The way to measure the universe was found by a woman hired at thirty cents an hour

She spent her days on photographic plates of the Magellanic Clouds and found 1,777 variable stars on them, stars that brighten and fade. As early as 1908 she saw that the brighter ones had longer periods, but there were too few to be sure. In 1912 she lined up 25 variables from the Small Cloud by period and wrote of a "remarkable relation" between their brightness and the length of their periods. Since they are probably all at nearly the same distance from Earth, the period seemed to be tied to how much light each star actually gives off.

That became a ruler for the cosmos, though at first without markings: her law gave only relative distances, and others later calibrated the scale. Leavitt never got the chance to take part, because Pickering kept her on other projects. Edwin Hubble used the ruler on the Andromeda nebula and showed it was a separate galaxy far beyond the Milky Way. Measuring more and more galaxies, he found the first observational evidence that the universe is expanding.

Leavitt never knew. She died of cancer on 12 December 1921. Today her discovery is called Leavitt's Law, and astronomers still use it to measure the distances to galaxies. In 1925 the mathematician Gösta Mittag-Leffler wrote to her at Harvard that he was seriously inclined to nominate her for the Nobel Prize in Physics. He did not know she had died.