The monthly sunspot number since 1749. The data values are represented by dots and the 12-month running average values are illustrated as a red shaded area. The sunspot cycle numbers are shown in the shaded area—number 1 starting in the 1755 and number 24 presently drawing to a close. Also shown in the figure are a set of vertical blue dashed lines that signify the magnetic activity cycle termination times that trigger the rapid growth of sunspot activity (McIntosh et al., 2019). The sunspot data used here are freely available and provided by the World Data Center-SILSO of the Royal Observatory of Belgium. We have used version 2.0 of the sunspot number (Clette et al., 2014, 2015) that is available at this website, identified as the “Monthly mean total sunspot number”: http://www.sidc.be/silso/.