2.1 Schwabe’s discovery
Although Christian Horrebow mentions this possible periodic variation in 1776 the solar (sunspot) cycle
was not truly discovered until 1844. In that year Heinrich Schwabe reported in Astronomische Nachrichten
(Schwabe, 1844) that his observations of the numbers of sunspot groups and spotless days over
the previous 18-years indicated the presence of a cycle of activity with a period of about 10
years. Figure 1 shows his data for the number of sunspot groups observed yearly from 1826 to
Figure 1: Sunspot groups observed each year from 1826 to 1843 by Heinrich Schwabe (1844). These
data led Schwabe to his discovery of the sunspot cycle.