The courthouse at the center of the Mason-Dixon line

The New Castle Court House Museum is the center of a circle with a 12-mile radius. In 1682, James, the Duke of York, gave William Penn a deed for land consisting of a 12-mile circle around the town of New Castle. That circle still defines most of the border between the states of Delaware and Pennsylvania, and parts of the borders between Delaware and New Jersey and Maryland.

A legal struggle over the borders lasted for 87 years between the Penn and Calvert families. A Commission led by Governor Patrick Gordon of Pennsylvania and Governor Samuel Ogle of Maryland met in the New Castle Court House four times in 1732 and 1733. By 1750, a Commission including William Allen and Benjamin Chew met in the Court House, where it was agreed that the cupola of the Court House building would be used as the center of the 12-mile circle.

From this point, Charles Mason and Jeremiah Dixon conducted the final survey of the borders in 1763. They ran a series of chains on a straight 12-mile line from the Court House to create the Mason-Dixon line. The British crown ratified the border and resolved the dispute in 1769.

Today, the building is a museum and a National Historic Landmark. You can walk through the space where the Pennsylvania colonial assembly met in 1700 and where the Delaware assembly chose William Rodney as their first Speaker in 1704. It remains a contributing property to the New Castle Historic District, still holding the center of the circle.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons