Astronomers have found that the level of the Sun's activity varies over the centuries. How did they come to realize that this is so:

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Astronomers have used all of these methods to determine the variability of the Sun's activity. Historical records of sunspots have been kept since the early 17th century, and these records show a cyclic pattern of activity. Measuring the amount of radioactive carbon in tree rings allows scientists to determine how much cosmic radiation is reaching the Earth, which in turn can be used to indirectly measure solar activity. Historical records of auroral activity also provide evidence of solar activity variations.

Astronomers Have Found That the Level of the Sun's Activity