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.