There's a good reason to write about a comet coming to Britain at the time of Arthur's death: there probably was one. Though we have no definitive records from the chroniclers of the period, we have other ways of determining what was happening in 6th century Britain, the period most commonly associated with King Arthur and the Matter of Britain.

Dendrochronology is the study of tree rings to provide evidence of ancient environmental conditions. Good years produce wide growth rings, while bad years appear as narrow ones. In Western Europe in the past two millennia, one particular dendrochronological pattern stands out. Between 536 and 545 AD, the weather in Britain wasn't good at all. In fact, this period is characterized by a world-wide set of crop failures and famines, and constant references in contemporary records to a "dry fog" or "dust veil".

The relevance of this to the Arthurian mythos is twofold. First, Arthur's death traditionally heralds the beginning of a time of famine and drought in Britain. Second, the Arthurian mythos is inextricably intertwined with that of the Grail, which also associated with famine. Now we have scientific evidence to indicate a widespread famine at precisely the period history has assigned to Arthur and the Grail.

Famine ‑- drought ‑- darkness ‑- what could have caused them? Since we're able to rule out volcanic eruption, the likeliest answer is the close approach of a comet. Passing through its tail could load Earth's atmosphere with enough cometary debris to produce the same "little winter" that occurred on a much smaller scale in Tunguska in 1908. Records of heavy meteor showers beginning around 400 and extending for the next two centuries suggest that Earth was at increased risk from cosmic interlopers during this period, and a comet would fit the few facts we have.

Roger of Wendover actually reports that a comet passed near the Earth somewhere in the 540s. He reports that it could be seen from Gaul and that it was "so vast that the whole sky seemed on fire". Roger also says that in the same year "there dropped real blood from the clouds.... and a dreadful mortality ensued". In The Ruin of Britain, written circa 540, the monk Gildas cites a long series of extracts from the Old Testament, as being similar to contemporary catastrophes. Among other things, Gildas specifically mentions that the land was made a wilderness, the stars were dimmed, the sun and the moon were shadowed, and that there were clouds, fog, and plagues.

Though Science can only speculate, the novelist is more fortunate. What could be more fitting than that Arthur should die at the appearance of a red dragon which blazed through the sky in a time of war and destruction, the death of which heralded the beginning of a dark age of famine and plague?

A comet.