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.
Career
Africa
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 by Manuel John Johnson of the East India Company's observatory on Saint Helena that the bright southern star Alpha Centauri had a large proper motion, and Henderson concluded that it might be relatively close. 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. Henderson did not immediately publish his results, however, and eventually he was beaten to the punch by Friedrich Wilhelm Bessel, who published a parallax of 10.3 light years for 61 Cygni in 1838. Henderson published his results in 1839, but was relegated to second place because of his lack of confidence. He later published confirming observations by Thomas Maclear. Alpha Centauri remained the nearest known star until the discovery of Proxima Centauri in 1915 by Robert T. A. Innes.
He married Alexander Adie's daughter Janet Mary Adie in 1836 and had one daughter, Janet Mary Jane Henderson. He died at home 1 Hillside Crescent in Edinburgh on 23 November 1844 and is buried in Greyfriars Kirkyard. The grave may be either in the grave of Alexander Adie or in a grave marked by the stone "to his memory". His name is not recorded on the Adie grave; Adie himself died 14 years after Henderson.
Recognition
A blue plaque is installed on his house at 1 Hillside Crescent. It refers to him as "Thomas J. A. Henderson", similar to middle names wrongly added to Wikipedia around 2007. A larger memorial is incorporated in the external wall of the City Observatory.
