How did Samuel Heinrich Schwabe discover the sunspot cycle?

The German amateur astronomer Samuel Heinrich Schwabe discovered the sunspot cycle in 1843 after 17 years of diligent, almost daily observations.12

Interestingly, Schwabe was not originally searching for sunspots. He was attempting to discover a planet located inside the orbit of Mercury. To track the potential passage of such a body across the face of the Sun, he made careful, systematic daily drawings of the solar surface. Through this rigorous process, he observed that the number of sunspots varied over a regular period rather than remaining constant.21

His findings were later confirmed by the Swiss astronomer Rudolf Wolf, who analyzed historical records to establish the average duration of the cycle as approximately 11 years.2