A physical interpretation of the 'red Sirius' anomaly

The controversy over whether the brightest star, Sirius (alpha CMa; HR 2491 ), was red in visual appearance some 2000 years ago, as suggested by Ptolem y amongst others, is re-examined from a physical perspective. Objections to hypotheses based on evolutionary events within the Sirius binary system it self are briefly reviewed. Scenarios that invoke reddening caused by foregr ound extinction in the interstellar medium or in the Earth's atmosphere are examined in detail to determine whether they offer viable alternatives. It is deduced that only atmospheric extinction is capable of producing approp riate changes in the colour and brightness of Sirius. This result concurs w ith the findings of Ceragioli, who deduced, from a re-evaluation of the his torical evidence and the cultural role of Sirius in Greek and Roman society , that 'red Sirius' refers to observations made at the heliacal risings and settings of the star. Both physical and historical evidence are thus consi stent with an interpretation of the 'red Sirius' anomaly based on reddening in the terrestrial atmosphere.