At the Harvard College Observatory in Cambridge, Massachusetts, three floors of metal cabinets house more than 550,000 glass plates, most of them eight by 10 inches, a photographic negative format dating from the mid-19th century. These plates recorded astronomical data from telescopes trained on celestial regions and objects. One side bears the print of light from distant stars; the other side had been marked with equations, arrows, circles, letters, and other notations by women who were hired to interpret the data.
From 1885 until the 1950s, hundreds of so-called women computers studied the plates. They discovered how variations in brightness of specific stars revealed their energy output, a relationship that provided a way to measure great distances.