Twelve-Mile Circle

Twelve-Mile Circle

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.

Its existence dates from a deed to William Penn from the Duke of York on August 24, 1682, which granted Penn:

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.