Jeremiah Dixon, land surveyor and amateur astronomer, was the son of George Dixon, a well-to-do Quaker coalmine owner, and his wife Mary Hunter. At school he acquired an interest in mathematics and astronomy. In 1760 he was chosen by the Royal Society of London to assist the astronomer Charles Mason* in observing the transit of Venus on 6 June 1761 at Bengkulu (then called Bencoolen), a small port on the south-west coast of Sumatra. Within hours after sailing on the Seahorse from Portsmouth in December 1760 their ship was attacked by a French frigate (the two countries were at war at the time) and had to return to port to refit. They were not at all keen to sail again, but changed their minds when the Royal Society threatened court action. They eventually reached the Cape of Good Hope on 27 April 1761, probably too late to reach their destination in time. Furthermore, they learned that Bengkulu had been taken by the French and decided to observe the transit from the Cape. This turned out to be a fortunate turn of events, as a second British expedition led by the astronomer Nevil Maskelyne sent to observe from the island of St Helena was clouded out.

Mason and Dixon erected a small observatory building and set up their equipment at Concordia Gardens in Cape Town, a public pleasure garden near Hope Street and Bouquet Street, behind the present St Mary's Cathedral. They had two reflecting telescopes of 600 mm focal length, a quadrant of 300 mm radius, and an astronomical clock. From 4 May to 27 September they observed the meridian passage of a number of bright stars to correct their clock, the zenith distances of the same stars to determine the latitude of their observatory, eclipses by Jupiter of its larger satellites to fix their longitude by comparison with similar observations in England, a total eclipse on 18 May, and of course the transit of Venus on 6 June. They measured the position of Venus against the sun's disc during the transit, as well as the time of internal contact with the sun's limb towards the end. Despite some interference from clouds their observations were successful and accurate. An account of the work, "Observations made at the Cape of Good Hope by Mr Charles Mason and Mr Dixon..." was published in the Philosophical Transactions of the Royal Society of London for 1761 (Vol. 52, pp. 378-394). By comparing their observations with those of fifteen observatories in the northern hemisphere, J. Short derived a mean solar parallax of 8,65 seconds of arc (Philosophical Transactions, 1762, Vol. 52, pp. 611-628). The modern value is 8,79 seconds of arc. An exhaustive review of all the observations during this transit, by 176 observers at 117 stations, was published in 1891 by Simon Newcomb*. The weights he assigned to Mason and Dixon's observations were among the highest that he allotted.

Sailing from the Cape on 3 October 1761 they travelled to St Helena, where Mason assisted Maskelyne in his researches. Dixon was sent back to the Cape with a pendulum clock to measure its rate there, so that the earth's gravitational attraction at the Cape could be compared with that at Greenwich and other places. They eventually reached England early in 1762. As a result of their success the two observers were chosen in 1763 to survey the boundary line between Maryland and Pennsylvania in the United States. The line was to run exactly east-west (along latitude 39º 43' 17.6" N) from a point south of Philadelphia. In November 1767, having surveyed 392 km with only some 58 km remaining, further progress towards the west was stopped as a result of opposition by the local natives. This so-called "Mason and Dixon line" became famous as the supposed boundary between slave-holding and non-slave-holding states. The work was written up under both their names as Field notes and astronomical observations... made in their survey of the boundary lines between the provinces of Pennsylvania, Delaware, and Maryland, 1763-1768 (n.d.). They also measured an arc of meridian for the Royal Society, at a mean latitude of 39º on the Delmarva peninsula in Maryland, using wooden rods and without any triangulation. Despite taking great care the results were not completely satisfactory. They reported this project in "Observations for determining the length of a degree of latitude in the provinces of Maryland and Pennsylvania, in North America", published in the Philosophical Transactions of the Royal Society in 1768. Some gravity observations were also made, using a pendulum clock. Their work was finally completed in June 1768. Both men were elected as corresponding members of the American Philosophical Society held at Philadelphia for Promoting Useful Knowledge.

At the time of the next transit of Venus in June 1769 (not visible from South Africa), Dixon was again employed by the Royal Society to observe the event, this time from the island of Hammerfest, in the north of Norway, with W. Bayley. However, the weather was unfavourable and the observations fragmentary. Dixon's "Observations made on the island of Hammerfost, for the Royal Society" were published in the Philosophical Transactions for 1769. Upon his return to Durham he resumed his work as a surveyor. As a bachelor he had no descendants, but the progeny of his brother have attained distinction as engineers and amateur astronomers.

List of sources:

Cope, T.D. The first scientific expedition of Charles Mason and Jeremiah Dixon. Pennsylvania History, 1945, Vol. 12, pp. 24-33.

Cope, T.D. Charles Mason and Jeremiah Dixon. The Scientific Monthly, 1946, Vol. 62, pp. 541-554.

Evans, D.S., Deeming, T.J., Evans, B.H. & Goldfarb, S. Herschel at the Cape... 1834-1838 (p. 93n). Cape Town: Balkema, 1969.

Fernie, J.D. Transits, travels and tribulations, II. American Scientist, 1997, Vol. 85, pp. 418-421.

Hurly, R.F. Astronomers and surveyors. Monthly Notes of the Astronomical Society of South Africa (MNASSA), 1979, Vol. 38(7-10), pp. 59-64.

Jeremiah Dixon s biography. In Oxford University Press, Dictionary of National Biography. Retrieved from http://masondixon.pynchonwiki.com/wiki/index.php?title=Jeremiah_Dixon's_Biography on 2016-8-18.

MacKenzie, T. Mason and Dixon at the Cape. Monthly Notes of the Astronomical Society of South Africa (MNASSA), 1951, Vol. 10(11), pp. 99-102.

National Union Catalogue, pre-1956 imprints. London: Mansell, 1968-1980.

Philosophical Transactions of the Royal Society of London, 1761/2, Vol. 52 (papers by Mason & Dixon and by Short); 1769, Vol. 59 (paper by Dixon).

Van Gent, R.H. Transit of Venus bibliography. 2003.

Wood, H.E. Astronomical relics at Touws River, C.P. South African Journal of Science, 1937, Vol. 33, pp. 127-129.

Compiled by: C. Plug

Last updated: 2024-02-09 11:07:50