"The mean center of U.S. population is determined by the United States Census Bureau from the results of each census. The Bureau defines it to be: the point at which an imaginary, flat, weightless, and rigid map of the United States would balance perfectly if weights of identical value were placed on it so that each weight represented the location of one person on the date of the census.[" - Amit Patel from Bookmarklet

It's still moving! - Amit Patel

My hometown was the center of population in the 1920 census. In the 1970's or so, the local high school mascot was the cops for Center Of Population. - Clare Dibble