June 1: calculation of Ri
Every first day of the month, the provisional International Sunspot Number is calculated based on observations of sunspots sent to us by observers all over the world.
Nederlandstalige versie /Version franaise posted on June 2, 2005

The SIDC is the World Data Center for the International Sunspot Number. One of the tasks is to calculate and distribute on the first of every month the provisional International daily and monthly mean Sunspot Number and the 12-months-ahead predictions. Observers, amateurs as well as professional observatories provide us with their daily observed Wolfnumbers of the passed month. The Wolfnumber equals by definition 10G+S, where G is the number of groups and S the number of sunspots and was developed by Johann Rudolf Wolf.
In a good statistical sense, the International Sunspot Number Ri is extracted from the input. Ri is a kind of weighted mean of the formula Ri= K(10G+S). K is a quality factor which adapts all input from an observer with the quality of the observation in mind. Every observer has its own personal K. The factor K makes every observer equal to Wolf, with the same eyes, same telescope, same observing conditions...
The numbers we broadcast the first of every month are provisional. Only when all observations have reached us, we are able to calculate the definitive Ri which is published in the quarterly 'SIDC News'.

The graph on the left pictures the provisional daily International Sunspot Number of May 2005 as they were calculated on June 1, 2005. The sunspot number is one of the indices to measure solar activity: if the Sun is spotted, we can expect more activity in the form of flares from it.