Johannes Kepler (1571–1630) was a German mathematician and astronomer. His major claims to fame are the discovery of his laws of planetary motion, perfecting the Copernician system, and paving the way to Newton's dynamics.

Johannes Kepler

Johannes Kepler

Kepler was born in Weil der Stadt, in the south-western German state of Baden-Württemberg, in a family whose fortune was in decline. Showing promise at a young age, he was educated at the expense of the Duke of Württemberg. In 1589, he went to study at the university of Tübingen, where he was exposed to Copernician astronomy by Michael Mästlin, professor of mathematics. Astronomy was not his primary interest yet but in 1594 he accepted a position of teacher of mathematics and astronomy in Graz.

In 1596, he published Mysterium Cosmographicum where he developed his famous Copernician-based model of the spheres of the planets inscribed in and circumscribed to the five regular polyhedra, matching available observations and explaining why there were exactly six (known) planets. He sent copies of the book to potential patrons and prominent astronomers, including Tycho Brahe.

Kepler felt the need for accurate data in order to refine his theories and hoped to gain access to Tycho's observations. In 1600, in the middle of growing religious tensions in Graz, he accepted Tycho's invitation to Prague, where Tycho had a chance to appreciate his skills and they negotiated an employment agreement. A few months later, banished from Graz because of his refusal to convert to catholicism, Kepler moved his entire household to Prague. After Tycho's untimely death in 1601, Kepler became his successor as imperial mathematician at the court of Rudolph II, Holy Roman Emperor, and gained unrestricted access to Tycho's observations.

Over the next decade, Kepler composed his greatest work, Astronomia Nova (1609) where he describes the discovery of his first two laws of planetary motion based on Tycho's data. His third law, discovered much later, appears in Harmonices Mundi (1619).

As imperial astronomer, Tycho Brahe had been put in charge by Rudolph II to compose new planetary tables in order to improve on the earlier Alphonsine tables, based on Ptolemy's model, and the Prutenic tables, based on Copernicus's model. He delegated the task to Kepler who worked on it for years after Tycho's death until finally publishing the Rudolphine tables (Tabulæ Rudolphinæ), based on his own model of the solar system, in 1627.

More biographical details can be found in Kitty Ferguson's book [3], which covers the lives of Tycho Brahe and Kepler.

The Tabulæ Rudolphinæ [5] contain a version of Tycho's manuscript catalog edited by Kepler, already covered on the Tycho Brahe page. To this, Kepler adds a list of 299 stars referred to as Secunda Classis [5, pp. 114–117], and a list of 136 southern stars referred to as Tertia Classis [5, pp. 118–119].

The Secunda Classis catalog

The Secunda Classis catalog covers for the most part the stars from Ptolemy's catalog that Tycho could not have observed at his latitude, corrected and precessed to the equinox of Tycho's catalog (the end of year 1600). To this, Kepler adds a few entries based on his own observations.

Kepler's description of the catalog is translated in Verbunt & van Gent [2, p. 1]. He mentions that the descriptions of the Secunda Classis stars that he observed himself and the ones from Ptolemy use different fonts. The difference is captured in kepler_2.dat.

The Tertia Classis catalog

Kepler also adds a catalog of 136 southern stars referred to as Tertia Classis , covering the twelve southern constellations introduced by the Dutch astronomer Petrus Plancius based on the observations of the Dutch explorers Pieter Dirkszoon Keyser and Frederick de Houtman. Kepler (translated in Verbunt & van Gent [2, p. 2] and quoting Bayer's Uranometria) and Baily [4, pp. 33–34] explain the origin of the catalog.

The positions of the southern stars were first collected and reduced by Keyser, from observations made by navigators during the 16th century. The resulting catalog was used by Bayer for the southern constellations in his Uranometria (1603), the first all-sky star atlas. The astronomer Jakob Bartsch (who would marry Kepler's daughter in 1630) published Planisphærii Stellati in 1624, containing celestial charts based on Bayer's tables used for Julius Schiller's Coelum Stellatum Christianum. He transmitted these charts to Kepler, along with a catalog of stars which Kepler seems to have used for Tertia Classis. This catalog was soon superseded by Halley's.

Ian Ridpath [1] examines some of the differences between Bayer's southern constellation map and Kepler's Tertia Classis catalog.

Kepler's Secunda and Tertia Classis catalogs have already been put in machine-readable form by F. Verbunt and R. H. van Gent as part of their study of early southern star catalogs [2], and made available as part of VizieR catalog J/A+A/530/A93.

I have independently digitized them from Kepler's Tabulæ Rudolphinæ [5] while working on Tycho Brahe's catalog, and made the results available on this page. The associated star maps may be of interest when comparing Kepler's catalogs with others, such as Ptolemy's and Halley's.

Catalog data

The following files are available.

File name Explanation
ReadMe File descriptions
kepler_2.dat Secunda Classis Catalog from Kepler's Tabulæ Rudolphinæ
kepler_3.dat Tertia Classis Catalog from Kepler's Tabulæ Rudolphinæ
corrs_2.dat Corrections to the Secunda Classis catalog
notes_2.dat Notes on Secunda Classis
notes_3.dat Notes on Tertia Classis
notes_a.dat Differences with Vizier catalog J/A+A/530/A93

The files kepler_2.dat and kepler_3.dat contain the raw catalog data. Their format, described in ReadMe, is compatible with Tycho's catalog files'. They are subject to automatic validation as long as the star descriptions (everything after column 46) are removed.

The file corrs_2.dat contains corrections of obvious errors in the Secunda Classis catalog in the overlay format described on the Tycho Brahe page. These corrections have been applied before drawing the maps.

The files notes_2.dat and notes_3.dat contain various observations about the originals.

Star designations

For the Secunda and Tertia Classis catalogs, I have extended the kno and tid numberings used for Tycho Brahe's catalog and described on the page dedicated to it. The kno numbers run from 1006 to 1304 for Secunda Classis and from 1305 to 1440 for Tertia Classis.

The tnum numbers within the constellation also extend the sequences started in Tycho's catalog for the constellations that already appear there, in order to avoid the duplication of tno values across Kepler's catalogs. There is however duplication of tno values for the 3 stars found in Tycho's manuscript catalog that don't appear in Kepler's version: 94 Gem 30, 675 Oph 38, and 913 Eri 20.

I accepted these duplications in order to avoid diverging from the numbering of VizieR catalog J/A+A/530/A93 unnecessarily. Even so, there are numbering differences (as there were already for Tycho's catalog) documented in notes_a.dat.


[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).

[3] Kitty Ferguson, Tycho and Kepler: The Unlikely Partnership that Forever Changed our Understanding of the Heavens, New York: Walker & Company, 2004.

[4] 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. Also available here.

[5] Johannes Kepler, Tabulæ Rudolphinæ, Ulm: Jonas Saur, 1627.


  • 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: Johann Kepler. Line engraving by F. Mackenzie. Credit: Wellcome Collection. CC BY