If you look at the picture here, it appears that the arc forms part of the land borders between Delaware and Pennsylvania, Delaware and New Jersey, and Delaware and Maryland (look at the parts in red).
Washington D.C. is only ten square miles.
No, it’s ten miles square - or rather, it was originally. Because some of it was returned to Virginia instead of being 100 square miles it’s only 68.3 square miles.
68.3 sq mi (177.0 km²), of which land is 61.4 sq mi (159.0 km²). It used to be a square with 10 miles on each side (100 sq mi), but part of the original square was retroceded to Virginia, and now forms Arlington and part of Alexandria.
Although, as already noted, you have to go through Canada to get there. 
In fact, the westernmost point of land in South America, (a bit Southwest of Talara, Ecuador, is farther East than Cleveland, Atlanta, or (most of) Jacksonville.
A slight tangent, but I’ve always found the term midwest to be misleading, seeing as how it’s largely in the eastern half of the country. Likewise Northwestern University is not where you’d expect it to be.
Not that I doubt you, but this is the first I’ve heard of border guards between the USA and Canada. Isn’t it supposed to be the longest undefended border in the world?
Not to mention Gretna FC, which is based in Scotland but used to play in the English Unibond League. They transferred to the Scottish Third Division, and at the End of last season achieved promotion to the Scottish Premier League after only 5 years.
Undefended by the military. There are still border guards at the crossings.
David Simmons, I grew up about forty miles from Kentucky Bend and heard references to the area, but no details. This is the first time that I have know about the part of Kentucky located in Missouri accessible through Tennessee. What a hoot! I’ve been to Reel Foot Lake, Tiptonville, and that section of the Mississippi – just never paid any attention to “the Bend.”
Now I want to go out of idle curiousity.
And thanks for bringing it to our attention, user_hostile.
The city of Lloydminster is in both Alberta and Saskatchewan. Now you might ask what so unusual about that? After all, Kansas City is in both Kansas and Missouri, Texarkana is in both Texas and Arkansas, Bristol is on the TN-VA border, Bluefield straddles the VA-WV line, and a number of much smaller places dot various state borders here and there around the country.
What makes Lloydminster different is that all those others are actually two separate municipalities with their own mayors and councils (if they’re incorporated – not all of them are). Lloydminster is a single municipality which both provinces consider to be entirely within their respective borders. Back in 1930 or thereabouts, the parliaments of the two provinces passed simultaneous laws setting up this condominium (as this situaton is called).
So residents of Lloydminster can attend public universities in either province and pay in-province tuition, for example. Not sure how legal issues having to do with an action being legal in one province and illegal in the other are handled. But as I understand it, criminal law in Canada is a federal function, so this problem would only apply to civil issues.
The Wiki article makes it sound as if the river course had already changed when the accurate survey was done, and they discovered the oddity. I know Kentucky joined the Union before 1811, but maybe they had only fixed rough “lines on a map” borders before that?
What can I say? I’m a Dapper Dan man.
I’m glad someone caught the reference. 
Shoot. I wish I could figure out how to link to a Google Earth page, but anyhow…
Here around the Twin Cities, MN, it is tough to make the claim that something is “the biggest thing East (OR West) of the Mississippi.” I usually have to check a map and say “Well, we are West of the Mississippi twice, and East of it once…” It gets all twisty up here!
The arc of the Twelve-Mile Circle is most of the Delaware-Pennsylvania border (a small portion extends west along the top of “The Wedge” to join the Circle to the Mason-Dixon Line separating Maryland from Delaware and Pennsylvania). But in addition:
(a) It also includes a very small shaving of what would otherwise be Maryland, where the curve of the circle extends marginally west of the north-south Mason-Dixon Line;
(b) Within 12 miles of old New Castle Courthouse the entirety of the Delaware River is within the state, which is not merely a trivia fact but important to shipping and riparian development – for example, if you own a riverside home in New Jersey, you need to get permission to build a dock from Delaware;
(c) The tip of one peninsula on the Jersey Shore of the Delaware River extends north across the 12-Mile Circle and hence is within the State of Delaware, even though east of the river. This last is what my “land boundary between Delaware and New Jersey” referenced.
I suppose the Kentucky-Tennesee border was established as along some line of latitude before the area had been completely explored. It must have been known before the earthquake that the Kentucky border ended on the bank of the Mississippi in the vicinity of some landmarks that survived the earthquake. When they got around to an accurate survey it turned out that that little piece was cut off but Kentucky wasn’t about to cede it. “Dammit, our line ends on the Mississippi near Pilot Rock and that’s where it’s a goin’ to stay.” Just a WAG
I was replying to someone who asserted exactly that something SHOULD refute something to be here. I was asserting it didn’t need to. Duh.
Spectre of Pithecanthropus explains above exactly why this was offered as “odd.” And I would tend to agree that most people outside of Southern Illinois wouldn’t think easily of the tip of Illinois as being in the backyard of the South. Whether that makes is “odd” or not is, of course, up to every person’s individual opinion. 
I don’t want Fop!
I’m not sure what the significance of this is supposed to be. On any given day, there will be a whole line of points where the Sun rises at the same point as at any given point. And what that line is for each point will vary through the year, so you can’t even absolutely state that three particular points are on the same line.
Not anything to float your boat I admit, but these points are not in line with each other it’s the elevation difference that makes it possible with Mt Washington about 200 miles west of Eastport ME.