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,
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.]
Meanwhile at the Cape Colony, the British Admiralty established
Observatory at the Cape of Good Hope.
The first astronomer (and director) was Fearon
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
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
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
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
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,
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
*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
who was assistant director at the Cape Observatory, filled his post as
Astronomer Royal for Scotland.
1832 - 1833: Director of Royal
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.
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,
"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
(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,
Brass barrel percussion pistol and powder flask that was used for the
time service is now housed in the South African Museum.
Koorts, W.: The 1882 transit of Venus: The British expeditions to South
Africa; MNASSA April 2004, Vol. 63 nos. 3 & 4, pp. 34 - 57.
Laing, J.D. (ed.), The Royal Observatory at the Cape of Good Hope 1820
- 1970 Sesquicentennial Offerings, pp. 9 - 10.
Moore, P. & Collins, P., Astronomy in Southern Africa, pp. 45 - 47.
Smits, P., A Brief History of Astronomy in Southern Africa. (Unpublished)
Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
SOUTH AFRICAN ARCHIVES, ROELAND STREET, CAPE TOWN [JHA8, pp. 220
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
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.