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Solar Cycle History
Sunspots are caused by solar storms, including flares and coronal mass ejections, produced by the Sun's magnetic field. Galileo Galilei made the first European observations of these spots in 1610. In 1848, Rudolf Wolf devised a method for counting them, producing what is known today as the Wolf number. The Wolf number is equal to the sum of the total number of individual sunspots counted and 10 times the number of overall sunspot groups counted. In 1849, continuous sunspot observations began at the Zurich Observatory.
The Solar Cycle
Based upon the monthly average of sunspots, scientists were able to discover a solar cycle. They found that sunspot activity rises and falls approximately every 11 years. The peak of sunspot activity is called the solar maximum, and the point of least sunspot activity is called the solar minimum. Each cycle is consecutively numbered. The cycle begins at solar minimum and is indicated by the first high-latitude, reversed polarity sunspot. Solar cycle number 24 began on January 4, 2008, with just such a sunspot. This sunspot cycle is related to the 22-year cycle of the sun's magnetic field reversal. However, scientists do not know why there is a solar cycle in the first place.
Maunder Minimum
While the solar cycle remains relatively constant, there have been exceptions. An extended period of sunspot inactivity occurred during the end of the 17th century, from around 1645 to 1715. This highly unusual 70-year period is called the Maunder Minimum. Its timing corresponded with the climatological period known as the "Little Ice Age." During this period, the Earth's average temperature dropped two to three degrees Fahrenheit; the Maunder Minimum is believed to have been a contributing factor, along with high volcanic activity.
Solar Cycle Number 24
In 2011, scientists indicated that the Sun appeared to be entering a second Maunder Minimum. Rather than increasing towards solar cycle 24's expected solar maximum around 2012, sunspot activity appears to be significantly calming down. To support their hypothesis, scientists point to a missing solar jetstream, fading spots and decreased activity at the sun's poles. Only continued observations will be able to determine whether this is a sign of another Maunder Minimum or simply a temporary deviation from the normal solar cycle.