Accuracy of magnitudes in pre-telescopic star catalogs

Abstract

Historical star magnitudes from catalogs by Ptolemy (137 AD), al-Sūfī (964), and Tycho Brahe (1602/27) are converted to the Johnson V-mag scale and compared to modern-day values from the HIPPARCOS catalog. 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 catalogs as it had previously been shown for the Almagest data by Hearnshaw, J. B., 1999, New A Rev., 43, 403. A slight dependency on the color index (B-V) is shown throughout the data sets and al-Sūfī's, as well as Brahe's data also give fainter values for stars of lower culmination height (indicating extinction). In all three catalogs, 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 approximate accuracy of the pre-telescopic magnitude estimates. The final converted and corrected magnitudes are available via the Vizier catalog access tool (Ochsenbein, F., Bauer, P., & Marcout, J., 2000, A&AS, 143, 23).