Zanotti, Eustachio

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(b. Bologna, Italy, 27 November 1709; d. Bologna, 15 May 1782)

as tronomy, geometry.

Like the astronomer Eustachio Manfredi, his godfather, Zanotti belonged to a prominent family distinguished in the arts, letters, and sciences. The son of Gian Pietro Zanotti and Costanza Gambari, he was educated by the Jesuits and endtered the University of Bologna, becoming Man fredi’s assistant at the Institute of Sciences in 1729. He graduated in philosophy in 1730 and obtained his first university post, as reader in mechanics at Bologna, in 1738, after presenting his trial lecture on the Newtonian theory of light. The following year he succeeded Manfredi as director of the Institute observatory, a post to which he dedicated himself almost exclusively for the next forty years, never marrying and declining all offers from other universities. He began teaching hydraulics at the university in 1760, having been requested by the government to supervise works on rivers and waterways. His publications in this field include a work on the characteristics of riverebeds near the sea (1760) that remained in print for almost a century. Zanotti wrote the last part of Manfredi’s Elementi della geometria, “according to the method of indivisibles” ; and his lucid and informative Traqttato teorico–pratico di prospettiva (1766) was intended for painters as well as as mathematicians.

Zanotti established a reputation as an astronomer even before Manfredi’s death, through the discovery of two comets, to the second of which (1739) he attributed a parabolic orbit. In 1741, under his direction, the new instruments that Manfredi had ordered from Sisson’s were installed at the Bologna observatory; a mural quadrant 1.2 meters in radius and a transit instrument with a focal length of about one meter. In 1780 he added a movable equatorial telescope made by Dollond’s.

With the acquisition of Sisson’s instruments, Zanotti’s observatory became one of the finest in Europe. In 1748 and 1749, with his assistants G. Brunelli and Petronio Matteucci, he carried out repeated obserbations of the sun and planets, and complied a catalog of 447 stars, all but thirty–three of them within the Zodiac. The work was published with additions in 1750 as an appendix to the new edition of Manfredi’s introductory volume to his ephemerides. Zanotti continued to publish the ephemerides with scrupulous care; three volumes covered the period 1751-1774, and a fourth was published posthumously by Matteucci in 1786.

Zanotti’s principal observations and descriptions, including some on occultations of stars by the moon, concern six comets (1737, 1739, 1742, 1743-1744, Halley’s comet of 1758, and 1769), four lunar eclipses (December 1739, January 1740, November 1745, June 1750), three solar eclipses (August 1738, July 1748, January 1750), the aurora borealis (December 1737, March 1739), and transits of Mercury (1743, 1753) and of Venus (1751) on the sun.

In 1750 Zanotti was invited by the Paris Academy of Sciences to participate in a major international research project, the main purpose of which was to measure the lunar parallax. His observations provided the program with some of its most accurate results.

Zanotti’s accomplishments also included the restoration in 1776 of Gian Domenico Cassini’s sundial in the church of San Petronio. The displaced perforated roofing slab forming the gnomon was raised slightly, restoring the instrument to its original height. The old deformed iron ship representing the meridian was removed and a solid foundation was laid as a base for new level marble slabs with the new brass meridian strip. Accurate geodetic and topographic measurements made in 1904 and 1925 have verified that the instrument has remained as Zanotti left it, in the position that perfectly reproduces Cassini’s original conditions of construction.

According to L. Palcani–Caccianemici, his collaborator and principal biographer, Zanotti was also a pioneer in the study of variable stars, a little understood phenomenon that was then considered to represent an error of vision or an effect caused by the intervening atmosphere. Zanotti, however, maintained that changes of light occur even when the possibility of such causes is entirely ruled out. “If you observe with a telecope two stars extremely near to each other,” he said, “you will see that one remains exactly the same and that the other, altered in intensity, no longer appears as before.”

Unfortunately, no trace of these observations appears in Zanotti’s published writings, possibly because he did not wish to seem to be questioning the incorruptibility and constancy of the heavens–a subject about which the Aristotelians who controlled the University of Bologna were particularly sensitive and uncompromising.


I. Original Works. Most of Zanotti’s astronomical memoirs appeared in the Commentarii of the Istituto e Accademia delle scienze di Bologna, beginning with 2, pt. 1 (1745); the last one appeared in 7 (1791)– see index. There are also three papers, in English, in Philosophical Transactions of the Royal Society: on the aurora borealis, 41 (1741), 593-601: on the comet of 1739. ibid., 809: and on the 1761 transsit of Venus, 52 (1761), 399-414.

His most important separately published works are Ephemerides motuum coelestium ex anno 1751 ad annum 1786, 3 vols. (Bologna, 1750-1774); Trattato teorico–pratico di prospettiva (Bologna, 1766); and La meridiana del tempio di San Petronio rinnovata l’anno 1776 (Bologna, 1779).

II. Secondary Literature. See the following, listed chronologically; L. Palcani–Caccianemici, De vita Eustachii Zanotti commentarius (Bologna, 1782), translated into Italian by G. A. Maggi as “Elogio di Eustachio Zanotti” and prefixed, with a bibliography, to the new ed. of Zanotti’s Trattato teorico–pratico di prospettiva (Milan, 1825) Vita di Eustachio Zanotti (n.p., n.d.), an extract from Giornale dei letterati (Pisa), 58 (1785), 175-197; G. Fantuzzi, Notizie degli scrittori bolognesi, VIII (Bologna, 1790; repr. 1965), 265-270; Noel Poudra, Histoire de la perspective (Paris, 1864), 529-533; and P. Riccardi, Biblioteca matematica italiana, II (Modena, 1873-1876; repr. Milan, 1952), 651-657.

More recent works include M. Rajna, “L’astronomia in Bologna,” in Memorie della Societa degli spettroscopisit italiani,32 (1903), 241-250, esp. 245; Federigo Guarducci, La meridiana di Gian Domenico Cassini nel tempio di San Petronio di Bologna riveduta nel 1904 e nel 1925 (Bologna, 1925); E. Bortolotti, La storia della matematica nella Universita di Bologna (Bologna, 1947), 177-178; G. Horn d’Arturo, Piccola enciclopedia astronomica, II (Bologna, 1960), 364; and Anna Maria Matteucci, Carlo Francesco Dotti e l’architettura bolongnese del settecento, 2nd ed. (Bologna, 1969), 44-49. Passim.

Giorgio Tabarroni