  As recently found, the distribution of sunspots is non-axisymmetric and spot
group formation implies the existence of two persistent active longitudes
separated by 180 degrees. Here we quantitatively study the non-axisymmetry of
sunspot occurrence. In a dynamic reference frame inferred from the differential
rotation law, the raw sunspot data show a clear clustering around the
persistent active longitudes. The differential rotation describing the dynamic
frame is quantified in terms of the equatorial angular velocity and the
differential rotation rate, which appear to be significantly different from
those for individual sunspots. This implies that the active longitudes are not
linked to the depth of sunspot anchoring. In order to quantify the observed
effect, we introduce a measure of the non-axisymmetry of the sunspot
distribution. The non-axisymmetric component is found to be highly significant,
and the ratio of its strength to that of the axisymmetric one is roughly 1:10.
This provides additional constraints for solar dynamo models.
