I had never heard this before. I was unable to find it in the Land Ordinance of 1785 or the Northwest Ordinance of 1787. Nor on any website. However, if you stretch that out to maybe 80 miles or so, it would seem to apply to former Ohio capitals Chillicothe, Zanesville, as well as Indianapolis, Springfield, Vandalia (old capital of Illinois), and Lansing. Two problems: Detroit was the capital of Michigan until 1842, and Madison???
I feel obligated to share this fact I found about the history of Columbus:
[quote]
Due to scandal in 1842 the legislature considered moving the capital to Newark or Lancaster; the capitol building would become an insane asylum. li[/li][/quote]
Behold, it has come to pass even without moving.