(b. Modena, Italy, 15 September 1844; d. Rome, Italy, 23 September 1919)
After graduating in natural sciences from the University of Modena and in engineering from the Milan Polytechnic, Riccò became an assistant at the Modena observatory and then at the Palermo observatory. In 1885, under the sponsorship of the Accademia Gioenia, he founded the astrophysics observatory at Catania, and in 1890 he became professor of astrophysics at the university, the first chair of that subject in Italy. Riccò created an astronomic and meteorological station on Mt. Etna, and with A. Secchi and P. Tacchini he founded the Society of Italian Spectroscopists in 1872; he was editor of its memories until his death.
At the Palermo observatory he began a regular series of direct and spectroscopic observations of the sun, using a 25-centimeter Merz refractor and a directvision spectroscope. He continued this research in Catania for forty years, obtaining important results on the frequency, position, and development of sunspots and solar prominences and on their influence on terrestrial phenomena. Riccò collaborated with G. E. Hale in an unsuccessful attempt to photograph the corona in full sunlight, using a spectro-heliograph invented by Hale and taking advantage of the altitude of Mt. Etna.
In 1913, at the fifth conference of the International Union for Cooperation in Solar Research, Riccò presented the results of his observations made from 1880 to 1912 on solar prominences and their structure. He showed that the cycle of prominences lasts about as long as that of the sunspots, although there is a certain delay in the appearance of the former, in addition to other differences. Riccò noted two kinds of prominences: those associated with very active sunspots are extremely variable and are composed of hydrogen, helium, calcium, and other metals; quiescent prominences are composed almost exclusively of hydrogen and migrate toward the poles during the course of the eleven-year sunspot cycle. Riccò was one of the first to explain that the so-called filaments are merely prominences seen projected against the solar disk.
Riccò led three expeditions to study total solar eclipses, to Algeria (28 May 1900), Spain (30 August 1905), and the Crimea (August 1914). All of these expeditions achieved important results on the flash spectrum, on prominences and their relationship to the corona, and on the emission spectrum of the corona. In 1882, at Palermo, Riccò had pointed out the delayed occurrence of terrestrial magnetic storms with respect to the presence of extensive groups of sunspots. Ten years later he reported to the Académie des Sciences his discovery that magnetic storms begin on the earth forty to forty-five hours after the passage of the spots, or groups of spots, across the central meridian of the sun, and that consequently the presumed agent of the storms must travel to the earth at a speed of about 1,000 kilometers per second. This was the first observation of what was later called the solar wind.
For the international Carte du Ciel, Riccò organized and directed the study of those parts of the sky between 46° and 55° N. latitude that had been assigned to the Catania observatory. He observed the Daniel, Morehouse, and Halley comets (1908–1910) and discussed their constitution with Horn d’Arturo according to hypotheses proposed by Righi. In the fields of geodesy and geophysics he determined the gravitational anomalies and the terrestrial magnetic constants for Sicily, especially in relation to seismic activity, and carried out observations and studies on the crater of Mt. Etna during the 1910 eruption.
A prolific writer, Riccò published mainly in the Memorie della Società degli spettroscopisti italiani. For a list of his papers (to 1900), see Royal Society Catalogue of Scientific Papers, VIII, 742; XI, 166–168; XII, 615; XVIII, 168–172. Subsequent writings are listed in Poggendorff, IV, 1241–1242; V, 1043; VI, 2165.
For a biography of Riccò, see G. Abetti, “Annibale Riccò, l’Accademia Gioenia e l’osservatoria astrofisico di Catania,” in Bollettino delle sedute dell’ Accademia Gioenia di scienze naturali in Catania, 4th ser., 3 (1955).