Introduction
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 [8] (1756), denoted by [EA] below, an article containing Lacaille's observations of 1935 of the brightest stars and nebulous objects south of the tropic of Capricorn.
☞ Sur les Étoiles Nébuleuses du Ciel Austral [7] (1761), an article describing 42 nebulous objects of the southern sky, referred to as [EN] below.
☞ Coelum Australe Stelliferum [6] (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 1750 equinox. 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 [1]).
L'Abbé de la Caille
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 [5].
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 1750 equinox rather than observed, does not give the detailed list of 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.
Catalog data
The following files provide the catalog data in machine-readable form.
| File name | Explanation |
|---|---|
| ReadMe | File descriptions |
| lac_data.dat | Lacaille's catalog from [CA] |
| obs_data.dat | Observations from [EA] |
| neb_data.dat | Nebulous objects from [EN] |
| lac_notes.dat | Notes on the source catalogs [CA], [EA], [EN] |
Lacaille's catalog from [CA]
Lacaille's catalog of 1942 stars from [CA] is contained in the file lac_data.dat. The file format 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 typos and features of the source catalogs that didn't make it to the digitized version.
According to the title, the positions given in [CA] are reduced ad annum 1750 ineuntem, the beginning of year 1750 (Gregorian calendar), which is
| Julian Day | 2,360,235 |
| Besselian epoch | B1750.0027… |
| Julian epoch | J1750.0068… |
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 described below. Discrepancies in letter assignments and gross differences in positions are described in lac_notes.dat.
Nebulous objects from [EN]
The file neb_data.dat contains the identifiers and coordinates of the 42 objects from [EN]. To avoid file clutter, it also includes the object descriptions, in French, UTF-8 encoded. Its format is specified in ReadMe and it is subject to automatic validation, provided that the descriptions starting at column 25 be trimmed off, since the Anafile package of the VizieR service doesn't support non-ASCII characters.
There is more information about Lacaille's nebulous objects at the SEDS page on Lacaille [3].
All the nebulous objects from [EN] have counterparts in [EA] and [CA], but a couple of nebulous objects from [EA], including one from [CA], don't appear in [EN]. The coordinates are sometimes a little different between these sources. These cases are documented in lac_notes.dat.
Star identifications
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 [2] for instance) but maybe not for non-lettered stars (at least, I haven't found any trace of it on the Web). It also establishes the correspondence between [EN], [EA], [CA] and gives modern identifiers (Messier, NGC, IC, etc.) for nebulous objects.
| File name | Explanation |
|---|---|
| lac_ident.dat | Cross-identifications of objects from [CA], [EA], [EN] |
| lac_bids.dat | Modern letter assignements to Lacaille stars |
For each object in the [CA] catalog, lac_ident.dat gives the corresponding number in [EA] and identifier in [EN], if any, as well as redundant modern identifiers of the corresponding star(s) (Bayer and Flamsteed designations; Bright Star, Draper, and Hipparcos catalog numbers). There are a few discrepancies with Wagman [2], documented in lac_notes.dat.
The correspondence with modern catalogs was established by finding all the stars from the Hipparcos, SAO, and Henry Draper star catalogs (adjusted to the 1750 epoch and equinox for proper motion and precession) within 5 minutes of arc of Lacaille's positions. There were clear Hipparcos matches in a large majority of cases. The larger SAO catalog (which has proper motions as well) provided a handful of additional matches not found in Hipparcos. Finally, the Henry Draper catalog was the only one to provide a match for 5 nebulous objects.
The instances of multiple matches were resolved manually, taking into account the proximity and magnitudes of the matches, using the sky photographs 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 lac_ident.dat also gives the position differences between [CA] and the modern catalog where the match was found (Hipparcos, SAO, or Henry Draper). If the match comes from the SAO catalog, the SAO number is mentioned in the star name column.
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 [2], and sometimes Gould's notes starting on p. 140 of Uranometria Argentina [4]. A plausible match was found in most cases; only 11 cases remain where no match could be found (they are flagged by a star in column 17 of lac_ident.dat).
Duplicate entries
There is only one case of duplication in the [CA] catalog. The entries 1735 and 1736 in [CA] with corresponding entries 1728 and 1729 in [EA] represent the same star, ν Oct (nu Octantis). Coincidentally (?), this star turns out to be a spectroscopic binary.
Modern letter assignements to Lacaille stars
While many of Lacaille's letter assignments are still in use today, many of them have been modified.
One one hand, similarly to Bayer, Lacaille has assigned 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 altered. Most of these alterations are due to Francis Baily who built on Lacaille's work, as mentioned above, and to Benjamin Gould, author of his own catalog of southern stars in Uranometria Argentina [4]. The changes are due to various causes: an author's 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 fate of Lacaille's letter assignments. I used Morton Wagman's book [2] 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. The same modern assignments are used in lac_ident.dat.
Accuracy of the catalog
The file lac_ident.dat 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 Lacaille's positions and modern ones (from the Hipparcos, SAO, or HD catalogs) adjusted to the 1750 equinox for proper motion and precession.
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 0.3 arcmin, vs. 0.6 arcmin for Flamsteed's catalog, and 2 arcmin for Tycho's. Using this value as a measure of accuracy, we could say that Lacaille's catalog is twice as accurate as Flamsteed's.
References
[1] F. Verbunt & R. H. van Gent, Early star catalogues of the southern sky, Astronomy & Astrophysics, 530, A93, 2011.
[2] Morton Wagman, Lost Stars, McDonald & Woodward Publishing Company, Blacksburg, Virginia, 2003.
[3] Hartmut Frommert & Christine Kronberg, Lacaille's "Catalog of Nebulae of the Southern Sky" at the Messier pages at SEDS.
[4] Benjamin Apthorp Gould, Uranometria Argentina, Buenos Aires, 1879.
[5] 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.
[6] 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.
[7] 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.
[8] 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.