HENDERSON,Thomas [FRS]

Professional Astronomer

Born: 28 December 1789, Dundee, Scotland
Died: 23 November 1844, Scotland

Index:
Education; Cape Observatory; Alpha Centauri; Career; Personal

Education:
Thomas Henderson was born in Dundee (U.K.) on 28 December 1798. He was educated at the Grammar School and Dundee Academy and he excelled in every subject. Henderson went on to study law and at age 15 he was apprenticed to a solicitor. In 1819 he moved to Edinburgh to complete his legal studies. His talents were noticed and he became Advocate’s Clerk-, and then Secretary to Lord Eldvin. [Warner – Astronomers, p.31.]
Henderson’s interest in Astronomy started in Dundee, and when he moved to Edinburgh he frequently visited Calton Hill Observatory.  Later he started using the instruments. From 1824 onwards, Henderson published a number of important papers, mostly of a computational nature. He became so well versed in practical Astronomy and associated theory of and methods of reduction, that he was considered as possible director of H.M. Nautical Almanac Office. [Laing, p. 10; Warner – Astronomers, p.32.]
Henderson had a weak constitution and an eye condition that at times made him nearly blind – although this never seems to have interfered with his observations (of bright stars) [Warner – Astronomers, p. 32.]

Cape Observatory:
  Meanwhile at the Cape Colony, the British Admiralty established the Royal Observatory at the Cape of Good Hope. The first astronomer (and director) was Fearon Fallows. It was his job to establish the Observatory and to initiate an observing program. However, on 25 July 1831 Fallows died of scarlet fever.
Upon receiving the news of the death of Fallows the Admiralty started looking around for suitable candidates for the post of director. Henderson was chosen and reluctantly he took up the post on 15 October 1831. (Reluctantly because he due to his poor health he did not like what he heard about the health hazards at the Cape Observatory. He was a Scott through and through, and was reluctant to leave Scotland. Henderson initially refused the position but his friends persuaded him to accept the position in order to further his career.) [Warner – Astronomers, p. 32.]
Just prior to this, the admiralty dispatched Lieutenant Meadows as Assistant Director. Meadows only learned on arrival at Cape Town about the death of Fallows. On paper Meadows was assistant to Fallows and Henderson, but in reality he was assistant only to Henderson.
Henderson arrived at the Cape on 22 March 1832 and took up residency at the Observatory on April 8th. He detested the place from the start. He it was who 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 places 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. [Koorts – British, p.36] Note: According to Warner- Astronomers, p.32 Henderson measured nearly ten thousand positions of stars (not six thousand as Koorts states). 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 improvements done, and the Admiralty refused his request. 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.
On 1 October 1834 he became the first Astronomer Royal for Scotland, and was also appointed to a professorship at Edinburgh University, a post that was combined with the directorship of the observatory on Carlton Hill.

Alpha Centauri:
However, Henderson’s most famous work was in connection with the distances of the stars – or, rather, of one stars in particular: Alpha Centauri, the brighter of the two Pointers to the Southern Cross. His investigations were of fundamental importance, and it was only by sheer bad luck that he missed the honour of making a great discovery.
On the voyage back to England the boat stopped over at St Helena Island. Henderson used the opportunity to visit Manual Johnson, an astronomer based on the Island, who also had a connection with Fallows. He informed Henderson that he noticed that Alpha Centauri has a large space motion and thus it must be close to Earth. Henderson had made a few observations of Alpha Centuari at the Cape and on his voyage back to England started to analyse the relevant information. [Warner-Astronomers, p.36.]
It had long been known that the stars are extremely remote. Men such as James Bradley, the third Astronomer Royal of England, had tried to measure their distances by what is termed the method of parallax – the slight apparent shifting of a comparatively nearby star over a period of six months, due to the real movement of the Earth from one side of its orbit round the Sun to the other. What had to be done was to measure the extremely small angular displacement of the target star against the background of stars, which were farther away. Bradley had failed; so had no less a person than Sir William Herschel – though in each case the attempts resulted in a discovery of quite a different sort: in Bradley’s case the aberration of light, in Herschel’s the detection of binary or physically-associated double stars.
One of the most promising stars was Alpha Centauri, because it was a wide binary and because it had unusually large individual or proper motion. Henderson’s measures were made at the Cape, but he returned home before he had reduced them, and he delayed for several years. In 1838 Friedrich Bessel, at Konigsberg, announced that he had carried out parallax measurements on a much fainter star (61 Cygni, in the Swan) and gave its distance as approximately 11 light-years. *It was only then that Henderson gave his results for Alpha Centauri, whose distance proved to be much less – a mere 4.3 light-years, or under 25 million million miles. Henderson’s estimate was very accurate, and Alpha Centauri is now known to be the nearest star in the sky – excluding the Sun, and also excluding Proxima Centauri, which is a faint companion of Alpha Centauri and which lies about I/I0 of a light-year closer to us.
It cannot be denied that the honour of priority must go to Bessel, but it is equally true that Henderson’s measurements were carried out first, so that the discovery was within his grasp. Not that there were any personal recriminations; the two astronomers were on the best possible terms, and later on they went together on a holiday trip to the Scottish Highlands.

*A light-year is the distance travelled by a ray of light in one year; it is equal to rather less than 6 million million miles, or about 9 million million kilometres. [Copied from Moore pp. 45 – 47.]

Henderson died on 23 November 1844. Charles Piazzi Smyth, who was assistant director at the Cape Observatory, filled his post as Astronomer Royal for Scotland.

Career:
1832 – 1833: Director of Royal  Cape Observatory.
Made parallax measurement of distance to Alpha Centauri, but didn’t publish his findings.
1834 (October 1): First Astronomer Royal for Scotland.
1838:  Bessel published his parallax measurement to 61 Cygni, after which  Henderson published his measurements.
Appointed Professor at Edinburgh University, a post which was combined with the directorship of Calton Hill Observatory.

Personal: 
1789 (December 28): Born at Dundee.  He was the youngest of five children of a respectable tradesman.
Henderson was said to acquire scientific knowledge by intuition.
1833 – 1834: Director of Cape Observatory (Look Career Section)
1842: Mrs Henderson dies.
1844 (November 23): Henderson dies. [Warner – Astronomers, p.60.]
Henderson had a weak constitution and an eye condition that at times made him nearly blind – although this never seems to have interfered with his observations (of bright stars) [Warner – Astronomers, p. 32.]
It appears that Henderson played no part in the intellectual life at Cape Town. (Most of the other 19th Century Astronomers played a great role in the social and intellectual life of Cape Town) [Warner – Astronomers, p. 32.]
“By South Africans, Henderson is sometimes regarded with a certain reserve, because he made no secret of his dislike of the surroundings in which he was placed. It may help the critics to understand Henderson better if we state that had he been appointed to the Cape a hundred and fifty years later he would probably have been able to cope with the situation easily enough. He was suffering from incipient heart trouble and doctors were scarce. He became depressed and on 27 April 1833 he wrote the following letter to his friends, fellow astronomers Herschel and Maclear (also to be his successor):
“I will tell – all about my residence in Dismal Swamp among Slaves and Savages – plenty of insidious venomous snakes. No one sets down a foot on the grounds of the Royal Observatory in the warm season, till he is certain that he is not treading upon a Snake in the grass. What would you think if on putting out your Candle to step into bed, you were to find one lurking beside the Bed?” [Moore p.47; Warner-Astronomers, p.34.]

Archival:
SOUTH  AFRICAN ARCHIVES, ROELAND STREET, CAPE TOWN [JHA8, pp. 220 – 221.]
Maclear-Mann Papers (Accession No. 515): this extensive accumulation of manuscripts and correspondence is contained in 139 files. They  derive from presentations made by members of the Maclear family and from donations from the Trigonometric Survey and from the Royal Observatory, Cape. Although mostly concerning Sir Thomas Maclear and William Mann, a considerable amount of material relates to Sir  John Herschel and to the early history of the Cape Observatory. Excluding miscellaneous files of accounts, testimonials, newspaper  cuttings, etc., the most significant references are:
Files 23 Includes some of Henderson’s observations.
119 Henderson’s reductions of transits 1832-33. Stars used in longitude  observations 1881.
Henderson’s reductions of mural circle observations.
121 Miscellaneous, including Henderson’s R.A. and Dec. reductions, Stone’s accounts. Gill’s expedition to Ascension.
130-132 Henderson’s observations 1832-33.

CAPE OF GOOD HOPE ROYAL OBSERVATORY PAPERS IN THE ARCHIVES OF THE  ROYAL GREENWICH OBSERVATORY [JHA 9 pp.74 – 75]
-Thomas Henderson (1831-33). Observing ledgers (transit and meridian circle). General note book (mostly instrumental adjustments), 2  vols. Official correspondence (bound with Fallows volume), 1832-33. Personal correspondence (1832-44) with Maclear, Airy and Brisbane.
– Miscellaneous notes, including a printed diary (astronomical notes), notes for a history of the Cape Observatory (principally of Henderson’s  own work) and an inventory.