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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.
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:
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.
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