### 2.2 Wolf’s relative sunspot number

Schwabe’s discovery was probably instrumental in initiating the work of Rudolf Wolf (first at the Bern
Observatory and later at Zürich) toward acquiring daily observations of the Sun and extending the records
to previous years (Wolf, 1861). Wolf recognized that it was far easier to identify sunspot groups than to
identify each individual sunspot. His “relative” sunspot number, R, thus emphasized sunspot groups with
where k is a correction factor for the observer, g is the number of identified sunspot groups, and n is the
number of individual sunspots. These Wolf, Zürich, or International Sunspot Numbers have been obtained
daily since 1849. Wolf himself was the primary observer from 1848 to 1893 and had a personal
correction factor k = 1.0. The primary observer has changed several times (Staudacher from
1749 to 1787, Flaugergues from 1788 to 1825, Schwabe from 1826 to 1847, Wolf from 1848
to 1893, Wolfer from 1893 to 1928, Brunner from 1929 to 1944, and Waldmeier from 1945 to
1980). The Swiss Federal Observatory continued to provide sunspot numbers through 1980.
Beginning in 1981 and continuing through the present, the International Sunspot Number has been
provided by the Royal Observatory of Belgium with Koeckelenbergh as the primary observer (Solar
Influences Data Analysis Center - SIDC). Both Wolf and Wolfer observed the Sun in parallel over a
16-year period. Wolf determined that the k-factor for Wolfer should be k = 0.60 by comparing
the sunspot numbers calculated by Wolfer to those calculated by Wolf over the same days. In
addition to these primary observers there were many secondary and tertiary observers whose
observations were used when those of the primary were unavailable. The process was changed
from using the numbers from a single primary/secondary/tertiary observer to using a weighted
average of many observers when the Royal Observatory of Belgium took over the process in
1981.