. The results were published in two volumes entitled Verification and extension of La Caille's arc of meridian at the Cape of Good Hope (London, 1866). It was Maclear's most important contribution to science. The results proved that De la Caille's measurements were accurate, but that his latitudes had been affected by the gravitational attraction of mountains, particularly at the northern point of his arc. The results also confirmed that there is no significant difference between the shape of the earth's southern and northern hemispheres. The survey was the first that could be used by later workers as a basis for mapping the region, and was incorporated as part of the geodetic network of the subcontinent established from 1880 onwards. Maclear was knighted in 1860 as a result mainly of his outstanding geodetic work at the Cape. After its publication he received the Lalande Medal of the Institute of France in 1867, and the Gold Medal of the Royal Society of London in 1869.
His responsibilities also included the supervision of regular meteorological observations at the observatory. The results for 1842 to 1856 were published in Cape Town in 1856, while those for 1842 to 1861 were published in the Proceedings of the British Meteorological Society in 1869. In later years the observations were supplied to the Cape of Good Hope Meteorological Committee (established in 1860), which published a summary in its annual reports. Maclear was a member of the committee to 1872. Articles by him on meteorology (1857, 1858, 1871), and more particularly on rain near Table Mountain (1858), the relation between daily maximum-minimum and mean temperatures (1858), how to observe temperatures (1859) and calculate the dew point (1859) appeared in the Cape Monthly Magazine.
Maclear played an active part in many other scientific and cultural activities. His interest in survey work extended to the origin and history of land measures at the Cape, of which he made a detailed study that was published in the Cape Monthly Magazine (1858, 1859). In June 1858 he was appointed by the governor as one of the commissioners (the others included M. Ruysch*, L. Marquard* and G.F. Childe*) who were to ascertain and fix the size of the unit of land measure used in the Colony - a local version of the Rhynland foot. The only standards available for this purpose were two measuring rods of six Rhynland feet each that had been used by L.M. Thibault* and were later bought by Ruysch. By comparing these rods to British standards the commission found that the land unit was equivalent to 1.033 British Imperial feet (314.85mm) and renamed it the Cape foot. It remained in use as the South African unit of land measure until replaced by the metre in the nineteen-sixties. Another of Maclear's contributions to the Cape Monthly Magazine dealt with local weights and measures (1858).
In August 1834 he became a member of the South African Literary and Scientific Institution and served as president (1838-1840), vice-president (1836-1837, 1841-1851), and member of its committee (to 1856). He had a strong interest in the geographical exploration of the sub-continent and from soon after his arrival until 1850 served on the committee of the Cape of Good Hope Association for Exploring Central Africa. The association sent Dr Andrew Smith*, who became a close friend of Maclear, on an expedition into the interior and Maclear served on the committee that drew up instructions for the expedition. In 1850 he met and befriended Dr David Livingstone*, taught him to use a sextant, and in subsequent years reduced the sextant and chronometer observations that Livingstone sent him during his travels. The resulting geographical positions were published by Maclear in the Journal of the Royal Geographical Society (1854, 1856) and in the Monthly Notices of the Royal Astronomical Society (1873). He was elected a Fellow of the Royal Geographical Society in 1859.
For many years he was an examiner for the South African College. When the South African Museum was re-established in 1855 he was elected as one of the trustees by the subscribers, and from 1857 to his death served as one of the trustees appointed by the government. He was also a trustee of the South African Library and served on the Coast Light Commission and the Weights and Measures Commission. To encourage public interest in science he wrote articles on scientific subjects for local newspapers. One of these explained in detail how he calculated the longitude of Grahamstown from a lunar occultation observed there by Dr W.G. Atherstone* on 17 April 1850 (South African Commercial Advertiser, 12 June 1850).
Maclear retired in October 1870 and settled in Mowbray, Cape Town. His eyesight deteriorated and by 1876 he was totally blind. Shortly after his death in July 1879 the House of Assembly adopted a resolution expressing its appreciation for his achievements in astronomy and geography, and his contribution to the welfare of the colony by the practical application of his research. He is commemorated in the name of the town Maclear in the Eastern Cape, Cape Maclear on the Cape Peninsula, and several other features.