A 'human computer' at Harvard found the ruler that measures the universe

Henrietta Swan Leavitt was hired to catalogue star brightness from photographic plates, not to theorise. In 1908 she noticed that pulsating stars called Cepheid variables blink slower the brighter they intrinsically are — a relationship precise enough to use as a distance marker. Edwin Hubble later used her period-luminosity law to show the universe is expanding.

— Henrietta Swan Leavitt, 1777 Variables in the Magellanic Clouds — Annals of Harvard College Observatory, 1908