Sun Spot

Watts Up With That?

Guest post by Dr. Leif Svalgaard

The official sunspot number is issued by SIDC in Brussels . The [relative] sunspot number was introduced by Rudolf Wolf  in the middle of the 19th century. He called it the ‘relative’ number because it is rather like an index instead of the actual number of spots on the Sun. Spots occur in groups [which we today call ‘active regions’] and Wolf realized that the birth of a new group was a much more significant event than the emergence of just a single new spot within a group, so he designed his index, R, [for any given day] to be a weighted sum of the number of spots, S, and the number of groups, G, giving the groups a weight of 10: R = S + 10*G. The number of 10 was chosen because on average a group contains about…

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