The sunspot number is the oldest index for solar activity. The International Sunspot Index is the sunspot number calculated
by the World Data Center for the sunspot index. The number
has different appearances depending on the extent of the used data, the date of calculation, the time period
over which we take a mean, the used method. We briefly present the different daily sunspot numbers.
Although they are all based on the same type of observations, i.e. visual sunspot counts as a measure of solar activity, they are obtained from different data sets, by different processing methods, with different accuracies and they serve different purposes. In the ideal case the three numbers are equal, but in practice it is evitable that there are small differences. Otherwise there would of course be no reason to have three numbers. It is thus useful to explain what does each specific index actually mean.