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Zurich Sunspot Numbers Slides
In 1848 Rudolph Wolf devised a daily method of estimating solar activity by counting the number of individual spots and groups of spots on the face of the sun. Wolf chose to compute his sunspot number by adding 10 times the number of groups to the total count of individual spots, because neither quantity alone completely captured the level of activity. Each daily mean is computed as a weighted average of counts made by a network of cooperating observers in an effort to reduce the influence of personal judgment. Sunspots are optically dark regions in the solar atmosphere of relatively low temperature. They mark areas where the sun’s magnetic field has intensified by 1 or 2 powers of 10. The frequency of occurrence of sunspots rises and falls with an 11-year periodicity.