Thomas James Henderson
Encyclopedia
Thomas James Alan Henderson (28 December 1798 – 23 November 1844) was a Scottish astronomer noted for being the first person to measure the distance to Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...

, the major component of the nearest stellar system to Earth, and for being the first Astronomer Royal for Scotland
Astronomer Royal for Scotland
Astronomer Royal for Scotland was the title of the director of the Royal Observatory, Edinburgh until 1995. It has since been an honorary title.The following have served as Astronomers Royal for Scotland:* 1834–1844 Thomas Henderson...

.

Early life

Born in Dundee
Dundee
Dundee is the fourth-largest city in Scotland and the 39th most populous settlement in the United Kingdom. It lies within the eastern central Lowlands on the north bank of the Firth of Tay, which feeds into the North Sea...

, Scotland, he was educated at the High School of Dundee
High School of Dundee
The High School of Dundee is an independent, co-educational, day school in the city of Dundee, Scotland which provides both primary and secondary education to just over one thousand pupils...

, after which he trained as a lawyer and a cartoonist, 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
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 to measure longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

 he came to the attention of Thomas Young
Thomas Young (scientist)
Thomas Young was an English polymath. He is famous for having partly deciphered Egyptian hieroglyphics before Jean-François Champollion eventually expanded on his work...

, superintendent of the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

'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.

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
Cape of Good Hope
The Cape of Good Hope is a rocky headland on the Atlantic coast of the Cape Peninsula, South Africa.There is a misconception that the Cape of Good Hope is the southern tip of Africa, because it was once believed to be the dividing point between the Atlantic and Indian Oceans. In fact, the...

 in South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

. 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
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

, 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
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 , meaning "alteration"...

, 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
United Kingdom of Great Britain and Ireland
The United Kingdom of Great Britain and Ireland was the formal name of the United Kingdom during the period when what is now the Republic of Ireland formed a part of it....

 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
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 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
61 Cygni
61 Cygni,Not to be confused with 16 Cygni, a more distant system containing two G-type stars harboring the gas giant planet 16 Cygni Bb. sometimes called Bessel's Star or Piazzi's Flying Star, is a binary star system in the constellation Cygnus...

 in 1838. Henderson published his results in 1839, but was relegated to second place because of his lack of confidence.

Scotland

In the meantime, his measurement work at the Cape had led him to be appointed the first Astronomer Royal for Scotland
Astronomer Royal for Scotland
Astronomer Royal for Scotland was the title of the director of the Royal Observatory, Edinburgh until 1995. It has since been an honorary title.The following have served as Astronomers Royal for Scotland:* 1834–1844 Thomas Henderson...

 in 1834. The vacant chair of astronomy at the University of Edinburgh
University of Edinburgh
The University of Edinburgh, founded in 1583, is a public research university located in Edinburgh, the capital of Scotland, and a UNESCO World Heritage Site. The university is deeply embedded in the fabric of the city, with many of the buildings in the historic Old Town belonging to the university...

 was given to him on the advice of Prime Minister Lord Melbourne. From 1834 he worked at the City Observatory, Edinburgh
City Observatory, Edinburgh
The City Observatory is an astronomical observatory on Calton Hill in Edinburgh, Scotland. It is also known as the Calton Hill Observatory....

 until his death. He is buried in Greyfriars Kirkyard
Greyfriars Kirkyard
Greyfriars Kirkyard is the graveyard surrounding Greyfriars Kirk in Edinburgh, Scotland. It is located at the southern edge of the Old Town, adjacent to George Heriot's School. Burials have been taking place since the late 16th century, and a number of notable Edinburgh residents are interred at...

. He also claimed a right to different types of astronomy.

In April, 1840 he was elected a Fellow of the Royal Society.
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