TIL that the Anglo-Saxon Chronicle recorded a "red crucifix" appearing in the sky in the year 774. Radioisotope analysis of tree rings from that year show a spike in Carbon-14 around the world. User Lawls91, in the Today I Learned (TIL) subreddit, 31 May 2016
An eerie "red crucifix" seen in Britain's evening sky in ad 774 may be a previously unrecognized supernova explosion — and could explain a mysterious spike in carbon-14 levels in that year's growth rings in Japanese cedar trees. User laurawheeler, in the History subreddit, 28 Jun 2012
Ancient text gives clue to mysterious radiation spike : Nature News User OldOhioPlayer, in the Reddit Science subreddit, 27 Jun 2012