American astronomer who specialized in the classification of stellar spectra (star).

Cannon graduated from Wellesley College in 1884. For several years thereafter she traveled and dabbled in photography and music. In 1894 she returned to Wellesley for a year of advanced study in astronomy, and in 1895 she enrolled at Radcliffe in order to continue her studies under Edward C. Pickering (Pickering, Edward Charles), who was director of the Harvard College Observatory. In 1896 she was named an assistant at the Harvard Observatory. There, joining Williamina P.S. Fleming (Fleming, Williamina Paton Stevens), she devoted her energies to Pickering's ambitious project, begun in 1885, of recording, classifying, and cataloging the spectra of all stars down to those of the ninth magnitude. The scheme of spectral classification by surface temperature used for the project and later (1910) universally adopted was largely work that Cannon had developed from earlier systems, and she eventually obtained and classified spectra for more than 225,000 stars. Her work was published in nine volumes as the Henry Draper Catalogue (1918–24).

In 1911 Cannon succeeded Fleming as curator of astronomical photographs at the observatory, and in 1938 she was named William Cranch Bond Professor of Astronomy. After 1924 she extended her work, cataloging tens of thousands of additional stars down to the 11th magnitude for the two-volume Henry Draper Extension (1925, 1949). The work was an invaluable contribution to astronomy, bearing strongly on countless other problems and areas of research and exerting major influence on the evolution of the science of astronomy from one of mere observation to one of great theoretical and philosophical content. In the course of her work Cannon also discovered some 300 variable stars and 5 novae.