Sunspots appear as dark areas on the surface of the Sun and have a lower temperature than the photosphere in which they occur.
Sunspots on the Sun were discovered early on by Galileo Galilei and Johannes Fabricius. They often have a complex structure. The smallest ones appear only as a simple dark dot, while the larger ones have a dark center called the umbra and a somewhat brighter zone called the penumbra.
The number and intensity of sunspots changes constantly. They often occur in groups and can be up to 300,000 kilometers long. They usually last only a few days, although larger groups can last for several months.
When viewed through a telescope, a large group of sunspots can be seen to move from east to west because of the rotation of the Sun.
Long-term observation has shown that the number of sunspots changes in cycles of about 11 years. This has been known since the early 18th century. However, it was not until 1908 that George Hale discovered that sunspots have strong magnetic_fields that lie in opposite directions (towards the interior and away from it) at the edge of a group.
There are two zones of spots, located about 40 degrees of latitude north and south respectively. This is where the first spots at the beginning of a new cycle appear. The spots increase in intensity and move slowly towards the equator. This is where the last few spots of an ending cycle can be seen before they disappear.
During the period it takes the sunspots to travel towards the equator, they increase in number. It is estimated that at the peak of activity there are approximately ninety spots present on the Sun.
