Astrolabes as records of previous eruptions of the recurrent nova T Coronae Borealis

Abstract

The recurrent nova T Coronae Borealis (T CrB) has recorded eruptions in 1946 and 1866 and a periodicity of 80 years renders the next eruption due shortly. A search for the earlier records of eruption (Schaefer 2023a) provided one as early as 1217 and another in 1787. Here we report the results of search in astrolabes, whose star dials, as we show, serve as unconventional sources of sightings. The catalogues (Gunther 1932; Sarma 2023) list the stars on the dials of astrolabes and provide a rich source of such records, though some pointers are left unidentified and sometimes misidentified by later studies. The procedure adopted for identification is explained with the help of the astrolabe which has the supernova SN1604 recorded to emphasise the difficulty involved. We were able to pinpoint three astrolabes as possible records of eruption of T CrB. The difficulties associated with this procedure and overcoming the disadvantages of poor resolution are discussed. One eruption around 1450CE is identified in a Spanish astrolabe and another sometime around 1570CE. The third is from second half of seventeenth-century. A long-term recurrence period of 82 years seems to be valid in the last eight centuries, putting the next eruption to 2028.