On June 1, 1908, Henrietta Swan Leavitt published her first systematic study of variable stars in the Small Magellanic Cloud — work that would eventually give astronomers a ruler capable of measuring the universe itself.

Leavitt was a "computer" at the Harvard Observatory, one of the women hired to catalog stars because they were paid less than men. Her job was to measure photographic plates, not to theorize. She did it anyway.

What she found in those plates was a pattern: certain stars, called Cepheid variables, pulsed brighter and dimmer in a precise rhythm. The slower the pulse, the more intrinsically luminous the star. This was not a coincidence — it was a law. And it meant that if you could measure a Cepheid's period, you could calculate its true brightness, compare that to how bright it appeared, and derive its distance.

She published the discovery. Her director, Edward Pickering, put his name on the paper. For decades, the method was called "Pickering's law."

Edwin Hubble used Leavitt's discovery to prove that other galaxies exist and that the universe is expanding. He later remarked that she should have received the Nobel Prize. She died of cancer in 1921, before she could be nominated.