Nicolas-Louis de Lacaille (1713–1762) was one of the main 18th century French astronomers. During a mission in the southern hemisphere from 1750 to 1754, he established an observatory at the Cape of Good Hope from which he observed and measured the positions of about 10,000 southern stars. This page focuses on the following three publications related to these observations.

Table des Ascensions Droites et des Déclinaisons Apparentes des Étoiles australes renfermées dans le tropique du Capricorne [1] (1756), denoted by [EA] below, an article containing Lacaille's observations of 1935 of the brightest stars and nebulous objects.

Sur les Étoiles Nébuleuses du Ciel Austral [2] (1761), an article describing 42 nebulous objects of the southern sky, referred to as [EN] below.

Coelum Australe Stelliferum [3] (1763), or [CA], a book containing the complete set of observations as well as a list of 1942 stars and nebulous objects, including all the ones from [EA], with their positions reduced to the epoch 1750. This list constitues the most substantial catalog of stars of the southern hemisphere of its time, after the previous attempts by de Houtman (1603), Kepler (1627), and Halley (1679) (see Verbunt & van Gent [8]).

Almost a century later, the reduction of the whole set of Lacaille's observations was undertaken under the direction of Francis Baily and published in 1847 as A catalogue of 9766 stars in the southern hemisphere [4].

In order to classify the newly observed stars, Lacaille creates 14 new constellations to fill the gaps between the existing ones (the Ptolemaic constellations and the southern constellations previously introduced by Plancius, Keyser and de Houtman). With the exception of Mensa, the Table Mountain, he names them after instruments used in the arts and sciences. These constellations are still in use today; they are part of the 88 constellations standardized by the International Astronomical Union in 1930. Here is their list, with the names as given in Latin on the planisphere at the end of [CA] and in French on the planisphere at the end of [EA].

Modern Latin name Lacaille's Latin name Lacaille's French name
Sculptor Apparatus Sculptoris l'Atelier du Sculpteur
Fornax Fornax chimiæ le Fourneau
Horologium Horologium l'Horloge
Reticulum Reticulus le Réticule Romboïde
Caelum Caelum Scalptorium les Burins
Pictor Equuleus Pictorius le Chevalet et la Palette
Pyxis Pixis Nautica la Boussole
Antlia Antlia Pneumatica la Machine Pneumatique
Octans Octans l'Octans de Réflexion
Circinus Circinus le Compas
Norma Norma l'Équerre et la Règle
Telescopium Telescopium le Télescope
Microscopium Microscopium le Microscope
Mensa Mons Mensæ Montagne de la Table

In the manner of Bayer, Lacaille assigns Greek and Latin letters to the stars in the new constellations and to those in the old ones that didn't have any. He also modifies Bayer letters already assigned in Ara, Centaurus, Lupus, Argo Navis and Piscis Austrinus, and he adds Latin letter assignments in Eridanus, Canis Major, Hydra and Sagittarius.

Running out of letters in Argo Navis, he distributes Greek letters to the brightest stars and then splits the constellation into three parts, Puppis, Carina, and Vela (the Stern, the Hull, and the Sails), where he separately assigns upper- and lowercase Latin letters.

Lacaille explains all this and more very clearly in the remarks that follow the catalog of observations in [EA]. Due to the historical interest of this document, I thought it would be worth making an English translation of it. A similar but more succinct text, in Latin this time, concludes [CA]. This one, besides mentioning that the coordinates are reduced to the epoch 1750 rather than observed, does not give the detailed list of the 14 new constellations, and omits the final scathing paragraph about Halley's new constellation Robur Carolinum.

Lacaille's constellations and many of his Greek and Latin letter assignments have remained part of the astronomical tradition to this day and originate in these documents. For this reason, I thought that the star catalogs in [EA] and [CA] deserved digitized versions, which I have compiled and made available on this page.

Data available on this page

The following files are available.

File name Explanation
ReadMe File descriptions
lac_data.dat Lacaille's catalog from [CA]
obs_data.dat Observations from [EA]
lac_notes.dat Notes on the source documents [CA] and [EA]
lac_ident.dat Cross-identifications of stars from [CA] and [EA]
lac_bids.dat Modern letter assignements to Lacaille stars
neb_data.dat Nebulous objects from [NE]
neb_descr.dat Object descriptions from [NE]
neb_ident.dat Cross-identifications of objects from [NE]

Lacaille's catalog from [CA]

Lacaille's catalog of 1942 stars from [CA] is contained in the file lac_data.dat. The format of this file is described in the ReadMe file and conforms to the the conventions for catalog descriptions of the VizieR service.

The file lac_notes.dat describes the few features and typos in [CA] that didn't make it to the digitized version.

Lacaille's observations from [EA]

The 1935 entries from [EA] are contained in the file obs_data.dat whose format is also described in ReadMe and subject to automatic validation.

The entries in [EA] are not numbered, but I have numbered them sequentially in the file from 1 to 1935 and established the mapping between [CA] and [EA] by finding the star(s) of [CA] whose position most closely matches the position given in [EA]. The positions of a given star in [CA] and [EA] are not supposed to be the same, but they should never differ by more than a few minutes of arc. A single obvious match was found in the vast majority of cases, and a simple visual inspection resolved the few ambiguous ones.

All the entries in [EA] have a match in [CA] but 7 entries in [CA] don't have a match in [EA]. The mapping between [CA] and [EA] is given in lac_ident.dat. Discrepancies in letter assignments and gross differences in positions are described in lac_notes.dat.

Cross-identifications of Lacaille stars

The main point of the file lac_ident.dat it to establish the correspondence between Lacaille's stars and modern star catalogs, which has been done before for lettered stars (in Wagman [7] for instance) but maybe not for non-lettered stars (at least, I haven't found any trace of it on the Web). For each star in the [CA] catalog, it gives the corresponding number in [EA] as well as the numbers of the corresponding star(s) in the Henry Draper and Hipparcos catalogs. There is a handful of discrepancies with Wagman [7], documented in lac_notes.dat, some of which plain typos.

The correspondence was established by adjusting the positions of the Hipparcos stars to the B1750.0 epoch according to their proper motion, precessing them to the B1750.0 equinox, and identifying for each Lacaille star all the Hipparcos stars within a radius of 5 minutes of arc. There was a single match in a large majority of cases. The instances of multiple matches were resolved manually, taking into account the proximity and magnitudes of the matches, using the sky pictures available from the Simbad service to help with the decision. There are probably a few debatable cases where the decision to include or exclude some close or weak component from the match could have been different. The file still lists all the 5' matches in the Hipparcos catalog and indicates which one(s) have been rejected.

The HD number(s) of all the Hipparcos matches are given, but I have not attempted to determine all the 5' matches in the Henry Draper catalogue. There may be more of these, since this catalog is larger, but it shouldn't affect the final choice of matching star. However, a small number of Lacaille stars that don't have a match in the Hipparcos catalog do have a valid match in Henry Draper catalog, and these have been retained.

There are a few cases where Lacaille's positions are wrong. Either the positions in [EA] and [CA] don't agree and some transcription or computation error must have occurred, or there is a clearly intended match close, but not close enough, to the given position. I tried to resolve these cases using common sense, Morton Wagman's book [7], and sometimes Gould's notes starting on p. 140 of Uranometria Argentina [5]. There are 61 such cases where a match is plausible (grep for PO in lac_ident.dat) and only 13 cases remain where no match could be found (grep for NM in lac_ident.dat).

Modern letter assignements to Lacaille stars

In many cases, similarly to Bayer, Lacaille assigns the same letter to several stars close to each other. Later astronomers have added indices in the manner of Flamsteed in order to distinguish them.

More conspicuously, many of Lacaille's letter assignments have been completely modified. Most of these changes are due to Francis Baily who, as mentioned above, built on Lacaille's work, and Benjamin Gould, author of his own catalog of southern stars in Uranometria Argentina [5]. The changes are due to various causes: the authors' own estimation of the relative importance of stars, modifications of the shapes of the constellations, restorations of well-estabished Bayer assignments, avoidance of capital letters after R that had become reserved for variable stars, etc.

The file lac_bids.dat describes in human- and machine-readable form the changes in letter assignments that have occurred. I used Morton Wagman's book [7] as reference for this part. For each lettered Lacaille star, the file gives the star number in [CA], the letter assignment by Lacaille, and the current letter (and index, and sometimes constellation) assignment (or a set of alternatives) or a mention that the star doesn't exist.

Nebulous objects from [EN]

The file neb_data.dat contains the identifiers and coordinates of the 42 objects from [NE]. Its format is specified in ReadMe and it is subject to automatic validation. The object descriptions (in French) are given separately in the UTF-8 file neb_descr.dat so that neb_data.dat can be validated automatically by the Anafile package of the VizieR service, which doesn't support non-ASCII characters.

There is more information about Lacaille's nebulous objects at the SEDS page on Lacaille [6].

Cross-identifications of Lacaille's nebulous objects

The file neb_ident.dat gives the correspondence between the nebulous objects from [EN], [EA] and [CA], their modern identifiers (Messier, NGC, etc.), if any, and additional information. Its format is formally defined in ReadMe and it is subject to automatic validation.

All the nebulous objects from [NE] have counterparts in [EA] and [CA], but a couple of objects from [EA], including one from [CA], don't appear in [NE]. The coordinates are sometimes a little different between these sources.

References

[1] M. l'Abbé de la Caille, Table des Ascensions Droites et des Déclinaisons Apparentes des Étoiles australes renfermées dans le tropique du Capricorne: observées au cap de Bonne-espérance, dans l'intervalle du 6 Août 1751, au 18 Juillet 1752, Histoire de l'Académie royale des sciences, Année 1752, pp. 539–592. Imprimerie royale, Paris, 1756.

[2] M. l'Abbé de la Caille, Sur les Étoiles Nébuleuses du Ciel Austral, Histoire de l'Académie royale des sciences, Année 1755, pp. 194–199. Imprimerie royale, Paris, 1761.

[3] Nicolas-Louis de la Caille, Coelum Australe Stelliferum seu Observationes ad Construendum Stellarum Australium Catalogum institutae, in Africa ad Caput Bonae-Spei, Guerin & Delatour, Paris, 1763.

[4] Francis Baily (ed.), A catalogue of 9766 stars in the southern hemisphere, for the beginning of the year 1750, from the observations of the Abbé de Lacaille made at the Cape of Good Hope in the years 1751 and 1752, Taylor, London, 1847.

[5] Benjamin Apthorp Gould, Uranometria Argentina, Buenos Aires, 1879.

[6] Hartmut Frommert & Christine Kronberg, Lacaille's "Catalog of Nebulae of the Southern Sky" at the Messier pages at SEDS.

[7] Morton Wagman, Lost Stars, McDonald & Woodward Publishing Company, Blacksburg, Virginia, 2003.

[8] F. Verbunt & R. H. van Gent, Early star catalogues of the southern sky, , Astronomy & Astrophysics, 530, A93 (2011).


Acknowledgment. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.