Giuseppe Piazzi (1746–1826), was an Italian priest who also studied mathematics and astronomy. He taught theology and mathematics, and became director of the observatory that he founded in Palermo, Sicily. I found more biographical details here.

Giuseppe Piazzi

Giuseppe Piazzi

Piazzi is known mainly for his 1801 discovery of the asteroid (now dwarf planet) Ceres, and for his star catalog [1] of 7646 stars (first edition 1803, second edition 1814). This catalog was "vastly superior to any that preceded it" according to Simon Newcomb [2, p. 381], thanks in no small part to the quality of the transit circle made for the Palermo observatory by English instrument maker Jesse Ramsden.

Several nineteenth and early twentieth century works mentioned on the Ptolemy and Tycho pages refer to Piazzi's catalog, which is how it became interested in it. The catalog doesn't seem to have been put in machine-readable format so far (except a sample of it, part of James Lequeux's study [3] of 18th century catalogs) and I thought I might spare the task of doing so to other people interested in ancient star catalogs.

Catalog data

The file piazzi.dat contains Piazzi's star catalog [1] (1814 version) in machine-readable form, taking into account the corrections listed at the end of the book. Its format is specified in the ReadMe file and it is subject to automatic validation using the Anafile package of the VizieR service. Various observations about the original are gathered in notes.dat.

File name Explanation
ReadMe File descriptions
piazzi.dat Piazzi's catalog
notes.dat Notes on Piazzi's catalog

The catalog's fields are described in Latin in the introduction to the catalog. Unfortunately, I haven't been able to decipher much of it and, rather than making things up, I have left some of the field descriptions quite vague in ReadMe.

The star positions correspond to the beginning of year 1800 (Gregorian calendar), or

Julian Day2,378,497
Besselian epochB1800.0024…
Julian epochJ1800.0054…

Star identifications

The file ident.dat collects redundant modern identifiers for Piazzi's stars. Its format is described in ReadMe as well.

File name Explanation
ident.dat Star identifications

The matches were obtained by finding the star(s) within 2 arcminutes of Piazzi's positions in the Hipparcos and SAO catalogs adjusted to the epoch and equinox of the catalog (January 1st, 1800).

Many Piazzi stars have no match in the Hipparcos catalog (about 320) or even the Henry Draper catalog (about 20). I used the Durchmusterung catalogs successfully as fallbacks at first, but settled on the SAO catalog since it handled all the cases and provides proper motions, resulting in better position matches.

There are no duplicate entries in the catalog.

Of the 7646 stars in Piazzi's catalog, only two remain that I haven't been able to identify (namely XII.169 and XX.408). Only five Piazzi star positions differ by more than 1 arcmin from the modern ones (XIV.260, XVI.280, XVII.044, XIX.009, XX.419), and every such case can be explained by a typographical error on a single coordinate digit that was not caught in the errata. I think this is a monumental achievement on the part of Piazzi and his collaborators.

Cross-references to other catalogs

The catalog specifies a constellation for each star, including some obsolete ones (Custos Messium, Frederici Honores, Quadrans Muralis, Taurus Poniatovii, Triangulum Minus, Turdus Solitarius). The Pleiades and Anser (of Vulpecula & Anser) appear as constellations. The names Regula and Navis refer to Norma and Puppis or Vela. I made up abbreviations for the non-standard constellations, documented in notes.dat.

The catalog includes cross-references to other catalogs in the form of Bayer, Flamsteed, Hevelius designations, or numbers from some the most accurate catalogs of the day: Lacaille's southern stars catalog, Lacaille's zodiacal stars catalog, and Tobias Mayer's zodiacal stars catalog [4].

Bayer designations use the letter assignments from Flamsteed's and Lacaille's respective catalogs. There are references to Flamsteed's non-existent stars. The Bayer and Flamsteed cross-references contain many errors: wrong case, confusion between similar Greek letters, etc. I have documented these in notes.dat.

There are some issues with Lacaille's southern and zodiacal star number assigments as well. I have documented these at the end of notes.dat. I plan to examine the cross-references to Hevelius's and Mayer's catalogs in the same manner as time permits.

Accuracy of the catalog

The file ident.dat also gives the differences in right ascension (multiplied by the cosine of the declination for normalization), the differences in declination, and the great circle distances, all in minutes of arc, between Piazzi's positions and modern ones (reduced to 1800).

The corresponding histograms below give an idea of the overall accuracy of the catalog (data outside the range of the x axis has been discarded).

Right Ascension Declination Distance
Right Ascension
          differences Declination
          differences Distances

The peak of the distance histogram occurs at about 0.06 arcmin = 3.6 arcsec, representing an accuracy improvement by a factor 5 over Lacaille's southern stars catalog and by a factor 2.5 over Lacaille's zodiacal stars catalog.


[3] James Lequeux, From Flamsteed to Piazzi and Lalande: new standards in 18th century astrometry, Astronomy and Astrophysics, Volume 567, July 2014. Related data gathered as VizieR's catalog J/A+A/567/A26.

[2] Simon Newcomb, A Compendium of Spherical Astronomy, New York: The Macmillan Company, 1906.

[4] Francis Baily, Mayer's Catalogue of Stars, corrected and enlarged; together with a Comparison of the Places of the greater part of them, with those given by Bradley; and a reference to every observation of every Star, Memoirs of the Astronomical Society of London, Vol IV, Part I, pp. 191–445. London: Priestley and Weale, 1830.

[1] Giuseppe Piazzi, Praecipuarum Stellarum Inerrantium Positiones Mediae Ineunte Saeculo XIX. Ex Observationibus Habitis in Specula Panormitana ab anno 1792 ad annum 1813. Palermo: Regia Typographia Militari, 1814.


  • This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in A&AS 143, 23.
  • This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.
  • Illustration: Giuseppe Piazzi. Line engraving by N. Bettoni. Credit: Wellcome Collection. CC BY