Italian astronomer who discovered six comets. He made important contributions to the early development of stellar spectroscopy and applied spectroscopic methods to the understanding of the nature of comets. He also studied cosmic meteorology.
Donati was born and educated in Pisa. From 1852 he worked at the observatory in Florence, becoming its director 1864.
During the 1850s Donati was an enthusiastic comet-seeker, and the most dramatic of his discoveries was named after him. Donati's comet, first sighted 1858, had, in addition to its major tail, two narrow extra tails.
Using the new technique of stellar spectroscopy, Donati found that when a comet was still distant from the Sun, its spectrum was identical to that of the Sun. When the comet approached the Sun, it increased in magnitude (brightness) and its spectrum became completely different. Donati concluded that when the comet was still distant from the Sun, the light it emanated was simply a reflection of sunlight. As the comet approached the Sun the material in it became so heated that it emitted a light of its own, which reflected the comet's composition.
Other areas of interest which engaged Donati's attention were atmospheric phenomena and events in higher zones, such as the aurora borealis.