Scientific cosmology is a tale of three stupendous realisations: the Earth is one of many planets, the sun is one of myriad stars and the Milky Way is just one of countless galaxies. The last of these mental expansions occurred in the early 1920s, and it hinged upon an earlier discovery that was made by a computer-not a machine, but a woman. In 1895 Henrietta Leavitt became part of a team of women who were employed as "computers" by the Harvard College Observatory. For $0.25 an hour, the computers (they were actually called this) pored over photographs of the sky, counting and classifying thousands of stars. One observatory director, Harlow Shapley, actually measured a project's difficulty by the number of "girl-hours" required. Yet out of this tedium came forth crucial data for Harvard astronomers.
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