Sunspot activity according to Greenwich observations

This article presents a survey of studies incorporating methods of evaluation of Greenwich observations. It also describes some extreme sunspot groups (i.e., groups with a very large area, a very long life, and/or in very high heliographic latitudes); data on the total areas of the largest sunspots are given, and a survey of the parameters concerning individual 11-year cycles, resulting from Greenwich Photoheliographic Results, is presented. (orig.)