Discovery that the Magnitudes in the Ancient Star Catalogs of Ptolemy, Al-Sufi, and Tycho Were All Corrected for Atmospheric Extinction

Details Discovery that the Magnitudes in the Ancient Star Catalogs of Ptolemy, Al-Sufi, and Tycho Were All Corrected for Atmospheric Extinction Discovery that the Magnitudes in the Ancient Star Catalogs of Ptolemy, Al-Sufi, and Tycho Were All Corrected for Atmospheric Extinction

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21911505s&link_type=abstract

American Astronomical Society, AAS Meeting #219, #115.05

Astronomy and Astrophysics

Astronomy

Scientific paper

The three ancient star catalogs of Ptolemy (c. 127, Alexandria Egypt), Al Sufi (c. 961, Isfahan Iran), and Tycho Brahe (c. 1600, Hven now in Sweden) all record independent measures of the visual magnitudes of close to a thousand stars over their entire visible sky. For stars culminating 60° from zenith to the south (around -29° declination for Alexandria), they should appear roughly a quarter or a third of a magnitude fainter than those at zenith, and this is easily detected with the many stars near this declination band, despite the quantization of the reported magnitudes to roughly one third of a magnitude. For stars near the southern limit, the dimming should be 1-2 mag. To seek this effect, I use stars culminating near zenith to set up a correspondence between the reported magnitudes and modern V magnitudes, compare the modern equivalent magnitude to the star's real magnitude, and looked to see the dimming as the southern horizon is approached. Surprisingly, no dimming towards the south is viewed in any of the three ancient star catalogs. A formal fit to the effective extinction coefficient for each catalog is +0.01+-0.01, +0.05+-0.01, and +0.01+-0.01 mag/airmass respectively. That is, the reported magnitudes have already been corrected for extinction. This new result is surprising because no astronomer or historian has previously reported the effect. This is also surprising because no written source before 1729 even mentions the existence of the phenomenon of extinction (although the effect is easily recognized by any studious visual observer), so the expectation would be that the pre-telescopic astronomers were not aware of the phenomenon, not interested, or not able to do the corrections. Nevertheless, this discovery that the ancient catalogers all corrected for extinction opens new horizons in `archaeophotometry’ and new recognition for the ability of pre-telescopic observers.

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