A Simple Method to Check the Reliability of Annual Sunspot Number in the Historical Period 1610-1847
Vaquero J.M., Trigo R.M., Gallego M.C.
Solar Physics 277:389-395, DOI 10.1007/s11207-011-9901-8
A simple method to detect inconsistencies in low annual sunspot numbers based on the relationship between these values and the annual number of active days is described. The analysis allowed for the detection of problems in the annual sunspot number series clustered in a few explicit periods, namely: i) before Maunder minimum, ii) the year 1652 during the Maunder minimum, iii) the year 1741 in Solar Cycle -1, and iv) the so-called "lost" solar cycle in the 1790s and the subsequent onset of the Dalton Minimum.