Tree rings can tell you a lot: The Japanese have discovered that, in the late eighth century, the earth was hit by a mysterious blast of cosmic rays (NOTE: Subscribers can still listen to this show). They discovered this by looking at cedar trees.
An analysis of two ancient trees found a surge in carbon-14, an isotope that arrives from space, which was deposited in cedar tree rings only in 774 and 775 AD. What happened?
One source of cosmic rays is the sun, which could have been having solar flares at that time. Another is a supernova--an exploding star that sends out gamma radiation before it burns out.
But there are no records of a supernova during that period. But the researchers plan to keep looking--they're planning a wider search of historical documents to see if, over 1,200 years ago, anyone noted a strange flare in the sky.
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