The 12-Mile Circle is an approximately circular arc which forms most of the boundary between the Commonwealth of Pennsylvania and the State of Delaware in the United States. It is not actually a circle, but rather a combination of different circular arcs that have been imperceptibly feathered together.
It is nominally a circle with a supposed—yet in fact only approximate and variable—12-mile radius, centered in the town of New Castle, Delaware. In 1750, the center of the circle was fixed at the cupola of the courthouse in New Castle (however, subsequent changes have also been based on a different center point, several thousand feet to the northwest of the courthouse). The 12-Mile Circle continues into the Delaware River. A small portion of the circle, known as the "Arc Line," also forms part of the Mason-Dixon line, separating Delaware and Maryland. Two other small portions, although not actually demarcated until 1934, form parts of the boundary between the states of Delaware and New Jersey. Although the 12-Mile Circle is often claimed to be the only territorial boundary in the United States that is a true arc (excepting those following arcs of latitude and longitude), the Mexican boundary with Texas includes several arcs, and many cities in the South (such as Plains, Georgia) also have circular boundaries.
|“||all that the Towne of Newcastle otherwise called Delaware and All that Tract of Land lying within the Compass or Circle of 12 Miles about the same scituate lying and being upon the River Delaware in America And all Islands in the same River Delaware and the said River and Soyle thereof lying North of the Southermost part of the said Circle of 12 Miles about the said Towne.||”|
The fact that the circle extends into the Delaware River makes for an unusual territorial possession. Most territorial boundaries that follow watercourses split the water course between the two territories by one of two methods, either by the median line of the watercourse (the Grotian Method, after Hugo Grotius) or, more often, the center of the main flow channel, or thalweg. However, due to the text of the deed, within the 12-Mile Circle, all the Delaware River to the low-tide mark on the east (New Jersey) side is territory of the state of Delaware.
New Jersey has often disputed this claim, as the rest of its territorial boundaries along the Delaware River are determined by the midline and thalweg methods. The dispute has been brought to the Supreme Court of the United States on three occasions (all titled New Jersey v. Delaware), most notably in 1934, and also in 1935 and 2007. The court's opinion for the first case contains an extensive history of the claims to this territory, and the second memorably enjoins New Jersey and Delaware from ever disputing their jurisdictions again.
Regardless of the Supreme Court's admonition to the two states against further litigation on this subject, they were back before the court as late as November 2005, when New Jersey's desire to approve plans by BP to build a liquefied natural gas terminal along the New Jersey shore of the Delaware River fell afoul of Delaware's Coastal Zone Act. The court on January 23, 2006, appointed a special master to study the border dispute, and on March 21, 2008, it upheld his report, which largely supported Delaware's authority. Meanwhile the Delaware House of Representatives considered a (symbolic) bill to call out the National Guard to safeguard the State's interests, while New Jersey legislators made comments about the battleship New Jersey, moored upriver from the site.
Surveying the Circle 
There is a persistent rumor that David Rittenhouse, the famous astronomer and instrument-maker of Pennsylvania, surveyed the circle around New Castle, but this is likely not correct. The circle was first laid out by surveyors named Taylor and Pierson in 1701. The 1813 "Memoirs of the life of David Rittenhouse" by William Barton surmises that Rittenhouse was involved in such a survey in 1763, based on a letter in which Rittenhouse mentions being paid "for my attendance at New Castle," but there is no clear record of what, exactly, Rittenhouse was paid for. His biographer, Brooke Hindle, guesses that Rittenhouse assisted with latitude or longitude calculations.
See also 
- Schenck, William S. (2007). "Delaware's State Boundaries". Delaware Geological Survey. Retrieved 2008-11-17.
- [Perry L., 1934, The circular boundary of Delaware, Civil Engineering: v. 4, no. 11, p. 576-580.]
- State of New Jersey v. State of Delaware, 291 US 361
- State of New Jersey v. State of Delaware, 295 US 694
- The state of Delaware, [is] perpetually enjoined from disputing the sovereignty, jurisdiction, and dominion of the state of New Jersey over the territory adjudged to the state of New Jersey by this decree; and the state of New Jersey, [is] perpetually enjoined from disputing the sovereignty, jurisdiction, and dominion of the state of Delaware over the territory adjudged to the state of Delaware by this decree.
- "NJ to file lawsuit against Delaware over Delaware River jurisdiction". Philadelphia Business Journal. July 28, 2005.
- "DE Receives Favorable Recommendation in U.S. Supreme Court Dispute with NJ over Regulation of Wharves on the Delaware River". Connolly Bove Lodge & Hutz LLP. April 2007.
- "New Jersey v. Delaware". Cornell University Law School. March 2008.
- Search archives of DelawareOnline.com for "BP gas terminal dealt setback", January 24, 2006
- Thomas D. Cope, "Some Local Scholars Who Counselled the Proprietors of Pennsylvania and their Commissioners during the Boundary Surveys of the 1760s," Proceedings of the American Philosophical Society, vol. 99, no. 4 (August 1955), pp. 268-276, p. 275.
- Brooke Hindle, David Rittenhouse, Princeton University Press, 1964, pp. 20-21.