Rudolf Wolf, first director of the Zürich Observatory around mid-19th century, recovered a large number of sunspot observations made by astronomers several solar cycles back. Based on that database, he defined the relative sunspot number from the number of sunspot groups and individual sunspots. He extended his daily and monthly series until 1749, whereas his yearly series to 1700 (Clette et al. 2014). Nowadays, the World Data Center Sunspot Index and Long-term Solar Observations is the responsible to maintain this sunspot number index. At the end of the 20th century, Hoyt and Schatten (1998) compiled more sunspot observations made by astronomers since the beginning of the 17th century. Thus, they created the group sunspot number index from the number of sunspot groups. Unlike the relative sunspot number, their series starts in 1610.
More recently, several works have detected some problems both in these two indices and the databases. For example, Vaquero et al. (2016) published a revised collection of sunspot group numbers correcting some of the mistakes found in the Hoyt and Schatten database, in addition to incorporate other unknown sunspot records. Currently, there is an ongoing global effort to improve the weakness of the database and recalibrate the indices. Some remarkable improvements to be carried out in future versions of the sunspot number databases have been made regarding the earliest sunspot observations recorded by astronomers such as Galileo and Scheiner, inter alia. Then, corrections of significant mistakes detected in the sunspot counting assigned to these observers in the existing databases are proposed as well as the incorporation of telescopic sunspot records made by the earliest observers not included in these databases.
The sunspot number series is the index including the longest direct solar observation set to study the long-term solar activity evolution and its influence on the Earth. Therefore, we need that the databases, in which these indices are based, are free of problematic observations and, moreover, to improve their observational coverage before mid-19th century. Thus, we will understand better past, present and future solar activity.
Clette, F., Svalgaard, L., Vaquero, J.M., Cliver, E.W.: 2014, Revisiting the Sunspot Number. A 400-Year Perspective on the Solar Cycle, SSRv 186, 35. DOI: 10.1007/s11214-014-0074-2.
Hoyt, D.V., Schatten, K.H.: 1998, Group sunspot numbers: a new solar activity reconstruction. Solar Phys. 179, 189. DOI: 10.1023/A:1005007527816.
Vaquero, J.M., Svalgaard, L., Carrasco, V.M.S., Clette, F., Lefèvre, L., Gallego, M.C., Arlt, R., Aparicio, A.J.P., Richard, J.-G., Howe, R.: 2016, A Revised Collection of Sunspot Group Numbers, Sol. Phys. 291, 3061. DOI: 10.1007/s11207-016-0982-2.