| In 1848 the Swiss astronomer Johann Rudolph Wolf introduced a daily measurement of
sunspot number. His method, which is still used today, counts the total number of
spots visible on the face of the sun and the number of groups into which they cluster,
because neither quantity alone satisfactorily measures sunpsot activity.An observer
computes a daily sunspot number by multiplying the number of groups he sees by ten
and then adding this product to his total count of individual spots. Results, however,
vary greatly, since the measurement stongly depends on observer interpretation and
experience and on the stability of the Earth’s atmosphere above the observing site.
Moreover, the use of Earth as a platform from which to record these numbers contributes
to their variability, too, because the sun rotates and the evolving spot groups are
distributed unevenly across solar longitudes. To compensate for these limitations,
each daily international number is computed as a weighted average of measurements
made from a network of cooperating observatories.
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