and maxima of solar cycles, a tapered-boxcar smoothing function is used. For a given month , with a monthly sunspot number of , the smoothed monthly mean can be expressed as where is the monthly sunspot number months away from month . Alternative series The accuracy of the compilation of the group sunspot number series has been questioned, motivating the development of several alternative series suggesting different behavior of sunspot group activity before the 20th century. However, indirect indices of solar activity favor the group sunspot number series by Chatzistergos T. et al. A different index of sunspot activity was introduced in 1998 in the form of the number of groups apparent on the solar disc. With this index it was made possible to include sunspot data acquired since 1609, being the date of the invention of the telescope. See also Solar variation Joy's law (astronomy) References External links The Exploratorium's Guide to Sunspots Solar Influences Data Analysis Center (SIDC) for the Sunspot Index NASA Solar Physics Sunspot Cycle page and Table of Sunspot Numbers (txt) by month since 1749 CE Stellar phenomena Solar phenomena de:Sonnenfleck#Sonnenflecken-Relativzahl