While driving from Philadelphia to Wilmington Saturday nite, I noticed on the map the rather unusual and interesting circular border with Pennsylvania and figured there must be a story behind it.
Apparently it is called the '12 mile boundary', and is the arc of a circle formed from the "New Castle Court House" located in the center of New Castle, Delaware, which was an early capital of the Delaware territory in the 18th century. The red star below is from mapquest and is in fact the town of New Castle.
So that explains that.
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