Thomas Henderson (astronomer)
|Thomas James Henderson|
Thomas James Henderson, c.1820s
Scottish Astronomer Royal
28 December 1798|
|Died||23 November 1844
|Institutions||City Observatory, Edinburgh|
|Known for||distance to Alpha Centauri|
Thomas James Alan Henderson FRSE FRS FRAS (28 December 1798 – 23 November 1844) was a Scottish astronomer and mathematician noted for being the first person to measure the distance to Alpha Centauri, the major component of the nearest stellar system to Earth, and for being the first Astronomer Royal for Scotland.
Born in Dundee, he was educated at the High School of Dundee, after which he trained as a lawyer, working his way up through the profession as an assistant to a variety of nobles. However, his major hobbies were astronomy and mathematics, and after coming up with a new method for using lunar occultation to measure longitude he came to the attention of Thomas Young, superintendent of the Royal Navy's "Nautical Almanac". Young helped the young Henderson enter the larger world of astronomical science, and on his death a posthumous letter recommended to the Admiralty that Henderson take his place.
Henderson was passed over for that position, but the recommendation was enough to get him a position at the Royal Observatory at the Cape of Good Hope in South Africa. There he made a considerable number of stellar observations between April 1832 and May 1833, including those for which he is remembered today. It was pointed out to him that the bright southern star Alpha Centauri had a large proper motion, and Henderson concluded that it might be a close star.
The 1830s version of the "space race" was to be the first person to measure the distance to a star using parallax, a task which is easier the closer the star. Henderson was thus in a good position to be this person. After retiring back to the United Kingdom due to bad health, he began analysing his measurements and eventually came to the conclusion that Alpha Centauri was just slightly less than one parsec away, 3.25 light years. This figure is reasonably accurate, being 25.6% too small.
Doubts about the accuracy of his instruments kept him from publishing, however (there had been previous, discredited attempts to claim a measurement of stellar parallax), and eventually he was beaten to the punch by Friedrich Wilhelm Bessel, who published a parallax of 10.4 light years (8.8% too small) for 61 Cygni in 1838. Henderson published his results in 1839, but was relegated to second place because of his lack of confidence.
In the meantime, his measurement work at the Cape had led him to be appointed the first Astronomer Royal for Scotland in 1834. The vacant chair of astronomy at the University of Edinburgh was given to him on the advice of Prime Minister Lord Melbourne. From 1834 he worked at the City Observatory (then called the Calton Hill Observatory) in Edinburgh until his death. In April, 1840 he was elected a Fellow of the Royal Society.
Personal life and death