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Published online by Cambridge University Press: 12 April 2016
In this contribution, preliminary results of the main component analysis of Bartels diagram of time series of daily values of sunspot group numbers for solar cycles 18, 19 and 20 are presented. The results obtained suggest that the most significant feature in the longitudinal distribution of sunspot activity is the existence of preferred solar hemispheres alternating with a mean period of 2.5 Bartels rotations.