Twelve-Mile Circle

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The Twelve-Mile Circle is an approximately circular arc that forms most of the boundary between Delaware and Pennsylvania. It is a combination of different circular arcs that have been feathered together. It is nominally a circle with a variable radius of approximately 12 miles (19 km) 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. The Twelve-Mile Circle continues into the Delaware River. A small portion of the circle, known as the "Arc Line," forms part of the Mason–Dixon line in the United States that separates 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. The Twelve-Mile Circle is one of few circular territorial boundaries in the United States, along with some cities in the South, such as Plains, Georgia, and state boundaries defined by circles of latitude. Its existence dates...

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