American astronomer and archaeologist who established the principles of dendrochronology (the dating and interpreting of past events by the analysis of tree rings). He coined the name of that study when, while working at the Lowell Observatory, Flagstaff, Ariz. (1894–1901), he began to collect tree specimens, believing that variations in the width of tree rings would show a connection between sunspot activity and the terrestrial climate and vegetation.

Douglass taught astronomy (from 1906) and dendrochronology (from 1936) at the University of Arizona and directed that university's Steward Observatory, Tucson (1918–38). Among his achievements in astronomy was the first photograph of the zodiacal light. He was also an authority in the study of Mars.