The actual period of rotation varies with the latitude of the sun. It is 25 days at the sun's equator and 27.4 days at a latitude of 40 . This lag occurs because the sun rotates as a gas, not as a solid body. Typical sunspots have a dark, circular center, called the umbra, surrounded by a lighter area, the penumbra. Rays issuing from the center of the umbra form the penumbra.
Sunspots vary greatly in size but are always small compared to the size of the sun.
When they appear in groups, they may extend over thousands of miles. The darkness of the umbras is a sign that the sunspots are cooler than the photosphere. Furthermore, when they approach the sun's edge, the umbras appear to be lower than the photosphere as well. Regular observations of sunspots have been made from 1750 to the present. They reveal that the spots appear and disappear in a definite cycle, and that they are limited to the two zones of the sun contained between latitudes 40 and 5 of its northern and southern hemispheres.