Giovanni Battista Donati, (born Dec. 16, 1826, Pisa [Italy]—died Sept. 20, 1873, Florence), Italian astronomer who, on Aug. 5, 1864, was first to observe the spectrum of a comet (Comet 1864 II). This observation indicated correctly that comet tails contain luminous gas and do not shine merely by reflected sunlight.
Between 1854 and 1864 Donati discovered six comets, one of which, first seen on June 2, 1858, bears his name. These discoveries led to his appointment as professor of astronomy and director of the observatory at Florence in 1864. He also contributed to early classification systems for stellar spectra. Donati was supervising the building of a new observatory at Arcetri, near Florence, when he died.