Recently, new estimates of the solar cycle length (SCL) have been calculated using the Zurich Sunspot Number (RZ) and the Regression-Fourier-Calculus (RFC)-method, a mathematically rigorous method involving multiple regression, Fourier approximation, and analytical expressions for the first derivative. In this short contribution, we show estimates of the solar cycle length using the RFC-method and the Group Sunspot Number (RG) instead the RZ. Several authors have showed the advantages of RG for the analysis of sunspot activity before 1850. The use of RG solves some doubtful solar cycle length estimates obtained around 1800 using RZ.
KeywordsFourier Solar Cycle Cycle Length Sunspot Number Group Sunspot
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