IDENTIFICATION OF THE GUEST STAR OF AD-185 AS A COMET RATHER THAN A SUPERNOVA

VERY few nearby supernovae have been bright enough to see with the naked eye. The only such case this century was supernova 1987A. Matching historical records of such events with presently observable remnants allows accurate estimates to be made of the age and incidence rate of supernovae; ancient Chinese astronomical records are a particularly valuable resource for this purpose. The Houhanshu(1) of the Later Han dynasty records the appearance of a 'guest star' in AD 185. This is widely regarded as the oldest supernova recorded historically, and several candidate remnants have been suggested(2,3), in particular the object RCW86(2). Here we show that a reinterpretation of the relevant passage in the Houhanshu is inconsistent with the supernova interpretation, but suggests instead that the guest star was a comet. Our findings indicate that some of the keywords used by Chinese astronomers in historical records must be interpreted with caution.