Cometary records revise Eastern Mediterranean chronology around 1240 CE

Eirene Laskarina, Empress of John III Batatzes of the exiled Byzantine Empire of Nicaea (1204-1261 CE), was an important Eastern Mediterranean figure in the first half of the thirteenth century. We reassess the death day of Eirene, which has been variously dated during the years between the end of 1239 and 1241, with the understanding that narrowing the range in which this event occurred contributes much to understanding the political situation in the area around 1240. George Akropolites, a famous official of the Empire, gives an account that connects Eirene's death to a comet that appeared "six months earlier," thus pointing to two comet candidates that were visible from the Eastern Mediterranean during the years 1239 and 1241, one recorded on "1239 June 3" and the other on "1240 January 31." Recent historians prefer the former, based on historical circumstances and without a critical assessment of the comet records. We revisit the historical records and reveal that the "1239 June 3" candidate was not a comet. On the other hand, the other candidate was a comet, as supported by multiple historical records in multiple regions, and is also a good fit with Akropolites' narrative. Therefore, we conclude that Eirene died six months after the comet that was seen on 1240 January 31, which places her death in the summer of 1240. Given that her death day is crucial for determining some other contemporary events across the Eastern Mediterranean, our results offer a solid basis for further research on the thirteenth-century Eastern Mediterranean.