The Butterfly Diagram
Detailed observations of sunspots have been obtained by the Royal Greenwich
Observatory since 1874. These observations include information on the sizes and
positions of sunspots as well as their numbers. These data show that sunspots do
not appear at random over the surface of the sun but are concentrated in two
latitude bands on either side of the equator. A butterfly diagram (appropriately
named because of its appearance) highlighting the positions of the spots for each
rotation of the sun since May 1874 shows that these bands first form at the sun's
mid-latitudes, widen, and then move toward the equator as each cycle progresses.
By the time the sunspots reach the equator, the cycle is at a minimum, and new
spots are beginning to form again at the mid-latitudes.