As mentioned previously, the U.S. Census Bureau recently announced the 2010 mean centre of the United States
The center is determined as the place where an imaginary, flat, weightless and rigid map of the United States would balance perfectly if all residents were of identical weight.
Records for this place go back to 1790. The precise location changing over time, reflecting immigration and migration west and south. The Census Bureau has now released an animated (Flash) map displaying the evolution of its position, indicating which States were in the Union at the time. Check it out below: