| 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.
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