Sunspots are a feature of the Sun that have been observed since ancient times (including by Galileo ). When viewed through a telescope, sunspots have a dark central region known as the umbra, surrounded by a somewhat lighter region called the penumbra. Sunspots are dark because they are cooler than the surrounding photosphere. They are the site of strong magnetic fields. The reason sunspots are cool is not entirely understood, but one possibility is that the magnetic field in the spots inhibits convection underneath them.

Dark, cool sunspots appear on the Sun's surface often in pairs, and typically grow over a few days and last anywhere from a few days to a few months. They have magnetic field strengths of 100-1000 gauss and lifetimes of 2 months. Observations of sunspots first revealed that the Sun rotates with a period of 27 days (as seen from Earth).

The number of sunspots on the Sun is not constant, but varies with a period of 11 y. During each cycle, sunspots migrate from high latitudes to the equator. Sunspots may affect Earth's climate, as deduced from the low number present during the little ice age of 1640-1710. During a solar cycle, all sunspot pairs in the northern hemisphere have the leading spot north and the trailing spot south (it is reversed in the southern hemisphere). Polarities are then reversed during the next solar cycle. An apparent 80-90 year cycle of long-term variations in solar activity is called the Gleisberg cycle.

The sun's output changes over the period of several days, as measured by the Solar Max satellite. The variation over the solar cycle was observed to be 0.04% between 1980 and 1986, when the flux at the Earth (known as the solar constant declined from 1368.5 to 1367 W m^-2. The flux may be influenced by sunspots crossing the disk. Large sunspot groups moving across the sun's equator temporarily decrease the flux. This occurred April 3-10, 1980 when the Solar Maximum Mission on Nimbus 7 revealed a decrease of about 1.5 W m^-2 during passage of a large sunspot group across the sun. However, faculae surrounding sunspots cause a general small solar flux increase!

Gleisberg Cycle, Maunder Minimum, Sun