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In late antiquity and throughout the middle ages, the positions of stars onthe celestial sphere were obtained from the star catalogue of Ptolemaios. Acatalogue based on new measurements appeared in 1437, with positions by UlughBeg, and magnitudes from the 10th-century astronomer al-Sufi. We providemachine-readable versions of these two star catalogues, based on the editionsby Toomer (1998) and Knobel (1917), and determine their accuracies bycomparison with the modern Hipparcos Catalogue. The magnitudes in thecatalogues correlate well with modern visual magnitudes; the indication `faint'by Ptolemaios is found to correspond to his magnitudes 5 and 6. Gaussian fitsto the error distributions in longitude / latitude give widths sigma ~ 27arcmin / 23 arcmin in the range |Delta lambda, Delta beta|<50 arcmin forPtolemaios and sigma ~ 22 arcmin /18 arcmin in Ulugh Beg. Fits to the range|Delta lambda, Delta beta|<100 arcmin gives 10-15 per cent larger widths,showing that the error distributions are broader than gaussians. The fractionof stars with positions wrong by more than 150 arcmin is about 2 per cent forPtolemaios and 0.1 per cent in Ulugh Beg; the numbers of unidentified stars are1 in Ptolemaios and 3 in Ulugh Beg. These numbers testify to the excellentquality of both star catalogues (as edited by Toomer and Knobel).

The star catalogues of Ptolemaios and Ulugh Beg: Machine-readable versions and comparison with the modern Hipparcos Catalogue