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Three pre-telescopic star catalogues contain about a thousand star magnitudeseach (with magnitudes 1, 2, 3, 4, 5, and 6), with these reported brightnessesas the original basis for what has become the modern magnitude scale. Thesecatalogues are those of Ptolemy (c. 137, from Alexandria at a latitude of31.2), Al Sufi (c. 960, from Isfahan at a latitude of 32.6), and Tycho Brahe(c. 1590, from the island of Hven at a latitude of 55.9). Previously, extensivework has been made on the positions of the catalogued stars, but only scantattention has been paid to the magnitudes as reported. These magnitudes will beaffected by a variety of processes, including the dimming of the light by ourEarth's atmosphere (atmospheric extinction), the quantization of thebrightnesses into magnitude bins, and copying or influence from priorcatalogues. This paper provides a detailed examination of these effects.Indeed, I find all three catalogues to report magnitudes that have near-zeroextinction effects, so the old observers in some way extinction corrected theirobservations.

The Thousand Star Magnitudes in the Catalogues of Ptolemy, Al Sufi, and Tycho Are All Corrected For Atmospheric Extinction