Introduction
This page focuses on Nicolas-Louis de Lacaille's zodiacal stars catalog from Éphémérides des mouvemens célestes (1765–1775) [3], based on observations performed after his return from the Cape of Good Hope and reduced by the French astronomer Jean Sylvain Bailly, a contemporary of Lacaille.
Jean-Sylvain Bailly
This catalog has the reputation of being one of the most accurate of its time. Piazzi's star catalog in Praecipuarum Stellarum Inerrantium [2] has cross-references to it, which is how it came to my attention.
Lacaille's zodiacal stars catalog has previously been digitized for the study by James Lequeux [1] of the accuracy of 18th century catalogs. The corresponding file is lacaizod.dat, part of VizieR catalog J/A+A/567/A26. It differs from the original by correcting the positions of many stars, adding star designations, and omitting some columns.
I have tried to stay as close as possible to the original and to confine additional information to auxiliary files, in the hope that my data files be reuseable for different projects.
Catalog data
The file zod_data.dat contains the machine-readable version Lacaille's zodiacal star catalog. Its format is documented in ReadMe and it is subject to automatic validation using the Anafile package of the VizieR service.
| File name | Explanation |
|---|---|
| ReadMe | File descriptions |
| zod_data.dat | Lacaille's zodiacal star catalog |
| zod_notes.dat | Notes on the source catalog |
The file zod_notes.dat documents a few issues affecting the catalog, such as erroneous coordinates or star designations, many of which likely typos.
Star identifications
The file zod_ident.dat gives redundant star identifications for the stars of the zodiacal catalog: Bayer / Lacaille and Flamsteed designations, Bright Star, Draper, and Hipparcos catalog numbers. Its format is specified in ReadMe.
| File name | Explanation |
|---|---|
| zod_ident.dat | Star identifications |
The matches were obtained by finding the closest star(s) in the Hipparcos catalog or in a couple of cases the SAO catalog , adjusted to the epoch and equinox of the zodiacal catalog, namely the beginning of year 1765 (Gregorian calendar), or
| Julian Day | 2,365,714 |
| Besselian epoch | B1765.0037… |
| Julian epoch | J1765.0075… |
There are no duplicate entries among the identifed ones in the catalog.
Some star positions are clearly erroneous, though. I was able to find a plausible correction and a star match for most of them, relying on the Bayer / Lacaille designations provided by the catalog when present. There remain 5 unresolved cases. These issues are documented in zod_notes.dat.
The star identifications in zod_ident.dat agree largely with VizieR file lacaizod.dat. I include a few more components of double stars and suggest position corrections for 5 unidentified entries in the study [1] but leave 405 (maybe 28 Sgr) unidentified. These are the only differences.
Accuracy of the catalog
The file zod_ident.dat also gives the differences between Lacaille's positions (reduced by Bailly) and modern positions of the corresponding stars from the Hipparcos or SAO catalog adjusted to the zodiacal catalog's epoch and equinox.
Specifically, it 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. 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 |
|---|---|---|
 |
 |
 |
The peak of the distance histogram occurs at about 0.15 arcmin, showing that the zodiacal catalog is twice as accurate as Lacaille's southern star catalog from Coelum Australe.
References
[1] 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] 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.
[3] Éphémérides des mouvemens célestes, pour dix années, depuis 1765 jusqu'en 1775, et pour le meridien de la ville de Paris, Tome VI, Paris: Jean-Thomas Heissant, 1763. Catalogue des 515 Étoiles Zodiacales, observées en 1760 & 1761, par M. l'Abbé de la Caille, & réduites au commencement de l'année 1765, par M. Bailly, de l'Académie Royale des Sciences, pp. lxv-lxxvij.