Description
This page addresses the identification of entries in Kepler's 2a classis and 3a classis catalogs. The general features of these identifications are described at length on the corresponding Ptolemy page and are not repeated here.
Identifications
The file ident_a.dat contains an expanded form of the identifications provided by VizieR catalog J/A+A/530/A93. The file ident_o.dat contains the identifications that I have retained for the maps, cross-references and distances. The file notes_id.dat comments on some of my choices.
| File name | Description |
|---|---|
| ReadMe | File descriptions |
| ident_a.dat | Identifications according to VizieR J/A+A/530/A93 |
| ident_o.dat | Identifications retained for the maps etc. |
| notes_id.dat | Explanation of some identification choices in ident_o.dat |
Cross-references
The following files cross-reference Kepler's catalogs with a few others and the reference star set, according to the identifications selected in ident_o.dat and the corresponding identifications file of the other catalogs. They are described in ReadMe as well. Their structure is explained on the corresponding Ptolemy page.
| File name | Explanation |
|---|---|
| keppto_oo.dat | Cross-references from 2a classis to Ptolemy's catalog |
| kephou_oo.dat | Cross-references from 3a classis to Houtman's catalog |
| kephal_oo.dat | Cross-references from 3a classis to Halley's catalog |
| keplac_o.dat | Cross-references from 2a and 3a classis to Lacaille's catalog |
| kepref_o.dat | Cross-references from 2a and 3a classis to the reference star set |
The file keppto_oo.dat shows the correspondence between Kepler's 2a classis catalog and Ptolemy's. Most 2a classis stars come from Ptolemy and are matched with their Ptolemy equivalents (I made sure that their identifications are the same). Identified entries that have no match in Ptolemy (flagged with an x) may have been observed by Kepler himself. A few unidentified entries (flagged with a *) still have clear matches in Ptolemy; they are documented in notes_id.dat.
VizieR's catalog J/A+A/530/A93 (in the file classis.dat) gives a Ptolemy star number equivalent for many Kepler 2a classis stars. Their assigments differ from mine in a few cases, documented in the file notes_a.dat.
The files kephou_oo.dat, kephal_oo.dat, and keplac_o.dat cross-reference Kepler's catalog with other southern stars catalogs of interest: Houtman's, Halley's, and Lacaille's respectively.
TODO: mapping to Ridpath [1].
Accuracy of the catalog
The file dists_o.dat, also described in ReadMe gives the differences between Kepler's positions and modern positions of the corresponding stars (according to ident_o.dat) from the Hipparcos or the SAO catalog, adjusted to Kepler's epoch and equinox (the same as Tycho's). The corrections from corrs_2.dat mentioned on the catalog page have been applied before computing the distances.
| File name | Explanation |
|---|---|
| dists_o.dat | Differences between Kepler's positions and modern ones |
The file gives differences in ecliptic longitude (multiplied by the cosine of the latitude), differences in ecliptic latitude, and great circle distances, in arcminutes. The corresponding histograms below have been drawn separately for 2a classis and 3a classis since the observations behind these catalogs come from completely different sources.
| Longitude | Latitude | Distance |
|---|---|---|
 |
 |
 |
For the 2a classis catalog, we have a peak of the distances histogram at about 40 arcmin, maybe surprisingly worse than the value for Ptolemy's catalog at 30 arcmin.
For the 3a classis catalog, the peak occurs at about 60 arcmin. The relatively poor precision of this catalog shouldn't be surprising given the rough conditions in which the observations were carried out (see [2, p.11]).
| Longitude | Latitude | Distance |
|---|---|---|
 |
 |
 |
The article by F. Verbunt and R. H. van Gent [2] has a detailed statistical discussion of the position and magnitude errors in Kepler's catalogs.
References
[1] Ian Ridpath, Identifying the stars on Johann Bayer's Chart of the South Polar Sky, Journal of the Society for the History of Astronomy, Issue 8, April 2014, pp. 97–108.
[2] F. Verbunt & R. H. van Gent, Early star catalogues of the southern sky, De Houtman, Kepler (second and third classes), and Halley, Astronomy & Astrophysics 530, A93 (2011).