Henrietta Swan Leavitt 

Henrietta Swan Leavitt (1868–1921) was an astronomer whose discovery of how to measure vast astronomical distances led to a shift in the understanding of the scale and nature of the universe. 

Born in Lancaster, Massachusetts, she was educated at Oberlin College (1886–1888) and at Harvard University’s Society for the Collegiate Instruction of Women (later Radcliffe College, B.A., 1892). Leavitt became a volunteer assistant at the Harvard Observatory in 1895. In 1902, she received a permanent staff appointment. She worked on the Observatory’s project, begun by Edward C. Pickering, to determine the brightnesses of all measurable stars. In this work, she was associated with Williamina Fleming and Annie Jump Cannon (2026 Legacy Recognition honoree). Leavitt soon advanced from routine work to a position as head of the photographic stellar photometry department. A new phase of the work began in 1907 with Pickering’s ambitious plan to ascertain photographically standardized values for stellar magnitudes. Leavitt’s North Polar Sequence was adopted for the Astrographic Map of the Sky, an international project undertaken in 1913, and by the time of her death she had determined magnitudes for stars in 108 areas of the sky. 

Her system remained in general use until improved technology made possible photoelectrical measurements of far greater accuracy. One result of her work on stellar magnitudes was her discovery of four novas and some 2,400 variable stars, the latter comprising more than half of all those known by 1930. Leavitt continued her work at the Harvard Observatory until her death in 1921. 

Leavitt’s outstanding achievement was her discovery in 1912 that in a certain class of variable stars, the Cepheid variables, the period of the cycle of fluctuation in brightness is highly regular and is determined by the actual luminosity of the star. The subsequent calibration of the period-luminosity curve allowed American astronomers Edwin Hubble, Harlow Shapley (elected to the Academy in 1920; President of the Academy from 1939–1944), and others to determine the distances of many Cepheid stars and consequently of the star clusters and galaxies in which they were observed. Leavitt was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, and the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers. Asteroid 5383 Leavitt and the crater Leavitt on the Moon are named after her.