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The Twelve-Mile Circle is an approximately
circular arc A circular arc is the arc of a circle between a pair of distinct points. If the two points are not directly opposite each other, one of these arcs, the minor arc, subtends an angle at the center of the circle that is less than radians (180 ...
that forms most of the boundary between
Delaware Delaware ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic and South Atlantic states, South Atlantic regions of the United States. It borders Maryland to its south and west, Pennsylvania to its north, New Jersey ...
and
Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...
. It is a combination of different circular arcs that have been feathered together. It is nominally a
circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre. The distance between any point of the circle and the centre is cal ...
with a variable
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
of approximately centered in the town of
New Castle, Delaware New Castle is a city in New Castle County, Delaware, United States. The city is located six miles (10 km) south of Wilmington, Delaware, Wilmington and is situated on the Delaware River. As of 2020, the city's population was 5,551. New Cast ...
. In 1750, the center of the circle was fixed at the
cupola In architecture, a cupola () is a relatively small, usually dome-like structure on top of a building often crowning a larger roof or dome. Cupolas often serve as a roof lantern to admit light and air or as a lookout. The word derives, via Ital ...
of the courthouse in New Castle. The Twelve-Mile Circle continues into the
Delaware River The Delaware River is a major river in the Mid-Atlantic region of the United States and is the longest free-flowing (undammed) river in the Eastern United States. From the meeting of its branches in Hancock, New York, the river flows for a ...
. A small portion of the circle, known as the "Arc Line," forms part of the Mason-Dixon line in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
that separates Delaware and
Maryland Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It borders the states of Virginia to its south, West Virginia to its west, Pennsylvania to its north, and Delaware to its east ...
. Two other small portions, although not actually demarcated until 1934, form parts of the boundary between the states of Delaware and
New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...
. Although the Twelve-Mile Circle is often claimed to be the only territorial boundary in the United States that is a true arc, some cities in the South, including
Plains, Georgia Plains is a city in Sumter County, Georgia, United States. As of the 2020 census, the population of the city was 573. It is well-known as the home of Jimmy Carter and his wife Rosalynn, who were the 39th president and first lady of the Un ...
, have circular boundaries. The Twelve-Mile Circle is, however, the only circular U.S. state boundary. Its existence dates back to 1681, when King Charles II granted a deed to
William Penn William Penn ( – ) was an English writer, religious thinker, and influential Quakers, Quaker who founded the Province of Pennsylvania during the British colonization of the Americas, British colonial era. An advocate of democracy and religi ...
north of the already chartered Maryland. Charles created an exception, consisting of 12 miles around New Castle and extending down the peninsula, since these lands were held by the
Duke of York Duke of York is a title of nobility in the Peerage of the United Kingdom. Since the 15th century, it has, when granted, usually been given to the second son of List of English monarchs, English (later List of British monarchs, British) monarchs ...
, who had won them in conquest from the Dutch colonists. Later, on August 24, 1682, the Duke granted these lands to Penn as well, giving him: The boundaries of the circle were the focal point of the 80-year
Penn–Calvert boundary dispute The Penn–Calvert boundary dispute (also known as ''Penn vs. Baltimore'') was a long-running legal conflict between William Penn and his heirs on one side, and Charles Calvert, 3rd Baron Baltimore and his heirs on the other side. The overlapping ...
. The fact that the circle extends into the Delaware River makes for an unusual territorial possession; within the 12-mile circle, all the Delaware River to the low-tide mark on the east (
New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...
) side is territory of the state of Delaware, leaving the river - and bridges - in sole possession of Delaware. 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 Hugo Grotius ( ; 10 April 1583 – 28 August 1645), also known as Hugo de Groot () or Huig de Groot (), was a Dutch humanist, diplomat, lawyer, theologian, jurist, statesman, poet and playwright. A teenage prodigy, he was born in Delft an ...
) or, more often, the center of the main flow channel, or
thalweg In geography, hydrography, and fluvial geomorphology, a thalweg or talweg () is the line or curve of lowest elevation within a valley or watercourse. Normally only the horizontal position of the curve is considered (as viewed on a map); the c ...
(the lowest point in the stream channel). A similar condition exists between the states of
Vermont Vermont () is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, New Hampshire to the east, New York (state), New York to the west, and the Provinces and territories of Ca ...
and
New Hampshire New Hampshire ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec t ...
. 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. In 1813, Delaware's legislature passed an act deeding
Pea Patch Island Fort Delaware State Park is a , Delaware state park on Pea Patch Island in the mid channel of the Delaware River near its entrance into Delaware Bay. It is a low, marshy island in New Castle County, Delaware, facing Delaware City on the Dela ...
to the United States government, and in 1820 New Jersey disputed that they owned the island since it was primarily on the New Jersey side of the river. Attorney General William Wirt sided with Delaware. In the 1840s, the Pea Patch Island disagreement led to two conflicting circuit court decisions—the circuit of Delaware ruling that the entirety of the river (and its islands) belonged to Delaware, and the circuit in New Jersey ruling that the island had belonged to New Jersey, which had deeded it to Dr. Henry Gale, a citizen of New Jersey. At the recommendation of President
James K. Polk James Knox Polk (; November 2, 1795 – June 15, 1849) was the 11th president of the United States, serving from 1845 to 1849. A protégé of Andrew Jackson and a member of the Democratic Party, he was an advocate of Jacksonian democracy and ...
, the parties agreed to arbitration, which resulted in a confirmation of Delaware's claim. The arbitration did not ultimately resolve the dispute, and it has been brought to the
Supreme Court of the United States The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all Federal tribunals in the United States, U.S. federal court cases, and over Stat ...
on several occasions (all titled '' New Jersey v. Delaware''), most notably in 1907, 1934, 1935, and 2008. The court's opinion for the 1934 case contained an extensive history of the claims to this territory, and the 1935 opinion enjoined 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 Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volume o ...
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 In the law of the United States, a special master is an official appointed by a judge to ensure judicial orders are followed, or in the alternative, to hear evidence on behalf of the judge and make recommendations to the judge as to the dispositi ...
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 The Delaware State House of Representatives is the lower house of the Delaware General Assembly, the state legislature of the U.S. state of Delaware. It is composed of 41 Representatives from an equal number of constituencies, each of whom is ...
considered a (symbolic) bill to call out the
National Guard National guard is the name used by a wide variety of current and historical uniformed organizations in different countries. The original National Guard was formed during the French Revolution around a cadre of defectors from the French Guards. ...
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 are persistent rumors that
David Rittenhouse David Rittenhouse (April 8, 1732 – June 26, 1796) was an American astronomer, inventor, clockmaker, mathematician, surveyor, scientific instrument craftsman, and public official. Rittenhouse was a member of the American Philosophical Society an ...
, 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, guessed that Rittenhouse assisted with latitude or longitude calculations.Brooke Hindle, ''David Rittenhouse'', Princeton University Press, 1964, pp. 20–21.


See also

*
Penn–Calvert Boundary Dispute The Penn–Calvert boundary dispute (also known as ''Penn vs. Baltimore'') was a long-running legal conflict between William Penn and his heirs on one side, and Charles Calvert, 3rd Baron Baltimore and his heirs on the other side. The overlapping ...
* Delaware Wedge *
Transpeninsular Line The Transpeninsular Line (at approximately 38°27′ N) is a surveyed line, the eastern half of which forms the north–south border between Delaware and Maryland. The border turns roughly north from the midpoint of the line towards the Twelve-M ...
* Killcohook National Wildlife Refuge * Mason-Dixon line


References

{{Reflist, colwidth=35em Border irregularities of the United States Borders of Delaware Borders of Maryland Borders of New Jersey Borders of Pennsylvania Geography of Cecil County, Maryland Geography of Chester County, Pennsylvania Geography of Delaware County, Pennsylvania Geography of Maryland Geography of New Castle County, Delaware Geography of Salem County, New Jersey Internal territorial disputes of the United States