  Historical star magnitudes from catalogues by Ptolemy (137 AD), as-Sufi (964)
and Tycho Brahe (1602/27) are converted to the Johnson V-mag scale and compared
to modern day values from the HIPPARCOS catalogue. The deviations (or "errors")
are tested for dependencies on three different observational influences. The
relation between historical and modern magnitudes is found to be linear in all
three catalogues as it had previously been shown for the Almagest data by
Hearnshaw (1999). A slight dependency on the colour index (B-V) is shown
throughout the data sets and as-Sufi's as well as Brahe's data also give
fainter values for stars of lower culmination height (indicating extinction).
In all three catalogues, a star's estimated magnitude is influenced by the
brightness of its immediate surroundings. After correction for the three
effects, the remaining variance within the magnitude errors can be considered
as approximate accuracy of the pre-telescopic magnitude estimates. The final
converted and corrected magnitudes are available via the Vizier catalogue
access tool (Ochsenbein, Bauer, & Marcout, 2000).
