I had recently posted a link to an article discussing an unusual spike in Carbon-14 levels in 774 A.D. without any known explanation. Now a possible answer from an ancient manuscript:
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. The link is suggested today in a Nature Correspondence by a US undergraduate student with a broad interdisciplinary background and a curious mind1.
A few weeks ago, Jonathon Allen, a biochemistry major at the University of California, Santa Cruz, was listening to the Nature podcast when he heard about a team of researchers in Japan who had found an odd spike in carbon-14 levels in tree rings. The spike probably came from a burst of high-energy radiation striking the upper atmosphere, increasing the rate at which carbon-14 is formed (see 'Mysterious radiation burst recorded in tree rings').
But there was a problem: the only known causes of such radiation are supernova explosions or gigantic solar flares, and the researchers knew of no such events in ad 774 or 775, the dates indicated by the tree rings.
Intrigued, Allen hit the Internet. "I just did a quick Google search," he says.
His long-standing interest in history was helpful, he notes. "I knew that going that far back, there's very limited written history," he says. "The only things I'd ever seen or heard of were religious texts and 'chronicles' that listed kings and queens, wars and things of that nature."
His search found the eighth-century entries in the Anglo-Saxon Chronicle at the Avalon Project, an online library of historical and legal documents hosted by Yale University in New Haven, Connecticut. Scrolling down to the year ad 774, Allen found a reference to a "red crucifix" that appeared in the heavens "after sunset".
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