The Solar Cycle
The Solar Cycle was not dicovered until l 1843 by Heinrich Schwabe. He noticed that the number of sunspots varied with a period near 10 years. In 1858, Wolf published his formula for determining the daily sunspot number:
R = k(10g + f)
g = number of spot groups
k = calibration factor that is different for each observer
f = number of spots seen on the surface (including those in groups).
Using this formula, observations from a large number of different observers could be compiled, thereby making sure that measurements of the sunspot number would be unbroken, regardless of inclement weather at some of the observatories on any particular day. This method of determining the sunspot number is still used today.