The monthly means of daily relative sunspot numbers, which are based upon counts of spots and groups of spots. D. F. Andrews and A. M. Herzberg, Data: A Collection of Problems from Many Fields for the Student and Research Worker, Springer-Verlag, New York, 1985.

The data was previously collected from different observers by the Swiss Federal Observatory, Zurich, Prof. M. Waldmeier, Director. M. Waldmeier, The Sunspot Activity in the Years 1610-1960, Schulthess, Zurich, 1961.

The data is currently collected by the Tokyo Astronomical Observatory, Tokyo, Japan.