Henrietta Leavitt (1868-1921), US astronomer. Leavitt graduated from Radcliffe College in 1892, and joined the Harvard College Observatory in 1895. Her early work on the photographic magnitudes of stars led to her work on Cepheid variables. Using plates made of the Magellanic Clouds, Leavitt discovered about 2400 new variable stars. She also noticed that the brightness variation in the Cepheids was extremely regular, and that the brighter stars had longer periods. By 1912, she had established that the apparent magnitude varied as the logarithm of the period. This was the foundation of the method now used to measure extreme cosmic distances.