Charles Mason (Oakridge Lynch, near Stroud, Gloucestershire, April 1728 – Philadelphia, October 25, 1786) was an English astronomer who made significant contributions to 18th-century science and American history, particularly through his involvement with the survey of the Mason-Dixon line, which came to mark the border between Maryland and Pennsylvania (1764–1768).
Early career 
While employed at the Greenwich Observatory, Mason became familiar with Professor Tobias Mayer’s Tables of the Moon. The Lunar Tables were designed to solve the problem of determining longitude at sea, a challenge that frustrated scientists and navigators for decades. Mason worked throughout his life to perfect the Lunar Tables as a method of improving navigation at sea. In 1787, Mason was awarded £750 by the Board of Longitude for his work on perfecting the Tables.
1761 transit of Venus 
In 1761, Mason was assigned to travel to the island of Sumatra to observe the Transit of Venus as part of an international effort to record data that would enable scientists to determine the distance from the earth to the sun. Mason was joined by Jeremiah Dixon, a surveyor and amateur astronomer from Cockfield in the County of Durham. Owing to an attack by a French man-of-war, they did not reach their destination in time for the transit and were forced to record their observations from the Cape of Good Hope. On the way back from the Cape they visited St. Helena where they made a series of observations with the astronomer Nevil Maskelyne.
The Mason–Dixon line survey 
From November 1763 to 1768, Mason and Jeremiah Dixon established the boundary line between the American provinces of Pennsylvania and Maryland. Colonial surveyors had been unable accurately to establish the boundary due to their poor training and inadequate scientific instruments. Mason and Dixon, accompanied by a large party of assistants, established three important boundaries: (1) the south boundary line of Pennsylvania separating it from Maryland and Virginia; (2) the west boundary of the three lower counties of Pennsylvania (now Delaware) separating it from Maryland; and (3) the south boundary of the three lower counties. The pair also conducted a number of experiments for the Royal Society such as measuring a degree of longitude. Mason’s journal provides the most complete record of the survey and its progress. The journal includes his astronomical observations and personal notes about the American frontier environment and his experiences in colonial America.
Mason and Dixon failed to measure the entire length of the south boundary of Pennsylvania as determined by its charter. In the summer of 1767, the surveying party crossed the Monongahela River and the Great Catawba War Path, violating a treaty limiting the westward expansion of English settlements. Not wishing to risk inciting native hostilities, Mason and Dixon were forced to return east after making their final observations at the crest of Brown's Hill.
Career after the Mason–Dixon line survey 
After completing the boundary survey in America, Mason returned to Greenwich where he continued work on Mayer's Lunar Tables. He also contributed to the Nautical Almanac, working under Nevil Maskelyne, Astronomer Royal.
On September 27, 1786, Mason wrote to Benjamin Franklin claiming to have returned to Philadelphia with his wife, seven sons, and one daughter. Mason was very ill and confined to his bed. Mason also shared with Franklin the design for an astronomical project. Mason provided no explanation for his return to America, and nothing more is known of Mason's proposed project.
Mason died on October 26, 1786, in Philadelphia.
Posthumous recognition 
- Cope, Thomas D. and H. W. Robinson. "Charles Mason, Jeremiah Dixon and the Royal Society." Notes and Records of the Royal Society of London. vol 9, no 7, 1951. p 75.
- Cope, Thomas D. "Some Contacts of Benjamin Franklin with Mason and Dixon in Their Work". Proceedings of the American Philosophical Society. 95 (1951) p 238
- Mason and Dixon Line Preservation Partnership—Information about Charles Mason and the Mason and Dixon Line]
- Mason's logbook for his 1761-2 voyage to observe the Transit of Venus. Digitized at the University of Pennsylvania
- Mason's biography from the American National Biography
- Account of Mason's Observations of the Transit of Venus in 1769 (PDF)
- Charles Mason at Find a Grave