Henrietta Swan Leavitt, American astronomer and academic (d. 1921)
Henrietta Swan Leavitt (; July 4, 1868 – December 12, 1921) was an American astronomer. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measure and catalog the brightness of stars. This work led her to discover the relation between the luminosity and the period of Cepheid variables. Leavitt's discovery provided astronomers with the first "standard candle" with which to measure the distance to faraway galaxies.Before Leavitt discovered the period-luminosity relationship for Cepheid variables, the only techniques available to astronomers for measuring the distance to a star were based on parallax and triangulation. Such techniques can only be used for measuring distances up to hundreds of light years. Leavitt's work allowed astronomers to measure distances up to about 20 million light years. As a result of this, it is now known that our own galaxy, the Milky Way, has a diameter of about 100,000 light years. After Leavitt's death, Edwin Hubble used Leavitt's period-luminosity relation, together with the galactic spectral shifts first measured by Vesto Slipher at Lowell Observatory, to establish that the universe is expanding (see Hubble's law).