Horn d’Arturo, Guido

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Horn d’Arturo, Guido

(b. Trieste, 13 February 1879; d. Bologna, Italy, 1 April 1967)


Horn d’Arturo graduated from the University of Vienna in 1902 and was, successively, assistant at the observatories of Trieste, Catania, Turin, Bologna, and Rome. From 1920 he was director of the Bologna observatory, in the old university center, and supervised its complete renovation. He also had built a branch observatory near Lojano, in the Tuscan Apennines between Bologna and Florence, at an altitude of 2,600 feet, furnishing it with a Zeiss reflector of sixty-centimeter aperture. Because he was of Jewish extraction he was removed in 1938 from the chair of astronomy and the directorship of the observatory; at the end of the war he was reinstated in both these posts at the University of Bologna, from which he retired in November 1954.

A capable observer with notable technical skills, Horn d’Arturo was active in positional astronomy, statistics, cosmography, and optical astronomy. With the Lojano telescope he and his co-workers observed variable stars, gaseous and planetary nebulae, and globular clusters and investigated the apparent distribution of nebulae and of the fixed stars. In optical astronomy he demonstrated how the density of photographic stellar tracks may be measured by using the diffraction of light. He clarified the effect on vision, especially in the astigmatic eye, of the suture of the eye lens and the formation of the so-called black drop.

In instrumental techniques Horn d’Arturo conceived of a conic lens (in place of the prism lens) in which each section passing through the axis acts as an infinitely thin prism. Stellar images obtained with this instrument exhibit concentric spectral lines (circular in the case of stars on the axis); in every other instance the lines are curves of the fourth order. Horn d’Arturo devised this instrument to obtain spectra of meteors, because by properly placing the camera and, with it, the conic prism for vertical reception it is possible to cover the entire sky from the zenith to 23° above the horizon and any azimuth.

As head of the Italian expedition sent to Somaliland to observe the eclipse of 14 January 1926, Horn obtained interesting photographs of the flash spectrum and of the prominences enveloped by the corona. On that occasion he was developing one of his theories on the phenomenon of “flying shadows” and on the perpetual eastern current of the very high equatorial atmosphere.

The last years of Horn d’Arturo’s scientific activity were devoted to the construction and use of his specchio a tasselli. With a diameter of 180 centimeters, the mirror was made up of many small mirrors arranged in a series of concentric circles. He mounted the horizontal mirror at the base of the university tower (the old Bologna observatory) so that it functioned only for the zenith. He was thus able to photograph stars to the eighteenth magnitude. Horn d’Arturo’s idea, while not applied to reflectors of the standard type, has been successfully adapted for other purposes.

Horn d’Arturo founded the popular astronomical magazine Coelum.


Horn d’Arturo’s work was most often published in both Pubblicazioni dell’Osservatorio astronomico della Università di Bologna and Memorie della Società astronomica italiana. Among the most significant are “Il fenomeno della goccia nera e l’astigmatismo,” in Pubblicazioni dell’Osservatorio astronomico della Università di Bologna, vol. 1 , no. 3 (1922); “Le ombre volanti,” ibid., no. 6 (1924); “Numeri arabici e simboli celesti,” ibid., no. 7 (1925); “L’eclisse solare totale del 14 gennaio 1926 osservata dalla missione astronomica italiana nell’Oltregiuba,” ibid., no. 8 (1926); “L’uso di una lente conica nella spettrograffia delle stelle cadenti,” ibid., vol. 2 (1934); “Primi esperimenti con to specchio a tasselli,” ibid., vol. 3 , no. 3 (1935); “L’aggiustamento dello specchio a tasselli effettuato dal centro di curvatura,” ibid., vol. 5 , no. 17 (1952); and “Lo specchio a tasselli di metri 1, 80 d’apertura, collocato nella Torre dell’Osservatoria Astronomico Universitario,” ibid., vol. 6 (1955).

An obituary is G. Mannino, L. Rosino, L. Jacchia, in Coelum, 35 , nos. 5–6 (1967).

Giorgio Abetti

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