Every first day of the month, the Monthly Sunspot bulletin is broadcast. Data of many worldwide distributed stations is used to calculate "The International Sunspot Number".
In 1849, Wolf of the Zürich Observatory proposed the now widely used formula: R=K(10 G + S), with S the number of sunspots, G the number of sunspot groups. The quality factor "K" was introduced later on to compare results from different observers, sites and telescopes. The factor "K" makes each observer equal to Wolf; as the observer would have the same eyes, the same telescope, the same environmental conditions. The task of the SIDC consists in collecting the observations from as many stations as possible worldwide, to determine the appropriate K factor for each of them and to extract an overall "International Sunspot number" Ri from all these observations in a good statistical sense.
The sunspot number is the oldest solar activity index. Its main interest results from its long-term behaviour and the length of its series. The SIDC manages a historical archive of the International Sunspot Number. In 2003, 82 stations sent us on a regular monthly basis the daily sunspot numbers. All stations have a valuable and appreciated contribution to the computation of the International Sunspot Number.
The figure above shows us were the stations in Europe are situated. In Belgium, 11 stations contributed regularly in 2003
.The figure above gives an overview of the geographical distribution of the station over the continents.
