Henrietta Leavitt, the woman who first saw beyond the limits of the Milky Way. Forbidden telescope time, she discovered the period-luminosity relation that allowed astronomers to measure the distance to extra-galactic stars. She gave us the first step on a ladder that we still use to measure cosmic distances today. Working at Harvard College Observatory in 1912 she showed a link between the brightness of a Cepheid variables star and how long it takes to brighten and dim. This property means Cepheid variables are immensely useful for measuring distances in the universe.