Giuseppe Piazzi and the Discovery of the Proper Motion of 61 Cygni

Bessel F. W. , “Uber den Doppel-Stern N.ro 61 Cygni”, Monatliche Correspondenz (hereafter: MC), xxvi (1812), 148–63.

For an overview of the attempts made over the centuries to measure stellar parallax, see for example Hoskin Michael A. , Stellar astronomy: Historical studies (Chalfont St Giles, Bucks, 1982), chap. A.1, pp. 5–25. To this book I owe the inspiration for this paper.

Piazzi Giuseppe (born in Ponte Valtellina in 1746, died in Naples in 1826), established in 1790 the Palermo Astronomical Observatory, which he directed until his death. Between 1787 and 1789 he travelled to France and England literally to learn the profession (when he had been appointed to the chair of astronomy in 1787 he had never made astronomical observations and could hardly be considered even an amateur). He then established close friendships with the leading astronomers of the time (Lalande, Messier, Herschel, Maskelyne, to mention just a few), and he succeeded in persuading Ramsden to make him a 5ft altazimuth circle, a superb instrument considered at the time “le plus bel instrument d'Astronomie qu'on ait fait jusqu'ici”, in the words of Lalande (Journal des scavans, August 1789, 572). Piazzi deserves a place in the history of astronomy for his discovery on 1 January 1801 of Ceres, the first of the minor planets, which gained him an immediate reputation, and for his two monumental star catalogues (1803 and 1814); but his astronomical work still awaits proper evaluation.

Piazzi Giuseppe , “Saggio sui movimenti proprj delle fisse”, Memorie dell'Istituto Nazionale delle Scienze, Classe di Fisica e Matematica, i (1806), 1–68, p. 2.

The works in which Piazzi's proper motions are published are: (i) “Ducenarum supra viginti quinque praecipuarum stellarum declinationes comparatae”, in Praecipuarum stellarum inerrantium positiones mediae (Palermo, 1803), 1–28; (ii) “Saggio sui movimenti proprj delle fisse” (ref. 4); (iii) “Delle posizioni medie di dugento venti stelle” in Libro sesto del Reale Osservatorio di Palermo (Palermo, 1806), 1–31; and (iv) Praecipuarum stellarum inerrantium positiones mediae (Palermo, 1814).

In Corrispondenza astronomica fra Giuseppe Piazzi e Barnaba Oriani (hereafter CA), Pubblicazioni del Reale Osservatorio di Brera in Milano, vi (1874), 74–75.

Op. cit. (ref. 4).

In a letter to Oriani dated 27 February 1804 and written a few days before sending his “Saggio”, Piazzi says: “I have found some movements that deserve full attention from astronomers, especially in the constellations of Cetus and Eridanus. The 40th of this last has −3″.589 according to Flamsteed's and −3″.64 according to my own observations, in dec., and an equally large motion in RA” (CA, 81). It would thus be very strange if he had already noticed 61 Cygni and did not mention it even as a possibility to be confirmed.

Piazzi writes: “I would like to know if and when they plan to publish it, because I want to correct all the RA according to my latest determinations of the positions of the fundamental stars, which I am again comparing with the Sun, and which I hope to establish with the greatest possible accuracy. At the same time I want to add the movements of another two hundred and more stars” (CA, 85). This work would result in the publication in 1806 of the catalogue of 220 stars (see text), and in 1814 in the publication of his second star catalogue.

See for example Hoskin , Stellar astronomy, 9.

Between 1806 and 1813 we have only five letters, all from Piazzi to Oriani (CA, 98–101). From the content of these letters it appears that in these years Piazzi was indeed isolated from the rest of the astronomical community.

MC, xxvi (1812), 295–6. In this “kleinen Nachtrag” to his previous paper, however, Bessel essentially emphasizes the agreement of Piazzi's observations with his own. The form of this acknowledgement has undoubtedly contributed to the scant recognition accorded to Piazzi's discovery, even in his own times.

MC, xvi (1807), 183–5. Oriani's letter is followed by some considerations by the editor, in which he uses further material attached to Oriani's letter (pp. 185–95).

This work resulted in the publication of “Ricerche di Giuseppe Piazzi sulla parallasse annua di alcune delle principali fisse”, Memorie della Società Italiana delle Scienze, xii (Modena, 1805), 40–61. For a brief discussion of Piazzi's results, see Hoskin , Stellar astronomy, 8–9.