Claudius Ptolemy (c. AD 100 – c. 170) was a Greco-Roman mathematician and astronomer who lived in Alexandria, in the Roman province of Egypt. He wrote a treatise on astronomy entitled Μαθηματικὴ Σύνταξις (Mathematical Treatise) known as the Almagest, after the Arabic version of its name, that remained the reference work on astronomy for about 1400 years until the time of Copernicus.

It contains the most ancient star catalog that we possess, thought to be an adaptation and possibly an extension of a more ancient catalog composed by Hipparchus more than two centuries earlier (see Rawlins [5]).



Ptolemy's catalog contains 1028 entries representing 1025 distinct stars or nebulous objects (in three instances, listed below, two entries refer to the same star in the context of different constellations).

It groups the stars into 48 constellations, the so-called 48 Ptolemaic constellations, thought of as imaginary figures drawn in the sky representing characters, mythical creatures, animals, or inanimate objects. Some stars mentioned in the catalog but not immediately part of the figures are referred to as ἀμόρφωτοι, informes, unformed.

For each star, the catalog provides the ecliptic coordinates, the magnitude and, as a device to identify it uniquely, the description of its situation with respect to the constellation figure. These descriptions have been more or less adhered to by later celestial cartographers such as Bayer [16], Flamsteed [13], Hevelius [15], Bode [12], Argelander [9].

A lot of ink has already been shed over Ptolemy's work by eminent scholars. I don't have anything new to add but I wanted to make available an electronic copy of the catalog as close as possible to the original, without "corrections" or additions of my own, as I have tried to do with the constellation boundaries and Flamsteed's catalog. This data has survived for more than 2000 years (if we think that it came from Hipparchus), first in manuscript form, then printed form, then scanned electronic form, and the natural next step seems to be an electronic format susceptible of treatment by computer programs.

Unfortunately, as explained in the introduction to Peters & Knobel [7], all the copies and translations of Ptolemy's catalog that have survived are plagued with transcription errors. Several authors have tried to restore a plausible version, with different results. I ended up working with six different versions of the catalog, in scanned or paper form.

  • The star catalog from Toomer's translation of the Almagest [4]. I encoded it from the original (taking the corrigenda on pp. xii–xiii into account) and compared the result with its previously digitized version, VizieR catalog J/A+A/544/A31 [17], for confirmation. The very few differences in star positions that I found are documented in notes_t.dat. See also the corresponding article by Verbunt & van Gent [2].
  • Peters & Knobel's version [7], because C. H. F. Peters has gone to great lengths to compile as many manuscripts as he could in order to figure out what the original data looked like. For Toomer [4, p. 14], it is "still the best treatment of the catalog as a whole..." although he adds "... though badly in need of updating and revision in certain respects". I copied it from scratch and compared the result with its previously digitized version [20], which allowed me to correct my initial typos (and find several others in [20] as well).
  • Manitius's version [8], referred to by Toomer and Peters & Knobel, and the source of VizieR catalog V/61 [19]. There are just a few differences in coordinates between Manitius and V/61, documented in notes_m.dat, but quite a few in star identifications, of which several look like typos in V/61. John Pratt's version [18], based according to his page on the translation of the Almagest by R. Catesby Taliaferro, is also very close to Manitius's. The few differences in star positions and magnitudes are documented in notes_m.dat as well.
  • Baily's version [10], since Peters & Knobel refer to it and Baily has critically examined several printed versions of the catalog. Baily also introduces a convenient global numbering of Ptolemy stars, also used by Peters & Knobel, that I have used consistently for all versions.
  • Halma's version [11], the first and so far only French translation of the Almagest from the Greek, based on manuscripts from the public library of Paris and one of Baily's sources [10, p. 6]. I didn't show the cross-identifications from this version because they contain too many typographical errors and are sometimes hard to make sense of.
  • Flamsteed's version contained in Historia Coelestis Britannica [14], because I wanted to compare its star identifications with the ones implied in Flamsteed's own catalog, which mentions Ptolemy catalog numbers when possible. The files to compare are cross_z.dat and cross_f.dat.

Catalog data

The files ptolemy-*.dat contain various versions of the catalog, with the star numbers, ecliptic coordinates, magnitudes and descriptions, but without the cross-identifications added by the authors. These are gathered in separate files cross-*.dat described below.

The descriptions are UTF8-encoded for Greek characters, accented and other special characters in French and German, and some special characters in Latin. The browser or text editor may need to be configured to use the Unicode or UTF8 encoding to display these files properly.

The files ptolemy-*.dat are described in the ReadMe file, but they should be stripped of the star descriptions (everything after column 32 included) before they pass the automatic validation of the Anafile package of the VizieR service, which doesn't allow non-ASCII characters. The files notes_*.dat are free-form files with various observations about the originals.

File name Explanation
ReadMe File descriptions
ptolemy_t.dat Ptolemy's catalog from Toomer
ptolemy_p.dat Ptolemy's catalog from Peters & Knobel
ptolemy_m.dat Ptolemy's catalog from Manitius
ptolemy_b.dat Ptolemy's catalog from Baily
ptolemy_h.dat Ptolemy's catalog from Halma
ptolemy_z.dat Ptolemy's catalog from Flamsteed
notes_t.dat Notes on Toomer's version
notes_p.dat Notes on Peters & Knobel's version
notes_m.dat Notes on Manitius's version
notes_b.dat Notes on Baily's version
notes_h.dat Notes on Halma's version
notes_z.dat Notes on Flamsteed's version

I have double-checked my work very carefully and I hope that no transcription errors are left. If any are found, I would appreciate the feedback.

Nebulous objects

Ptolemy's five nebulous objects are summarized in the following table. Only 191 Per 1 and 449 Cnc 1 correspond to actual non-star objects.

PtolemyBayer / Fl.Comment
191 Per 1 Per h Double cluster NGC 869 / NGC 884
Per χ
449 Cnc 1 Open cluster Praesepe, M44, NGC 2632
567 Sco ~1 Sco G Marked nebulous maybe because of the proximity of
NGC 6441 (Peters & Knobel)
577 Sgr 8 32 Sgr ν1 There are several small stars close by (Peters & Knobel)
35 Sgr ν2
734 Ori 1 29 Ori λ Marked nebulous maybe because it forms a small cluster
with 37 Ori φ1 and 40 Ori φ2 (Peters & Knobel)

On the other hand, the entry 855 Cen 21 corresponds to the globular cluster ω Cen (NGC 5139) and 559 Sco 14 most likely to the open cluster NGC 6231 [6], but they are not identified as a nebulous objects in Ptolemy's catalog.

Star identifications

The files cross_*.dat listed below summarize the star identifications according to various versions of the catalog. The file format is described in ReadMe as well and they are subject to automatic validation.

File name Explanation
cross_t.dat Star identifications according to Toomer
cross_p.dat Star identifications according to Peters & Knobel
cross_b.dat Star identifications according to Baily
cross_m.dat Star identifications according to Manitius
cross_z.dat Star identifications according to Flamsteed
cross_f.dat Star identifications extracted from Flamsteed's catalog

All the catalog versions associate contemporary star designations with Ptolemy stars, in the form of Bayer letters, Flamsteed numbers, Bright Star (HR) numbers, or designations more or less fallen into disuse (e.g., Piazzi, Groombridge, Hevelius, Heis, Lacaille, Ambronn).

Letter assignments have shifted over time due to the contributions of several authors and should be interpreted in their context. Most notably, Baily and Flamsteed use the original Bayer lettering in Argo Navis, Centaurus, Lupus, Ara, and Corona Australis, while Toomer, Peters, and Manitius use a "post-Lacaille" lettering. Baily [10, p. 16] uses the Bayer letters that he retained in his corrected and enlarged version of Flamsteed's catalog in An Account of Flamsteed. Flamsteed uses letter assignments that he introduced in his own catalog.

I have used the notes interspersed between catalog pages in Baily's version [10] and the notes, pp. 106–113, in Peters & Knobel's book [7] to extract additional information about star identifications. The table pp. 114–119 of Peters & Knobel compares uncertain star identifications in several versions of the catalog, including Peters's, Baily's, and Manitius's.

There are many uncertainties in the identification of Ptolemy's stars and considerable effort has been spent over the centuries to elucidate them. I will not add to the general confusion by attempting my own identifications, but just mention two other relevant articles: Keith Pickering's [3] and René Bourtembourg's [1]. The latter suggests that one of the uncertain stars may correspond to an observation of the planet Uranus.

Duplicate entries

As mentioned earlier, there are three pairs of duplicate entries in Ptolemy's catalog. They are the following.

Ptolemy designationsModern equivalents
96 Boo 9 = 147 Her 29 Boo ν = Her ψ
147 Her 29 = 96 Boo 9 Her ψ = Boo ν
230 Aur 11 = 400 Tau 21 23 Aur γ = 112 Tau β
400 Tau 21 = 230 Aur 11 112 Tau β = 23 Aur γ
670 Aqr 42 = 1011 PsA 1 79 Aqr = 24 PsA α
1011 PsA 1 = 670 Aqr 42 24 PsA α = 79 Aqr

Summary files

The files pos_all.dat and mag_all.dat summarize the star positions and magnitudes of all Ptolemy stars in all versions (except Flamsteed's catalog, which has its own coordinates and magnitudes).

File name Explanation
pos_all.dat Summary of star positions across all versions
mag_all.dat Summary of star magnitudes across all versions

They make differences and commonalities between versions immediately apparent. Flamsteed's and Halma's versions, in particular, contain some egregious positions.


[1] René Bourtembourg, Was Uranus observed by Hipparchus?, Journal for the History of Astronomy, Vol. 44, pp. 377–387, 2013.

[2] F. Verbunt and R. H. van Gent, The star catalogues of Ptolemaios and Ulugh Beg, Astronomy & Astrophysics, 455, A31 (2012).

[3] Keith A. Pickering, A Reidentification of some entries in the Ancient Star Catalog, DIO, Vol. 12, pp. 62–63, 2002.

[4] Ptolemy's Almagest translated and annotated by G. J. Toomer, with a foreword by Owen Gingerich, Princeton University Press, 1998 (pp. 341–399). Available in print.

[5] Dennis Rawlins, An Investigation of the Ancient Star Catalog, Publications of the Astronomical Society of the Pacific, Vol. 94, p. 359, 1982.

[6] William B. Ashworth, Jr, Halley's Discovery of NGC 6231 and the Hazards of Early Star Nomenclature, Journal for the History of Astronomy, Vol. 12, pp. 1-10, 1981.

[7] Christian Heinrich Friedrich Peters & Edward Ball Knobel, Ptolemy's Catalogue of Stars - A Revision of the Almagest, The Carnegie Institution of Washington, 1915 (pp. 27–50). Available here and here.

[8] Karl Manitius, Des Claudius Ptolemäus Handbuch der Astronomie, B.G. Teubner, Leipzig, 1912-13 (Tome 2, pp. 32–63). A German translation of the Almagest available here.

[9] Friedrich Argelander, Uranometria nova, Berlin: Schropp, 1843.

[10] Francis Baily, The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, Hevelius, Deduced from the Best Authorities. With Various Notes and Corrections, and a Preface to Each Catalogue. To Which is Added the Synonym of each Star, in the Catalogues of Flamsteed of Lacaille, as far as the same can be ascertained. Memoirs of the Royal Astronomical Society, Vol. 13, London, 1843. Available here and here.

[11] Nicholas Halma, Composition mathématique de Claude Ptolémée, traduite pour la première fois du grec en français, sur les manuscrits originaux de la bibliothèque impériale de Paris. H. Grand, Paris, 1816 (Tome 2, pp. 33–83). A Greek version and French translation of the Almagest available here.

[12] Johann Elert Bode, Uranographia sive Astrorum Descriptio, Berlin, 1801. Plates available on Felice Stoppa's Atlas Coelestis site.

[13] John Flamsteed, Atlas Coelestis, London, 1729. High resolution plates visible here.

[14] John Flamsteed, Historia Coelestis Britannica, London, 1725. Available here.

[15] Johannes Hevelius, Firmamentum Sobiescanum, in Prodromus Astronomiae, Gdańsk: Johann Zacharias Stoll, 1690. High resolution plates visible here.

[16] Johann Bayer, Uranometria, Augsburg: Christoph Mang, 1603. High resolution plates from a 1655 edition are available here.

Online versions of the catalog

[17] VizieR catalog J/A+A/544/A31 by F. Verbunt and R. H. van Gent (2012), an electronic version of Ptolemy's catalog based on the translation by G. J. Toomer.

[18] John P. Pratt's page on Ptolemy's Star Catalog, and his electronic version of the catalog (2015).

[19] VizieR catalog V/61 (1987), an electronic version of Ptolemy's catalog based on the translation by K. Manitius, according to the ReadMe file.

[20] An electronic version (in HTML) of Ptolemy's catalogue on Brian Tung's site, also mentioned here. This is a transcription of Peters & Knobel's version, with additional data fields. It also appears on the Atlas Coelestis site of Felice Stoppa.


  • 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:Claudius Ptolemaeus (Ptolemy). Woodcut by T. Stimmer, 1587. Credit: Wellcome Collection. CC BY