(1868-1921) US astronomer who in 1912 discovered the period–luminosity law, which links the brightness of a Cepheid variable star to its period of variation. This law allows astronomers to use Cepheid variables as “standard candles” for measuring distances in space.
Leavitt was born in Lancaster, Massachusetts, and studied at what was to become Radcliffe College. She joined the Harvard College Observatory 1902, and was ultimately appointed head of the department of photographic stellar photometry.
Leavitt's work grew out of director Edward Pickering's research program at the observatory, toward the establishment of a standard photographic sequence of stellar magnitudes. She discovered a total of 2,400 new variable stars and four novae.
Leavitt's period–luminosity curve for the Cepheids enabled Danish astronomer Ejnar Hertzsprung and US astronomer Harlow Shapley to read the data in terms of absolute rather than apparent magnitude. By comparing a Cepheid's apparent magnitude with its absolute magnitude, the distance of the star from the Earth could be deduced. Until then it had not been known that they are outside our Galaxy.