Did Ptolemy observe the Almagest star catalogue or adapt Hipparchus's?

The Almagest catalogue is the oldest fully preserved star catalogue and the basis of Western and Islamic stellar astronomy until Ulugh Beg. Whether it records new observations bears on Ptolemy's reliability as a witness to his own practice, on the reconstruction of Hipparchus's lost catalogue, on the value of ancient positions for measuring precession and proper motion, and on broader arguments about how ancient scientists used data.

Ptolemy dates the catalogue to the first year of Antoninus (AD 137) and implies he observed it, but its longitudes are systematically about 1° too small for that date, consistent with adding a precession of 1° per century to positions for Hipparchus's epoch. Statistical tests of epoch, observer latitude, rounding patterns and error structure have yielded conflicting verdicts because the catalogue may mix sources and because results depend on modelling choices. Until 2022 there were almost no direct Hipparchan coordinates against which to compare.

Recovery of enough of Hipparchus's own coordinates (from palimpsests or other witnesses) to show for a substantial sample of stars whether Ptolemy's entries equal Hipparchan values shifted by 2°40′ in longitude, or differ in ways that require independent measurement; or a pre-registered statistical analysis of the whole catalogue, using modern astrometry, that discriminates between the two hypotheses for defined subsets of stars.

• Agents can rebuild the competing statistical tests from a single machine-readable edition of the catalogue and modern Hipparcos and Gaia positions, making every assumption explicit and reporting results by constellation and source subset.
• In parallel, they can systematically compare newly recovered Hipparchan readings with Ptolemy's entries.
• Further progress depends on multispectral imaging of the remaining undertext of the Codex Climaci Rescriptus and other Sinai palimpsests by imaging scientists, and on collaboration with astronomers to model proper motion and refraction.

Drafted by Claude Opus 5.5, 2026-10-08

1. 1925
   Heinrich Vogt, Versuch einer Wiederherstellung von Hipparchs Fixsternverzeichnis, Astronomische Nachrichten 224 (2–3): 17–54.Reconstructed Hipparchan coordinates from the Commentary on Aratus and found that many of Ptolemy's entries could not be simple copies of them.
2. 1977
   Robert R. Newton, The Crime of Claudius Ptolemy, Baltimore: Johns Hopkins University Press.Charged Ptolemy with fabricating observations, including deriving the star catalogue from Hipparchus.
3. 1984
   G. J. Toomer (trans.), Ptolemy's Almagest, London: Duckworth (repr. Princeton University Press, 1998).Standard English translation with the full catalogue, used as the textual basis for most modern analyses.
4. 1987
   James Evans, On the Origin of the Ptolemaic Star Catalogue (Parts 1 and 2), Journal for the History of Astronomy 18 (3): 155–172; 18 (4): 233–278.Argued that Ptolemy did observe, using an instrument calibrated with a faulty solar theory, which would explain the systematic longitude error.
5. 1990
   Gerd Graßhoff, The History of Ptolemy's Star Catalogue, New York: Springer.Statistical study concluding that the catalogue rests substantially on Hipparchan data adjusted by Ptolemy, while rejecting simple fraud.
6. 1990
   M. Shevchenko, An Analysis of Errors in the Star Catalogues of Ptolemy and Ulugh Beg, Journal for the History of Astronomy 21 (2): 187–201.Compared error structures across catalogues and argued for a Hipparchan origin of much of Ptolemy's data.
7. 2000
   A. K. Dambis and Yu. N. Efremov, Dating Ptolemy's Star Catalogue through Proper Motions: The Hipparchan Epoch, Journal for the History of Astronomy 31 (2): 115–134.Used fast-moving stars to date the observations to roughly the second century BC, consistent with Hipparchus rather than Ptolemy.
8. 2005
   Bradley E. Schaefer, The Epoch of the Constellations on the Farnese Atlas and Their Origin in Hipparchus's Lost Catalogue, Journal for the History of Astronomy 36 (2): 167–196.Dated the Farnese Atlas globe to about 125 BC and linked it to Hipparchus's catalogue; the identification has been contested.
9. 2011
   Olaf Pedersen (rev. Alexander Jones), A Survey of the Almagest, New York: Springer (first ed. 1974).Standard guide to the Almagest, with Jones's revision summarizing the catalogue debate.
10. 2012
    F. Verbunt and R. H. van Gent, The Star Catalogues of Ptolemaios and Ulugh Beg, Astronomy & Astrophysics 544: A31.Published machine-readable catalogues and found that the error pattern supports Hipparchan origin for most entries.
11. 2022
    Victor Gysembergh, Peter J. Williams and Emanuel Zingg, New Evidence for Hipparchus' Star Catalogue Revealed by Multispectral Imaging, Journal for the History of Astronomy 53 (4): 383–393.Recovered Hipparchan equatorial coordinates for stars of Corona Borealis from the undertext of the Codex Climaci Rescriptus, the first direct sample of his catalogue.

• Museum of the Bible, Washington, DCpartly digitized Codex Climaci Rescriptus (Green Collection), Syriac palimpsest with Greek astronomical undertextHolds most leaves of the palimpsest that yielded Hipparchan coordinates; multispectral images have been made for part of the codex.
• UCLA Library and St Catherine's Monastery, Sinaipartly digitized Sinai Palimpsests Project (multispectral images of palimpsests from St Catherine's)Open-access multispectral images of Sinai palimpsests, among which further Greek scientific undertexts may survive; free registration required.
• Biblioteca Apostolica Vaticana, Vatican Citydigitized Vat. gr. 1594 (ninth-century Almagest) and other Greek Almagest manuscriptsOne of the oldest complete Greek witnesses of the catalogue, fully digitized on DigiVatLib.
• Bavarian Academy of Sciences, Munichpartly digitized Ptolemaeus Arabus et Latinus (database of Arabic and Latin Ptolemaic manuscripts)Inventory of Arabic and Latin witnesses of the Almagest, whose catalogue variants help establish the text's original values.
• European Space Agencydigitized Hipparcos and Tycho cataloguesModern astrometry with proper motions used to compute where each star stood in 130 BC and AD 137.

Compiled by Claude Opus 5.5, 2026-10-08