Sunspots are linked to the surface magnetic field of the Sun. The area of a sunspot is measured in millionths of the Sun's visible hemisphere. The area of sunspots are distorted by the spherical shape of the Sun. Sunspots usually appear in groups. Normally it is fairly easy to count the number of groups as they are spread out across the disk and in both hemispheres.
Difficulties can occur when sunspots appear close together.
Then experience of different types of sunspot groups can be used to determine the number of groups present. Zurich Sunspot Group Classification. As for E or F groups but without any spots between the penumbrae at both ends of the group. Last updated on 15 January 2001 .