Johann Rudolf Wolf (1816-1893), as a Swiss astronomer and astronomical historian, he is noted for his pioneering understanding of sunspot activity. Extending the work of Schwabe's discovery of the sunspot cycle, he amassed all available data on sunspot activity as far back as 1610 and calculated a period for the cycle of 11.1 years. He devised a system of gauging solar activity by counting sunspots and sunspot groups, which are known as Wolf's sunspot numbers (1849). This system is still in use for studying solar activity by counting sunspots and sunspot groups. Wolf was one of four people to independently, and more or less simultaneously, notice the coincidence between the 11-year sunspot cycle and the cycle of geomagnetic activity (1852).
- National Geophysical Data Center. Sunspot Numbers.
- O'Connor, J.J. and E. F. Robertson, 1996. Johann Rudolf Wolf. University of St. Andrews, Scotland.