The Ptolemy Star Catalog

by John P. Pratt
Mon 14 Dec 2015 (1 Water)

A downloadable translation of Greek astronomer Ptolemy's star catalog, with stars identified using modern designations, in standard spreadsheet format.

Claudius Ptolemy was a Greek mathematician, astronomer, and geographer who wrote about 150 AD. His work on astronomy known by its Arab name The Almagest was the standard of astronomy for some fifteen hundred years after his death. It is the only record extant of the constellations as known by the earlier Greeks. Even his record was preserving the actual measurements of stellar positions made some three centuries earlier by Hipparchus. Most of the star names used today are of Arab origin and are essentially Arab translations of Ptolemy's designations for the stars. While his catalog is mostly overlooked today, it was preserving a tradition that may yet to proven to be of great worth.

The star tables in Ptolemy's Almagest have been presented very cleverly in a manner that allows for mistakes to be mostly discovered and corrected. They present two different descriptions of the stars, which seem to divide neatly into "left brain" and "right brain" descriptions. That provides a method of double checking their correctness, even as the two halves of the brain appear to be intended to provide two views of the world which hopefully agree as two witnesses.

The Cambridge Star Atlas
The Cambridge Star Atlas
On the left brain side, there is a list of measured (ecliptic) latitudes and longitudes for every star in the catalog. As stated, this is the only such list still extant from the ancient world and is invaluable. But it has great room for errors, both in measurement, in recording, in translation, in the final printing, and in transcription into this catalog being presented. Indeed, there are several entries where there are no stars near the positions indicated in what we have today. How can such errors be detected?

Fortunately for the right brain, along with the numbers are descriptions of the figures of the constellations as seen in the sky. These have at least two features. First, the tell us how to draw the pictures: this star is the head, this one the elbow, etc. But secondly, the say things like, "There are three bright stars in a straight line forming the arm". If one looks at the indicated place either in the actual sky or in a star atlas such as the recommended The Cambridge Star Atlas, then one indeed sees three bright stars in a straight line. But when looking at the numbers, one of them might have an incorrect number by several degrees. In those cases, the error seems to be clearly in the numbers because there are indeed three stars in a straight line. In other cases it seems clear that he has switched "eastern" with "western" when describing two stars. As each star was reviewed for having been accurately identified, there was only one star which remained uncertain after applying this test. It was the last star listed in Hydra, being outside the constellation. The identifications provided in this work were all verified by this author and occasionally differ from others. Words in brackets [] in the descriptions are my attempt to correct obvious errors in the translation.

Because part of the reason for producing this star catalog is not only to study the accuracy of ancient measurements, but also to know how the ancient figures appeared, it is important to understand that Ptolemy freely admits he has changed them, as had done Hipparchus. He states: "We have not used for each of the stars altogether the same formations as our predecessors, just as they did not use the same as their predecessors. But often we use others according to the greater propriety and fittingness of the configurations, as for example, when those stars which Hipparchus places in the shoulders of the Virgin, we call the sides because of their distance from the stars in the head appears greater than from the hands, and thus they better fit the sides" (from the translation used for this work, The Almagest, Great Books of the Western World, Enclyclopaedia Britannica, U. of Chicago, 1952, vol. 16, pp. 233-234, translated by R. Catesby Taliaferro).

Col.Contents
AConstellation Number
BConstellation Name
CStar Entry Number
DIn the Figure? (Yes or No)
EStar Location
FMultiple Entry?
GImplied Entry?
HEcliptic Longitude (°)
IEcliptic Longitude (')
JLatitude (North or South)
KEcliptic Latitude (°)
LEcliptic Latitude (')
MStar Magnitude
NSidereal Longitude (°)
OSidereal Longitude (')
PZodiac Longitude
QZodiac Sector
RBright Star Number
SNumber of Double Star
TModern Star Designation
UZodiac Longitude
VZodiac Sector
WLatitude (rounded up)
XMagnitude (both if double)
Columns A-Q are Ptolemy,
R-X are star identification.
It was surprising to me to what extent he felt justified in modifying the figures. He separates the stars listed for each constellation in to two categories: those in the actual figure and those in the surrounding neighborhood. In his descriptions, he states that he removed the brightest star in Aries (Hamal) out of the constellation entirely, which Hipparchus had placed in the head. To me it sounded like he felt that was an achievement of some sort, rather than verging on insanity. As if those who had first drawn the figures had said, "Those stars look like a Ram, except let's leave the bright one out of it." Another example is that he removed the brightest star in the entire northern spring sky, Arcturus, entirely out of the constellation of Bootes (the Herdsman) and placed it between his legs. In the much earlier descriptions of Aratus, that very star is explicitly mentioned as being just below the belt.

There was a discovery made in the preparation of this work which might be helpful to those interested in restoring the original figures. In several cases, the stars listed as outside the constellation can clearly be seen as once having been in the constellation but removed by someone who thought the river was too long, or shouldn't stretch into another area that was the turf of the neighboring constellation. Astronomers tend to want a list of stars and to use the constellations, if at all, only to help find stars. Those who originated the figures saw great interaction between the figures and had myths that accompanied them from earliest times.

In many cases there are very many fairly bright and even more dim stars surrounding a constellation. Many of these have been made into the later constellations to fill in gaps. But Ptolemy does not seem interested in mapping out all the stars in the neighborhood. For about half of the constellations he lists no stars nearby whatsoever. In other cases, he skips many fairly bright stars but lists a few very dim ones surrounding the constellation. Why would he do that? In nearly every case, those stars listed can be seen to have most likely formed part of an earlier version of the figure. One example is Bootes, where the only star listed as nearby is Arcturus! Another example is the stars of Libra, The Balance. That constellation was apparently entirely lost to the Greeks even though it had been known in ancient Babylon. To the Greeks it was only The Claws of the scorpion. But some of the stars listed as between the claws and not in the figure are the very ones which seem to clearly have formed the scales. They may be treated in a future article which attempts to restore the original forms.

Another thing that irritates and even confuses astronomers is when two constellations share the same star. Ptolemy indeed had trouble being consistent on this point as have others who have translated his list and numbered his stars. It appears to have been his desire to make a table of stars, so his usual method is to note that a certain star is part of two constellations, but he only lists them once where he mentions both figure positions. He does that seven times. But three times he actually lists the stars in both constellations, so he is not consistent in his method. Thus most people say that his table lists 1,028 stars because it has that many entries, whereas there are really only 1,025. Because part of my purpose is to understand how the figures looked, in the version presented here, the stars are listed in both constellations. There are two new columns added to keep track of this. First there is a multiple entry (ME) column where there are ten pairs of entries labeled a to j which shows which stars are the same in two constellations. There is also a column labeled "I" for "Implied" stating that this entry was added to his list and only implied by his earlier statement. The numbering of each star in the constellation proper has not been changed; the implied entries come at the end of his list. They do, however, change the star numbers of the stars listed as not in the constellation. Thus, there are 1,035 entries in the tables offered here which describe only 1,025 stars.

To actually locate the stars in this catalog, it is usually necessary to look up their positions on a star map to see their positions relative to others in the sky. The spiral bound version of The Cambridge Star Atlas by Wil Tirion is recommended to be used in conjunction with these tables. The maps indicate not only the usual Bayer Greek letters and Flamsteed number designations, but also the upper and lower case English alphabet designations used extensively in the southern skies. The maps are also in color and lay flat with no stars lost in the binding. Moreover, it has stick figure constellations shown which are extremely useful to find stars quickly, but which are not found in almost any other detailed star atlas. Other atlases are more like a map of the U.S.A. would be with only cities shown as dots without the outlines of the states drawn in.

Hopefully this version of the Ptolemy Star Catalog will be useful to others doing star table analysis. It can be downloaded free by clicking the above "download" button. Then double click on the file, which should bring up your default spreadsheet program. Then select "comma" as the column separator. The columns are described in the table. Columns A-M refer directly to Ptolemy's actual table. Columns N to Q convert his longitude measurements to the equivalent sidereal longitude by reckoning them from his longitude value of Pi Vir, the origin of sidereal longitude. Zodiac Longitude is merely sidereal longitude broken into the 30 degree zodiac sectors. That is, a sidereal longitude of 41 degrees is 11 Lib (subtract off 30 for Virgo). Then columns R-X refer to the modern star identification and provide its sidereal longitude, latitude and magnitude for comparison. The sidereal coordinates are taken from the Mag 5 Star Catalog.