What the Long-Term Sunspot Record Tells Us About Space Climate
Direct observations of sunspots span the nearly 400 years since the time of Galileo. Dedicated observing programs at several observatories over the last 150 years have provided detailed information not only on the number of sunspots but on their sizes and positions as well. The data acquired by those original observers, and by those who have more recently brought those observations to light, provide important clues about the nature of the solar cycle and its contribution to space climate. The period of the cycle, the equator-ward drift of the active latitudes, the asymmetry between the rise to maximum and the fill to minimum, shifting asymmetries between northern and southern hemisphere activity, the tilt of active regions, and the increasing amplitude of the cycles since the Maunder Minimum are all well established. Other, less well established characteristics such as multi-cycle and short-term periodicities, often depend upon the method of data analysis. The strong correlation between sunspot statistics and other measures of solar activity, coupled with the length of the sunspot record, make these observations extremely valuable for characterizing and understanding space climate.