The Twelve Mile Circle, Part I

On November 6, 2008 · 2 Comments

Today is the one-year Anniversary of this website, the Twelve Mile Circle. However I’ve yet to explain the Twelve Mile Circle behind the name. I promised myself I would do so if I could keep this site active for a year, and I’ve now met that goal. So here goes.

As many regular readers have already guessed, it refers to the arc that forms the preponderance of the border between Delaware and Pennsylvania. I find it particularly fascinating. Borders are generally based upon straight lines that have been formally delineated by surveyors or upon natural phenomena such as rivers, ridgelines, or coasts. Imagine, though, the sheer wackiness of a boundary based on a circle.


Delaware & Pennsylvania Twelve Mile Circle
SOURCE: Wikimedia Commons, which notes that the map was obtained from an edition of the National Atlas of the United States, which is in the public domain.


This image gives a pretty good overview of the border between the two states. Clearly it is a perfect arc. This one happens to have a 12-mile radius. Notice the little red dot at New Castle which is the exact center. This unusual arrangement has roots in colonial times. Very simply put, the Duke of York was granted all lands within a circle of twelve miles surrounding New Castle. Eventually he conveyed his Delaware holdings to William Penn who decided to keep these lands distinct from Pennsylvania even though they were now both part of his domain. The circle survived.



View Larger Map

This is the exact center of the circle. It’s the old courthouse in New Castle. Specifically it’s the cupola right at the top of the courthouse. The circle may have been well-defined by land records but it’s precise measurement on the ground was problematic due to its unusual shape. The 22.57 mile arc has been surveyed several times including 1710 by Taylor and Pierson and again in 1892-1893 by modern surveyors using more sophisticated techniques.



View Larger Map

Imagine the challenge of surveying this border. Rather than tracking a straight line, the surveyors needed to account for an infinite number of twelve-mile spokes emanating from New Castle.

In upcoming post I’ll go into further detail on the numerous anomalies of the Delaware border. This is just a brief introduction of one aspect. Perhaps you’ve already noticed another anomaly (hint: look immediately to the left of the border arc). Stay tuned!

On November 6, 2008 · 2 Comments

2 Responses to “The Twelve Mile Circle, Part I”

  1. even ignoring flaws in the survey work
    this was actually not a perfect arc at all
    but at least 3 different arcs of different radii
    only one of which was centered on the new castle courthouse

    but all were imperceptibly feathered together
    as if they were a single perfect arc

    close observation may perhaps distinguish as many as 6 utterly different arc segments involving all the 4 states

    & tho rare in the world
    arc boundaries are less rare than say quintipoints

    for example
    plains georgia
    like many other georgia cities
    is theoretically a perfect circle

    but arcs both relatively perfect & imperfect are found even in some international boundaries

    i mean without considering geodetic arcs
    such as of latitude or longitude

    & of course there have always been surveyors tricks that greatly simplify what looks so terribly complicated to the layperson

    but i still agree with your basic premise

    this so called 12 mile so called circle
    even when it is closer to 11 or 13 miles
    & especially where it is flagrantly irregular
    is the most fascinating bit of state line we have

  2. […] most visible manifestation of the so-called Twelve Mile Circle is an arc-shaped portion of the border between Delaware and Pennsylvania, as noted in the previous entry. However it’s not the only impact discernible. The oddity […]

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12 Mile Circle:
An Appreciation of Unusual Places
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