Almagest Star Catalogue

for Guide Users

Edited by Eduardo Vila-Echagüe

August 2000

Click here for Italian version


Contents of the Catalogue
Star Identification
Sources and Implementation of the edition for Guide users
Instructions for the Guide user
The Editor


The Almagest Star Catalogue was published in Alexandria around 150 AD, as part of the famous Almagest, the book in which the great Greek astronomer Claudius Ptolomaeus presented his geocentric theory of the planets. There are reasons to believe that most of the stars of the catalogue were observed by Hipparchus, between the years 160 BC to 130 BC. In the later Middle Ages and in the Renaissance the Almagest had a tremendous impact on European astronomy. Copernicus, Tycho Brahe and Kepler made an extensive use of this book. Almost an exact copy of the Star Catalogue was included by Copernicus in his De Revolutionibus.

Contents of the Catalog

The catalogue itself is made up of 1028 star entries. The stars are listed by constellations, which in turn are grouped in Northern, Zodiacal and Southern. For each star the following information is given. Positions are generally given in sixths of a degree (10' of arc), except for a few latitudes which are given in fourths (15' of arc). The sky is covered from the North Pole to about 52° of Southern Declination (for 150 BC), which was the practical limit of observation from the latitude of Alexandria (31° N) .


An important point is to establish the epoch for which the positions are given. Though Ptolomaeus implies that the catalog should be dated near 138 AD, the best fit when compared to modern observations gives an epoch of 43 AD. The explanation is that Ptolomaeus seems to have used Hipparchus' positions and precessed them to his own times, by adding 2°40' to each longitude. But as he followed Hipparchus in using a wrong value for the precession, 1°/century, instead of the real value, 1.4°/century, his longitudes are too small. If we restore Hipparchus' original catalogue by subtracting 2°40' to all Ptolomaeus' longitudes, the resulting values correspond to an epoch around 140 BC, quite compatible with the known dates when Hipparchus performed his observations.

Star Identification

One of the tasks of the modern historian of astronomy is to link the catalogue entries with existing stars. For that purpose he uses the tabulated positions, the magnitudes and the descriptions of the place of the stars within the figure of their constellations. These must be compared with the data of a modern star catalogue, expressed in the same coordinate system, for the same epoch and including corrections for proper motion. This may result in a very tiresome process when done without an adequate graphical tool as that provided by this Almagest Catalogue for Guide users. When comparing the identifications done in the past, one is amazed to see the discrepancies between the different efforts, showing that there is still an opportunity for improvement in this process.

For most of the catalogue entries, the identifications are pretty straight forward. Only one star of a similar magnitude is found near the place tabulated in the Almagest. In these cases the average difference between the Almagest position and modern values is about 30' of arc in each coordinate. But in about 10% of the situations, identification is not that easy. Either the position does not agree with the catalogue description, or there is a great difference in magnitude, or there are no stars or too many near the tabulated values. Additionally the positions themselves may have become corrupted during the many centuries of manuscript transcription. There is nothing as boresome as copying long strings of numeric data. As a consequence for many stars different values are found in the various sources. Some may be easily spotted and corrected, as when the longitude is expressed with a wrong Zodiacal sign, or when North and South are interchanged. Others are less obvious, and the investigator must look into the Greek figures to understand the ways these transcription errors are made. For instance, in many cases a latitude is found as 33° in a source and as 30° 20' in another one. This is easier to understand when you know that the first number is written in Greek as λγ (30 + 3) and the second as λγ' (30 + 1/3).

But in some cases the blame cannot be put on the copyists. Hipparchus and specially Ptolomaeus did made mistakes. Some of them are systematic and can be accounted for. An example is offered by the stars of the Southern Cross, which are shifted almost 4° to the South of the real positions. In other cases we presume an error when all the sources agree in a position distant several degrees from the star corresponding to the description. There are some extreme cases  where no identification is possible with any degree of probability: there are no actual stars matching either the descriptions nor the tabulated positions.

Sources and Implementation of the edition for Guide users

I have found two online versions of the Almagest Star Catalogue: Additionally I have used the English translation of the Almagest included in Volume 16 of the collection of the Great Books of the Western Worlds (Encyclopaedia Britannica Inc., 1984) and, in a few cases, the version of Copernicus' De Revolutionibus included in the same volume.

This edition for Guide users is a combination of the two internet sources with minor inclusions from the text sources. Usually the positions are taken from the Centre de Donnés Astronomiques, except when there is an obvious typo (there are several of them) or where the other sources offer a much better choice. The ecliptic positions used are shown in the 'More Info' window, as well as a note indicating if there are differences with those present in this source.

As Guide does not handle catalogues other than those with equatorial coordinates, these have been computed in the following way:

It must be noted that the obliquity of the ecliptic does not change significantly from century to century, so that the results would have been almost the same if I had used that of 150 BC, or even of 150 AD. Hipparchus and Ptolomaeus value for the obliquity was significantly larger (23° 51' 20"), but there is no reason to think that they have used it to derive their ecliptic coordinates.

Magnitudes are taken from the same source, but are expressed in the way the original catalogue does (see above). To comply with the needs of the Guide implementation, magnitudes 7 and 8 are reserved for the stars called obscure and nebulous respectively. The words 'major', 'minor' (see above), 'obscura' and 'nebula' are presented when applicable as notes in the 'right click' and the 'More Info' windows.

The descriptions (in Latin) are taken from The Astronomy Corner source; I have made no attempt to translate them into English (they can be found in the Great Books), but have corrected some misspelling in the Latin. This language was chosen because it was the only one for which a soft copy was available. Additionally its syntax is much closer to the original Greek than our modern languages.

In the identifications I have followed the practice of both sources, labeling the real stars with the Yale Bright Star numbers. In a couple of cases the 'stars' are really nebulae or clusters and I have used the NGC denominations for that purpose.

The identifications of the Centre de Donnés Astronomiques are plagued with errors. In many cases I have followed those of The Astronomy Corner source, while in a few I have preferred my own. These situations have been marked as notes in the 'More Info' window.

Instructions for the Guide user

The Almagest stars are identified in Guide 7 with the symbol of a white star with a pink 'X' superimposed, or just with a pink 'X' in Guide 6. The size of the star symbol corresponds to the magnitude in the catalogue, while the size of the 'X' is also related to the brightness. The symbols are better seen in levels 4 (20°) and 5 (10°).  Obscure and Nebulous stars may be misrepresented, as they will look as stars of the seventh and eighth magnitude, respectively.

I recommend to switch to 'off' the display of every other objects except stars, and to set the limiting magnitude for the latter at 6 or 6.5. The Almagest objects should be set to 'on', so that all of them can be seen.

In order to take account of the proper motions of the stars, you should set the date in the Time box at the time you assume the observations were made. 140 BC is a reasonable choice. Only for a few stars this will make a difference larger than the catalogue's accuracy. See, for example, α Centauri or ο² Eridani. This last case is one of the best arguments for dating the catalogue at Hipparchus times.

To verify identifications it is advisable to print a hard copy from The Astronomy Corner source. The Almagest  labels should be turned on. This is done by pointing to any Almagest symbol, right clicking the mouse, pressing the Display button and checking the label box in the opening window. At the same time all the real star naming or numbering should be checked off in the Star Display submenu, with the exception of the Yale Bright numbers which should be turned on. As a consequence all the Almagest symbols will have a label in white with an 'A' followed the number in the catalogue, and the real stars will be seen with their YBS numbers in gray. Using the 'Goto .TDF' submenu, located in 'Goto' menu, you can go directly to any symbol in the catalog. Right click on it and you will see the suggested YBS identification. The real star will probably be seen nearby and you can make your own judgement about the validity of the identification.

The great benefit of Guide is that with just one look it allows you to see a lot of Almagest symbols together with their corresponding real stars. It will be seen immediately than in many areas the differences in their positions are not random, but that the symbols have about the same displacements (both in distance and position angle) relative to their stars. The cause of this is that the ancient observers performed better when measuring relative positions than when obtaining absolute ones. With this knowledge further analysis can be performed to try to establish their methodologies and even the relative timing of their observations. An example of this is that the positions of the entries in the Northern constellations have a better fit when the Epoch of the catalogue is set to the year 150 BC (which can be done by editing the Almagest.TDF file), while 130 BC is better for the Zodiacal and Southern ones. This makes sense if we assume that Hipparchus worked from North to South, approximately in the same order of the constellations in his catalogue.

I am confident that this Almagest Star Catalogue for Guide users will help both researchers in refining their investigations and also amateurs in introducing themselves into this fascinating treasure of ancient star lore.

The Editor

Eduardo Vila-Echagüe has been a devoted amateur for the last 50 years. He has been a member of amateur astronomy societies in Argentina and in Chile, where he currently resides. He combines a passion for the history of the ancient world with his love for astronomy. Some of his writings can be found at his web site at He can be contacted at edovila at vtr dot net (address modified to evade spammers.) .