You can tell the age of a tree by
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its rings, but these records of a tree"s life really say a lot more. Scientists are using tree rings to learn what"s been happening on the sun"s
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for the last ten thousand years. Each ring
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a year of growth. As a tree grows, it acids a layer to its trunk, taking up
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elements from the air. By looking at the elements in the rings from a given year, scientists can tell what elements were in the air that year. Doctor Stevenson is
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one element—carbon-14, in rings from both living and dead trees. Some of the rings go back almost ten thousand years to the end of the Ice Age. When Stevenson followed the carbon-14 trail back in time, he found carbon-14 levels change with the
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of solar burning. You see the sun has cycles. Sometimes it burns
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at other times it"s relatively calm. During the sun"s
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periods, it
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charged particles in fast moving streams called "solar winds". The particles
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the formation of carbon-14 on earth. When there"s more solar wind activity, less carbon-14 is produced. Ten thousand years of tree rings show that the carbon-14 level rises and falls about every 420 years. The scientists concluded that solar wind activity must follow the same cycle.