Sir Thomas Maclear

Sir Thomas

Sir Thomas Maclear (1794-1879) served as Her Majesty's Astronomer and Director at the Royal Observatory at Cape of Good Hope (in what is now South Africa), from 1833-1870.


Maclear's Beacon

Maclear's Beacon marks the highest point on Table Mountain in Cape Town, South Africa at 1086m (3563 feet). It is named for a stone-cairn beacon built there in December 1844 by Sir Thomas as a triangulation station to assist in measuring the curvature of the Earth.

The South African Astronomical Observatory notes:
"The Arc of the Meridian is an imaginary line on the meridian (runs true North - South), drawn anywhere on earth. The Scientific value of such a line is that, if the line is long enough (at least 70 km (43.4 mile)) and surveyed with great accuracy, then the curvature of the earth can be measured. Add to that principle the accurate measurement of the latitudes (how far north or south you are from the equator) at the two far ends of the line, and you measures the distance of the line by independent means to the physical measurement. This gives you an extra value to test measurement against. In other words: you can measure the curvature of the Earth, and more than that, the size and shape of the Earth. But for the method to work, measuring the latitudes, or in more layman's terms, getting the vertical correct is critical.
Lacaille (also known as Abbe De La Caille) was sent to Cape Town to do the measurements. Due to a mistake, his measurements showed that the earth was pear shaped, round on top (Northern Hemisphere), but bulged at the bottom (Southern Hemisphere). Maclear re-measured the Arc of the Meridian and discovered the mistake, proving that the earth is round."

Maclear's Beacon

The SAAO has further detailed information regarding the life and achievements of Sir Thomas. See also History of geodetic surveying in South Africa.

Obituary from The Times

Wednesday, August 6, 1879, Page 5, Column 6

"Sir Thomas Maclear, F.R.S., Astronomer Royal, died recently at the Cape of Good Hope. It may not be generally known that Sir Thomas was a member of the medical profession, having passed his examinations and been admitted a member of the Royal College of Surgeons of England so long ago as 1815, under the presidency of another veteran in that profession, the late Sir William Blizard. The age of Sir Thomas is not mentioned; but seeing he must have been at least 22 on becoming a member of the college, he had reached the great age of 86 years.

He commenced the practice of the medical profession in Bedfordshire, but, being very partial to astronomical studies, he made the acquaintance of the late Admiral Smyth, well known as an astronomer, and by him was persuaded to go out to the Cape to make investigations, which were crowned with signal surveys, and deservedly obtained for him the appointment of Astronomer Royal and the fellowship of the Royal and other learned societies at home and abroad. The deceased was knighted by patent and in June, 1863, a pension of 160 [pounds] per annum was granted him."

Dictionary of National Biography

Dictionary of National Biography 20th Century

Vol 35, pp204-205, Vol 12, pp648-649, Ed. Sidney Lee, London, 1893

"MACLEAR, SIR THOMAS (1794-1879), astronomer, was the eldest son of James Maclear of Newtown Stewart, co. Tyrone, , where he was born on 17 March 1794. His refusal to enter the church led to a breach with his father, and he was sent to England in 1808 to be educated for the medical profession, under the care of his maternal uncles, Sir George and Dr. Thomas Magrath. Having studied in Guy's and St. Thomas's Hospitals, and passed distinguished examinations, he was admitted in 1815 a member of the Royal College of Surgeons, under the presidency of Sir William Blizard. He then accepted the post of house-surgeon to the Bedford Infirmary, where he became acquainted with Admiral Smyth; and studied astronomy and mathematics.

In 1823 he entered into partnership with his uncle at Biggleswade in Bedfordshire, and married in 1825 Mary, daughter of Mr.Theed Pearse, clerk of the pence for that county. The Astronomical Society lent him in 1829 the Wollaston telescope for the purpose of observing a series of occultations of Aldebaran, calculated by himself, and he set it up with a thirty-inch transit in a small observatory in his garden at Biggleswade (Menoirs of Royal Astr. Society, vi. 147). Succeeding Thomas Henderson [q. v.] in 1833 as royal Astronomer at the Cape of Good Hope, he arrived there on 5 Jan.1834, ten days before Sir John Herschel, whose zealous co-operator and attached friend he became.

Maclear was indefatigable in the duties of his office. His activity, indeed, as an observer outran the computing powers of his small staff, and most of the valuable materials he had accumulated were left by him unreduced. He published, however, in 1840 a volume of observations made in 1834. From 1837 he was occupied with the remeasurement and extension of Lacaille's arc. The field operations, conducted with remarkable skill and energy in the midst of most deterrent difficulties, were completed in 1847, and the results appeared in two 4to volumes, edited by Sir George Airy, in 1866. For this great work, still fundamental in the survey of the colony, Maclear received the Lalande prize in 1867 and a royal medal in 1869. Bradley's zenith-sector was sent out to the Cape for use in the arc-measurement, and returned uninjured to Greenwich in 1850. A seven inch equatorial by Merz was mounted at the Cape in 1849, and a large transit-circle, a facsimile of that at Greenwich, in 1855. Maclear's determinations of Alpha Centauri in 1839-40 and 1842-8 confirmed Henderson's parallax of about one second (ib, xii. 329). He observed the maximum of Mu Argus in 1843, and the meteoric shower of 1866. His cometary observations, regularly communicated to the Astronomical Society, were of great value. They included prolonged series on Halley's and Donati's comets, besides numerous places of Encke's, Petersen's, and others. His observations of Mars during the opposition of 1862 were employed by Stone, Winnecke, and Newcomb in fresh determinations of the sun's distance, but a fine set of measures by him of southern double stars remains unpublished. His observations, between 1849 and 1852, of all the southern stars in the 'British Association Catalogue' supplied materials for the' Cape Catalogue for 1850,' published by Dr. Gill In 1884. The Cape Catalogue for 1840,' containing 2,892 stars, and the' Cape Catalogue for 1860,' containing 1,159 stars, both published by Stone, embodied the results of Maclear's observations in 1835-40 and 1856-61 respectively.

Much care was devoted by him to the collection of meteorological, magnetic, and tidal data; and he set on foot in 1860 the communication of time-signals by electricity to Port Elizabeth and Simon's Town. Lighthouses were through his aid established in South Africa. He sat on a commission of weights and measures, promoted sanitary improvement, and contributed in innumerable ways to the welfare of the colony. African exploration interested him keenly. Livingstone was his intimate friend, and was instructed by him in the use of the sextant.

Maclear visited England, Paris, and Brussels in 1859, and was knighted in June 1860.

A severe afliction befell him in the death of his wife in 1861. He retired from the observatory in 1870, and took Up his abode at Grey Villa, Mowbray, near Cape Town. In 1876 he became totally blind, but was attended by a devoted family, and retained unabated interest in public matters, leaving his house for the last time to welcome Mr. H. M. Stanley at a meeting in Cape Town. He died on 14 July 1879, and was buried with his wife in the grounds of the Royal Observatory.

Three days later the House of Assembly at Cape Town passed a resolution expressing their sense of his signal services to the colony. He was a member of the Astronomical Society from 1828, of the Royal Society from 1831, and was elected in 1863 a corresponding member of the Institute of France. He was besides associated with the Academy of Sciences of Palermo, and the Imperial Geographical Institution of Vienna. Maclear's life was one of unflinching devotion to science.

[Monthly Notices, xl. 200 (Gill) ; Proceedings Royal Society, vol. xxix. p. xviii; Nature, xx. 365) ; Obsersvatory, iii. 154; Times, 6 Aug. 1879 ; Memoires couronnes par I' Academie des Sciences, Bruxelles, 1873, xxiii. 77 (Mailly) ; Andre et Rayet's L'Astronomie Pratique, ii.'68 ; Grant's History of Astronomy,pp.138,149,552; Madler's Geschichte der Himmelskunde, Bd. ii. ; information from Miss Maclear.] A. M. C."

Lunar Crater

A moon crater 20 km diameter, at the coordinates of Latitude 10.5N, Longitude 20.1E, is named after Sir Thomas. Images of the area are available at this Lunar Atlas site and similarly for Rimae Maclear (lava channel).
Sir Thomas and Lady Maclear