Make a rigid map of the United States and add a weight for each person in the country. Then try to balance it on your finger.

In 1790, you’d find a neat equilibrium point under Chestertown, Md. — but you’d have to keep shifting as settlers moved west. By 1850 the balance point had slid into West Virginia; in 1900 it reached Indiana.

It’s still moving. During the 20th century, the point shifted another 324 miles to the west, into Phelps County, Mo. That includes a dramatic jump of 10 miles in a single year — in 1960, when Alaska and Hawaii first appeared on the census.