Geographic Center of US Population
The Census Bureau has created an interactive map that shows the geographic center of United States population as of each census, starting in 1790, through the last census (2010). The map also visually demonstrates the addition of states through the census years.
As you can see from the map, the American population has shifted west and the south over the past 120 years. This reflects the sweep of the American population to the frontier, pushing it onward as it was settled and new lands opened up.