Victorian Celebrities: Thomas Henderson, First Person to Measure the Distance to Alpha Centauri

220px-Thomas_James_Henderson,_1798-1844_Henderson-01r A Scottish astronomer and mathematician, Thomas James Alan Henderson (28 December 1798 – 23 November 1844) was 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.

Early Life
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 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. Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις (parallaxis), meaning “alteration.” Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances.

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.{The parsec (symbol: pc) is a unit of length used in astronomy, equal to about 3.26 light-years, or about 30.9 trillion kilometres (19.2 trillion miles).}

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.

Henderson became a member or fellow of several distinguished societies, including the Royal Astronomical Society (1832) and the Royal Society of Edinburgh (1834).

Personal Life and Death
He was married in 1836 and had one daughter. He died in Edinburgh on 23 November 1844 and is buried in Greyfriars Kirkyard.


About Regina Jeffers

Regina Jeffers is the award-winning author of Austenesque, Regency and historical romantic suspense.
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