•Physics|ABL•

In 1908, Henrietta Leavitt was working as a computer at Harvard College Observatory, paid about 30 cents an hour to catalogue stars on photographic plates. Studying thousands of images of the Small Magellanic Cloud, she noticed that the pulsating stars called Cepheids followed a rule: the slower they pulsed, the brighter they truly were. By 1912 she had made the rule precise enough to measure distances across space. Her rule became the first rung of the cosmic distance ladder. Edwin Hubble used it to show the universe is expanding, and today it sits at the centre of cosmology's biggest dispute: measurements built on Cepheids give an expansion rate of about 73, while the early universe predicts about 67. The James Webb Space Telescope rechecked the stars and found no error, though a rival team using different stars gets about 70 and doubts the gap is real. Leavitt died in 1921; three years later, a member of the Swedish Academy wrote to her, hoping to nominate her for a Nobel Prize. A rule found by a clerk on 30 cents an hour now decides whether our model of the universe is complete.