Since their discovery by Galileo in 1609, astronomers have learned that they are regions, about the size of the Earth, where powerful magnetic fields are concentrated. They appear black because they emit less light than the sun. In fact, if they were suspended in the night sky, they would glow a bright red color and be brighter than the full moon. The sunspot cycle has been seen since about 1670 and has a period of about 11 years. Before 1670, no such cycles were seen and this time corresponded to the 'Little Ice Age' in Europe.
Scientists now think that the solar activity influences the Earth's weather in some way.