Sunspot number

Astronomers have been observing the Sun recording information about sunspots since the advent of the telescope in 1609.

[1] However, the idea of compiling the information about the sunspot number from various observers originates in Rudolf Wolf in 1848[2] in Zürich, Switzerland.

The international sunspot number series is still being produced today at the observatory of Brussels.

Traditionally, a scaling of 0.6 was applied to all sunspot counts after 1893, to compensate for Alfred Wolfer's better equipment, after taking over from Wolf.

This scaling has been dropped from the revised series, making modern counts closer to their raw values.

Since Wolf was the primary observer for the relative sunspot number, his observatory factor was 1.

[13] However, indirect indices of solar activity[14][15][16] favor the group sunspot number series by Chatzistergos T. et al.[12] A different index of sunspot activity was introduced in 1998 in the form of the number of groups apparent on the solar disc.

[17] With this index it was made possible to include sunspot data acquired since 1609, being the date of the invention of the telescope.