Mason, Charles ( Apr. 1728-25 Oct. 1786), and Jeremiah Dixon (27 July 1733-22 Jan. 1779), British astronomers and surveyors, were responsible for establishing the Mason-Dixon Line. Charles Mason was born at Wherr (now Weir) Farm, Oakridge Lynch, Gloucestershire, England, the son of Charles Mason, a baker and miller, and Anne Damsel Mason. He attended Tetbury Grammar School and received additional tutoring from mathematician Robert Stratford. He lived near the astronomer royal, Dr. James Bradley, and Reverend Nathaniel Bliss, Savilian Professor at Oxford. It was through these local connections that Mason's prowess as a mathematician came to the attention of Bradley, who in 1756 offered him the position of assistant (or "labourer") at the Royal Observatory, Greenwich, with a salary of £26. At about this time Mason married Rebekah (maiden name unknown), with whom he had two sons.

At the observatory Mason compiled tables of lunar distances for deriving longitude, based on the work of Tobias Mayer. A congenial person and a meticulous observer of nature and geography, Mason was elected a corresponding member of the American Philosophical Society in 1767.

Dixon was born at Bishop Auckland, County Durham, the son of George Dixon, a Quaker colliery owner of Cockfield, and Mary Hunter Dixon. He and his elder brother George were educated at a school in Barnard Castle run by John Kipling. Dixon became friends with Hurworth mathematician William Emerson and the famous London instrument maker John Bird of Bishop Auckland. Of his early career as a land surveyor little is known. He may have learned the profession from his brother George. In 1760 he was expelled from the Quaker meeting house for excessive drinking. As a Quaker, albeit ethically weak but physically strong, slavery offended Dixon. The proposal for electing Jeremiah Dixon a corresponding member of the American Philosophical Society went forward with Mason's but for unknown reasons he was not elected until 1768.

In 1760, the Royal Society of London was preparing to observe the first transit of Venus for a hundred years. Dixon was recommended to the society, probably by John Bird, as one of the observers. Charles Mason was teamed with the Cambridge astronomer Reverend Nevil Maskelyne, FRS, to observe the Transit from the island of Saint Helena. However, the society also required observers at the East India Company's trading post at Bencoolen (Bengkulu, Sumatra). On 11 September 1760, Mason was offered the job with Dixon as his assistant; Dixon agreed "to accompany Mr. Mason, and be under his directions." Their contract with the society (each was to receive £200) was signed on 25 October, and thus was born the famous Mason-Dixon partnership.

They sailed from Portsmouth on 8 January 1761 aboard HMS Seahorse. In the late morning of 10 January the Seahorse was attacked by the French L'Grand. During the battle, the Seahorse was damaged, and it returned to Plymouth for repairs. It sailed on 3 February 1761 and arrived in Cape Town on 27 April. There a temporary observatory was erected where the astronomers successfully observed the transit of Venus; they remained in Cape Town until 3 October then joined Nevil Maskelyne on Saint Helena. Dixon returned briefly to South Africa to make gravity observations while Mason assisted Maskelyne with astronomical and tidal measurements. It was Mason and Dixon's observations in Africa that established their reputations for excellence and won them the praise of the scientific community and, especially, that of the future astronomer royal, Nevil Maskelyne.

In 1763, Mason and Dixon were chosen to complete a survey to define the borders of two colonies in the Americas: Pennsylvania and Maryland. Pennsylvania's charter of 1681 had triggered a dispute between William Penn and the Calverts (Lords Baltimore) of neighboring Maryland. After many attempts at negotiation, the matter was referred to the English court of chancery. Mason and Dixon arrived in Philadelphia on 15 November 1763, where they received instructions from the commissioners for Pennsylvania and Maryland. Their first task was to find the latitude of Philadelphia. On 7 January 1764, they headed west to find a point having the same latitude as Philadelphia, arriving at the farm of John Harlan (Stargazers' Farm, Embreeville), which became their headquarters. They made observations and took measurements, then measured fifteen miles due south to establish "the Post mark'd West," establishing a point on the Pennsylvania-Maryland border. On 12 June they reported to the commissioners that "the Post mark'd West" lay in the latitude of 39 degrees 43 minutes 18.2 seconds north. From this spot, the famous Mason-Dixon Line would extend due west and east. The work of laying out the line remained to be done.

They then set out to ascertain Lord Baltimore's eastern border (modern Delaware). It was this so-called Tangent Line, running for over eighty miles from the Delaware Middle Point (established 1751) to where it grazed the curious 12-mile circular border line centered on New Castle, Delaware, that had perplexed the proprietors' American surveyors. Mason devised a strategy to run a perfectly straight line from the Middle Point to the Tangent Point, using as his guide a star in the tail of the Great Bear (Ursa Major) constellation. The results of their work were astonishingly accurate and the line deviates only a few feet at the midway point.

On 5 April 1765 they began surveying the Maryland-Pennsylvania border (Mason-Dixon Line). The first section of the West Line was completed on 28 May as far as the Susquehanna River, after which they completed Lord Baltimore's eastern boundary from the Tangent Point due north to the West Line. Returning to the Susquehanna on 21 June, Mason and Dixon with their large team of laborers and assistants, including American surveyors, began the second part of the West Line. By 21 October, they had reached the foot of North Mountain, which point marked the limit of their work for 1765. Work resumed on 1 April 1766 and by 18 June had reached as far as Savage Mountain. The Savage Mountain point marked the line of the 1763 royal proclamation that forbade settlement beyond the Alleghenies (the so-called dividing mountains). At this point work had to stop until the Native American chiefs of the Six Nations agreed to its continuance. General Sir William Johnson, the government's agent for Indian affairs, had the task of negotiating terms.

While Mason and Dixon awaited the outcome, they began on another task, unrelated but historically important: the first measurement in North America of a degree of latitude. This measurement, for the Royal Society, would add to the work of the French Académie Royale des Sciences in defining the size and shape of the Earth. The 1766 season concluded with Mason and Dixon extending the West Line eastward to the Delaware River. They spent the winter months at Harlan's farm making the first gravity observations in America, using a pendulum clock made by Jackson of Philadelphia and the same John Shelton clock used by Maskelyne at Saint Helena.

Finally, the Six Nations consented to the work's proceeding and provided "deputies" to act as supervisors and provide protection. In addition, the commissioners sent Captain Hugh Crawford, a war veteran and renowned trader and explorer, as the survey's guide. Work recommenced on 7 July 1767 and proceeded as far as the Cheat River, which some deputies believed marked the end of their commission. Shortly after, the survey team had the first of several encounters with unfriendly Native American bands. A few miles west of the Monongahela River in October 1767, a warpath was reached which the deputies refused to pass. This marked the end of Mason and Dixon's famous line. (U.S. surveyors including David Rittenhouse and Andrew Ellicott completed the work in 1785.) Mason and Dixon's final task for the commissioners was to draw maps of all the borderlines and have them officially printed. In the intervening period, they were able to complete their measurements of a degree of latitude. The commissioners, entirely satisfied, discharged Mason and Dixon on 27 August 1768. In 1769 the two were again employed by the Royal Society to observe the 1769 transit of Venus. Mason went to Cavan, in Ireland, while Dixon went to Norway with William Bayly, who later sailed with James Cook.

In 1770 Dixon retired to his home in Cockfield, a "gentleman." He continued to practice as a land surveyor, making maps of the estates of Lanchester Common and Auckland Castle. He died unmarried at Cockfield on 22 January 1779, leaving his personal fortune to Margaret Bland and her two daughters. He was buried in an unmarked grave in Friends' ground (the Quaker cemetery) at Staindrop.

Mason continued to work for the Royal Observatory, the Board of Longitude, and the Royal Society. In 1770 he married Mary Williams, daughter or sister of his friend Robert Williams of Tetbury (who may have acted as guardian to Mason's sons while Mason was in America); the couple had five sons and one daughter. Three years later, Mason traveled in Scotland to locate a suitable mountain for Maskelyne's gravity experiment, identifying the peak of Schiehallion. He was offered the chance to conduct the experiment himself, but the offer was derisory and he declined. From his home at Bisley, Mason continued to refine the lunar tables and made substantial improvements to the Nautical Almanac, which continued to appear many years after his death. He applied unsuccessfully to the Board of Longitude for the £5,000 longitude prize under the terms of the 1774 Act of Parliament but received only £1,317, which, according to the French astronomer Jérôme Lalande, fell well short of his expectations.

In July 1786, Mason returned with his family to America, writing on 27 September from the Sign of the George on Second Street, Philadelphia, to his friend Benjamin Franklin. He was by then very ill and passed his scientific papers to his friend Reverend John Ewing, provost of the University of Pennsylvania, asking him to publish an American version of the Nautical Almanac; sadly, Ewing did not pursue the opportunity. Mason died in Philadelphia and is buried in Christ Church burying ground.

Mason and Dixon's partnership was a perfect merging of scientific astronomy with land surveying, laying the foundations of modern geodetic field survey. While the Mason-Dixon Line is their lasting memorial, all their scientific accomplishments were of the highest caliber. Neither man was honored with a memorial, nor did they receive any public recognition for their contributions to science and peace.

The libraries of the National Maritime Museum and the Royal Society in London are the principal sources for what little exists. The library of the Religious Society of Friends in Britain has a copy of Penny's book (noted below) and much other supportive information about early Quakers. The Sackler Archive Resource at the Royal Society provides reliable (but short) sketches of fellows, and the web site of the Harlan family in America (http://www.harlanfamily.org) is full of interesting snippets and worth a visit. This researcher has relied much upon interviews and correspondence with, among others, Jeremiah Dixon's descendent George Dixon; Eugene Smith of Pennsylvania; artist David H. Naylor, owner of Mason's birthplace; and John Loosley, who collected details about the Mason family.

The main body of information on Mason and Dixon's work in America is in A. Hughlett Mason's transcription "Journal of Charles Mason and Jeremiah Dixon," Memoirs of the American Philosophical Society 76 (1969) (whole volume). Thomas D. Cope was an authority and wrote several papers, including "The Stargazers' Stone," Pennsylvania History 6, no 4 (Oct. 1939); "Charles Mason and Jeremiah Dixon," Scientific Monthly 62 (June 1946); "Collecting Source Material about Charles Mason and Jeremiah Dixon," Proceedings of the American Philosophical Society 92, no. 2 (1948): 111-14; "Charles Mason, Jeremiah Dixon and the Royal Society," Notes and Records of the Royal Society of London 9 (Oct. 1951): 56-78; and "Some Contacts of Benjamin Franklin with Mason and Dixon and Their Work," Proceedings of the American Philosophical Society 95 (June 1951): 232-38. For a source of short biographic sketches (including many early American scientists), recommended is Whitfield J. Bell, Jr., "Patriot-Improvers: Biographical Sketches of Members of the American Philosophical Society, 1743-1768," Memoirs of the American Philosophical Society 226 (1997). The Dictionary of National Biography and its Missing Persons supplement provide some useful details but also include some errors. An authoritative source for Jeremiah Dixon is Norman Penny, My Ancestors (1920), while Derek Howes's excellent Nevil Maskelyne: The Seaman's Astronomer (1989) contains some details about Charles Mason's later life. A complete treatment of the men and their work is found in Edwin Danson's Drawing the Line: How Mason and Dixon Surveyed the Most Famous Border in America (2000).