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Thomas Henderson
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Dundee, Scotland
Edinburgh, Scotland
Biography
Thomas Henderson was the son of Thomas Henderson (1763-1801) and Isabell Rollo (1758-?). Thomas Henderson was a tradesman in Dundee who married Isabell Rollo in the Church of Scotland in Dundee on 18 January 1788. Isabell had been born on 19 June 1763 in Forgan, Fife, to William Rollo and Mary Muir. Thomas and Isabell Henderson had five children, two boys and three girls: John Henderson (baptised on 19 April 1789 in the Church of Scotland in Dundee); Catharine Henderson (baptised on 15 May 1791 in the Church of Scotland in Dundee); Janet Henderson (baptised on 28 April 1793 in the Church of Scotland in Dundee); Isabell Henderson (baptised on 28 October 1795 in the Church of Scotland in Dundee); and Thomas Henderson, the subject of this biography (baptised on 6 January 1799 in the Church of Scotland in Dundee). John Henderson had a legal career, first with Mr Small, a legal writer in Dundee, and later became a noted advocate in Edinburgh. He died in Leith South, Edinburgh, at the age of 38 on 10 April 1827.Thomas Henderson was only two years old when he lost his father who died in Dundee on 2 March 1801. Isabell, his mother, had to bring up her five children on her own. Thomas attended the Dundee Grammar School which, with its origins dating back to 1239, still flourishes today as the High School of Dundee. When Henderson was a pupil, the school shared a building in School Wynd with the English School. In 1811 he moved to Dundee Academy, a school in Nethergate which had opened in 1785. This school had been founded by the Town Council and specialised in mathematics and the sciences. In 1801 the mathematician Thomas Duncan (1777-1858) became the rector. Duncan was highly regarded as a teacher and Henderson flourished in his school excelling in mathematics and science. Duncan described Henderson as "remarkable for everything that was good." We note that in 1820 Thomas Duncan became the Regius Professor of Mathematics at the University of St Andrews and wrote several excellent mathematics textbooks.
At the age of fifteen, Henderson left the Dundee Academy and, despite his love of mathematics and science, decided to follow a legal career like his elder brother John. He became an apprentice with Mr Small, an attorney, in whose office his elder brother John had been a partner. After six years as an apprentice in Dundee, spending most of his time classifying the burgh records, Henderson moved to Edinburgh. He was appointed to a position in the office of a writer to the signet in Edinburgh, where his abilities came to the notice of James Gibson Craig a partner in the law firm Craig, Dalziel & Brodie. It was on Craig's recommendation that Henderson was appointed secretary to the judge John Clerk, Lord Eldin, who had an extensive law practice. He then became secretary to James Maitland, 8th Earl of Lauderdale. Lauderdale was a member of the House of Lords in London and spent longs spells there taking Henderson with him. Finally he was secretary to Lord Advocate Jeffrey who was dean of the Faculty of Advocates in 1829 and then had a career in parliament.
Although Henderson only had formal training in mathematics with Thomas Duncan at Dundee Academy, after he left the Academy and began his training as an apprentice in the law firm in Dundee he took up both mathematics and astronomy as a hobby. Henderson moved to Edinburgh in 1819, the year in which William Wallace became Professor of Mathematics in Edinburgh and John Leslie, who had been Professor of Mathematics, took the more prestigious chair of Natural Philosophy. Henderson became friendly with both Wallace and Leslie and they quickly realised that, despite not having formal training, he was knowledgeable and eager and quick to learn. Henderson was equally keen to be involved in the forefront of astronomy and joined the Astronomical Institution of Edinburgh. This Institution had been founded in 1811 and had set about raising funds for extending the observatory on Calton Hill. This observatory, first proposed by Colin Maclaurin in the first half of the 18th century, had, however, made slow progress [27]:-
... in 1776 a foundation stone was laid on the Calton Hill site, on land provided by the City Council. The Observatory building was completed in 1792, much reduced from the original plans. In 1811 a group of interested private individuals founded the Astronomical Institution of Edinburgh to raise funds for a new or extended observatory on Calton Hill. Their fund-raising was successful and in 1818 work commenced on the new observatory, to a neo-classical design by William Playfair. During his visit to Edinburgh in 1822 George IV conferred a Royal Charter on the still-incomplete observatory, which hence became a Royal Observatory, responsible to both the University and the Crown.Once a member of the Astronomical Institution of Edinburgh, Henderson was able to use the instruments available at the Calton Hill Observatory [5]:-
He showed special dexterity in the computing processes of practical astronomy, and he forwarded to Dr Thomas Young 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' (1825), and was followed by similar communications. In a paper 'On the Difference of Meridians of the Royal Observatories of Greenwich and Paris', 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 ...In this paper Henderson discusses how he uses the theory of probabilities to reduce errors. He begins this discussion as follows [11]:-
These various results differ so little from each other, that their arithmetical mean may be assumed to be near the truth. But it may not be improper to ascertain the most probable value, and its probable error, by the calculus of probabilities as practised by Gauss, Bessel, etc to serve as a rule for other investigations of a similar nature, in which it may be more requisite. Each night's result is liable to an error occasioned by the errors in the observed times of the signals, and of the transits of stars, whereby the clocks were compared with the heavens. The probable error of a single observation of a signal and a transit, appears from a considerable number of observations, to be one tenth of a second; and this divided by the square root of the number of these phenomena observed at any station, gives the probable error of the mean of the observed times at that station. But the results are exposed to other causes of error, such as the small deviations of the transit instruments from their meridians, the peculiar state of the eyes of the different observers, atmospheric circumstances, and various others which fluctuate from night to night, but may be supposed constant during the same night. Each night's result is equally liable to these errors, which have no tendency to be diminished by an increased number of observations upon that night. ...Henderson had very poor eyesight, so he was keen to master the mathematical techniques necessary to reduce the observations and compute orbits of comets, eclipses, occultations etc. We noted above that his job as a legal expert often took him to London for long spells and he made friends with several professional astronomers there and he was given the use of James South's Campden Hill Observatory.
Robert Blair was the Regius Professor of Practical Astronomy at the University of Edinburgh but had not contributed much in that role as explained in [27]:-
... in 1785 the University of Edinburgh had founded a Chair of Practical Astronomy, principally to provide instruction in navigation for officers serving on merchant ships operating out of the nearby port of Leith. The Chair was a Regius one, that is, it was a Royal appointment. The first incumbent was Robert Blair (1748-1828), though the choice was not a happy one. He was principally a medical and naval man (and continued to make significant contributions to these fields), but he refused to give lectures at the University on the, not entirely unreasonable, grounds that he had neither an observatory nor instruments. He was away from Edinburgh for long periods of time and died in post in 1828.Thomas Young supported Henderson as the best person to fill the Chair of Practical Astronomy which became vacant when Blair died in December 1828. There was a problem about filling the chair, however, since for over 40 years it had contributed little due to Blair's inactivity in the role. Young died in May 1829 and, shortly before his death, had sent a document to Stephen Rigaud, the Savilian Professor of Geometry at Oxford and about to become the Savilian Professor of Astronomy, indicating that Henderson was the best person to succeed him as superintendent of the Nautical Almanac. It was decided, however, that the astronomer royal, at that time John Pond, should take on that position. Henderson was offered a position of sharing the role of superintendent of the Nautical Almanac with Pond but declined, choosing to continue with his legal position and helping Pond, which he already did, in an informal way.
Fearon Fallows (1788-1831) had been appointed by the Admiralty to be the astronomer at the Government Observatory at the Cape of Good Hope in 1820 and had served there from 1821 until his death in Simon's Town, South Africa, in July 1831 from scarlet fever. Henderson was offered the position of astronomer royal at the Cape of Good Hope but was reluctant to accept. His friends encouraged him to accept and, although reluctant to leave Britain, he finally accepted having been persuaded by the argument that this was a way to become a professional astronomer. William Meadows, a Lieutenant in the Royal Navy, had been appointed as Fallows' assistant at the Cape of Good Hope on 30 May 1831 and had learnt of Fallows' death while on his journey from Portsmouth, England to the Cape. He was already at the Cape Observatory when Henderson arrived in March 1832.
Henderson had begun his voyage to the Cape in early 1832 but his ship was becalmed at Falmouth. While waiting for a fair wind, he wrote to his friend, the amateur astronomer Thomas Maclear (1794-1879) who was in the medical profession. Henderson wrote [33]:-
.. I received my official instructions. They prescribe a sufficient amount of work. In the Southern hemisphere I am asked to do as much as now occupies twelve or twenty Observatories in the northern. However I am not at all daunted. Patience and perseverance can do much. I am required to pay particular attention to the Transit of Mercury; but as nothing is said with regard to those of Venus, I conclude that it is held a settled point that I shall not be alive at the first one that happens, which I believe is in 1874.Henderson did not find the Cape a happy experience. He said he was in a "dismal swamp among slaves and savages", and he described Meadows as "bickering". Henderson's successor at the Cape, Thomas Maclear, described Meadows and his wife as [34]:-
... the most melancholic, discontented croaking helpless couple I ever met with.Despite these difficulties, Henderson made a remarkable number of observations while at the Cape for around a year. His task was made much more difficult by having poor, essentially faulty, equipment. We quote from [13] concerning his observations at the Cape observatory:-
Henderson arrived at the Cape on 22 March 1832 and took up residency at the Observatory on 8 April. He detested the place from the start. He named it the Dismal Swamp. He stayed for thirteen months and ever afterwards maintained that this was a year too long. Yet it is a mark of his exceptional qualities that during his brief tenure of office he made five or six thousand observations of the positions of southern stars, observed Encke's and Biela's Comets and the transit of Mercury of 5 May 1812; he studied stellar occultation's and the eclipses of Jupiter's satellites, as well as making special parallax' observations of Mars and the Moon. ... These constituted the first large body of accurate fundamental positions in the Southern Hemisphere. Early in 1833 Henderson started a new time service. With a brass barrel percussion pistol and a pocket chronometer he climbed each night onto the roof of the Observatory and fired a charge of black powder at an advertised time. The flash was bright enough for any sailor to see (if his telescope was correctly aimed). The brass pistol, and its powder flask, is now in the South African Museum. The Admiralty were loath to spend money on the Observatory. Henderson asked to have so many improvements done, but the Admiralty refused his requests. In a fit of rage Henderson resigned his post in May 1833 without troubling to make more than a flimsy excuse, and lost no time in returning to his native Scotland.The Astronomical Institution of Edinburgh had owned the Calton Hill observatory in Edinburgh but, as noted above, King George IV had visited Edinburgh in 1822 and gave it the title "Royal Observatory of King George the Fourth". The government supplied money for instruments and by 1834 this was complete. Francis Baily (1774-1844) was president of the Royal Astronomical Society and in the summer of 1834 he was approached by Lord Melbourne, First Lord of the Treasury, asking the Society to recommend a person to fill the role of Regius Professor of Astronomy at the University of Edinburgh and also the role of Astronomer Royal for Scotland. Since a quick response was required, Baily did not go to the Council, but asked four others to join him in making the decision. These four were George Biddell Airy, John Herschel, Francis Beaufort, hydrographer to the Navy famed for the Beaufort scale, and William Henry Smyth, a naval officer and astronomer. They recommended Henderson for the two positions based on his outstanding work at the Cape Observatory. He was appointed by Royal Warrant of 18 August 1834. In addition to being Astronomer Royal for Scotland and Regius Professor of Astronomy at the University of Edinburgh, Henderson also became the director of the Calton Hill Observatory.
Following Henderson leaving the Cape, Thomas Maclear had been appointed Her Majesty's astronomer at the Cape of Good Hope. Soon after his appointment in Edinburgh, Henderson wrote to Maclear on 29 September 1834:-
I am devoting as much time as possible to the reduction of the Cape Observations, but the labour is immense, and my interruptions have been frequent. I am investigating several points which have been scarcely attended to in English Observations, for instance the magnitude of the numerical Coefficients which enter into the formulae of refraction. I expect to have the first great division of the work, the Declinations, ready about the new Year. The Right Ascensions will follow next ...He took much longer, however, to work on his Cape observations than he anticipated. Partly this was due to the large amount of work he was undertaking at the Calton Hill observatory.
On 16 November 1836 Henderson married Janet Mary Adie (1807-1842). Janet had been born on 16 February 1807 in Edinburgh to Alexander James Adie and Marion Ritchie. Alexander Adie was a maker of medical and optical instruments and a meteorologist. He invented a type of barometer that was widely used on ships in the 19th century. Thomas and Janet Henderson had a daughter Janet Mary Jane Henderson, born 14 November 1842. Sadly Henderson's wife died December 1842.
We have not yet mentioned Henderson's most famous achievement. Manuel John Johnson (1805-1859) had an observatory on St Helena and, shortly before Henderson left the Cape he received a letter from Johnson saying that the star α Centauri had a large proper motion, something that he had seen by comparing his own position of the star with that given by Nicolas-Louis de Lacaille in 1751. Henderson had realised that if α Centauri had a large proper motion, it must be nearer to the sun that the vast majority of stars. It was therefore a good star to attempt to determine its parallax. Let us briefly explain why this was important.
Hipparchus had used a parallax method to determine the distance of the moon from the earth. When there was a total eclipse at Hellespont and a partial eclipse at Alexandria (probably on 14 March 190 BC) then knowing the distance from Hellespont to Alexandria and some trigonometry he could compute the distance to the moon in earth radii. Giovanni Cassini and Jean Richer measured the distance to Mars by a parallax method in 1672 with a base line on the earth, and this gave the distance from the earth to the sun. If the earth orbited the sun then Tycho Brahe realised that one could use the diameter of the earth's orbit as a base line by taking observations six months apart. His failure to find any star showing a parallax with this method was one of the main reasons he believed the sun orbited the earth. Robert Hooke made telescope observations with this parallax method in 1669 but failed as did James Bradley in 1729. John Brinkley published several papers in the 1820s claiming to have computed positive parallax observations of four stars but, after a long controversy, it was shown that his results was wrong caused by faulty equipment. Certainly Brinkley's incorrect papers on this topic made Henderson be very cautious about claiming to have obtained a positive parallax result for α Centauri.
Wilhelm Bessel had undertaken the monumental task of determining the positions and proper motions of over 50000 stars and, from this list he chose the relatively dim star 61 Cygni as having the largest proper motion so he deduced it was a good candidate for computing parallax. He published the result in 1838. It was the first correct claim to have measured the distance to a star and was immediately praised as a "most glorious triumph." Henderson could have published his parallax measurement for σ Centauri several years before Bessel but had chosen to wait for additional confirmations. He sent his paper to the Royal Astronomical Society on 11 January 1839, two months after Bessel's paper appeared, and it was published in 1840. It begins [12]:-
The two stars designated and Centauri, are situated within 19" of space of each other. On comparing the observations of Lacaille with those of the present time, it has been found that, although the two stars have not sensibly changed their relative positions, each has an annual proper motion of 3.6 seconds of space. It thus appears that they form a binary system, having one of the greatest proper motions that have been observed; and from this circumstance, and the brightness of the stars, it is reasonable to suppose that their parallax may be sufficiently sensible to powerful instruments. On reducing the declinations from his observations at the Cape of Good Hope, Mr Henderson remarks, that a sensible parallax appeared, but he delayed communicating the result until it should be seen whether it was confirmed by the observations of Right Ascension made by Lieutenant Meadows, with the transit instrument. He now finds that these observations also indicate a sensible parallax. It is to be observed, that the observations both of right ascension and of declination were not made for the purpose of ascertaining the parallax, but of determining the mean places of the stars with a proper degree of accuracy. Had the author been aware of the proper motion at an earlier period, a much greater number of observations, and of such as would have been better adapted for ascertaining the parallax, would have been made, and the result thereby rendered more secure.The Royal Astronomical Society chose to award their Gold Medal to Bessel rather than Henderson. See THIS LINK.
In 1842 Bessel visited Britain and spent a week with Henderson and Carl Jacobi partly in Edinburgh and partly in the Scottish Highlands.
Henderson was honoured with election to the in 1834 and to the
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There is also the following commemorative tablet on the wall of Calton Hill observatory:
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Written by J J O'Connor and E F Robertson
Last Update March 2026
Last Update March 2026