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The Sunspot Cycle
In the last few decades, the forces behind sunspots are becoming better understood, but we've known for over a 150 years that sunspots appear in cycles. The average number of visible sunspots varies over time, increasing and decreasing on a regular cycle of between 9.5 to 11 Sunspot Cycle 1900-1993years, on average about 10.8 years. An amateur astronomer, Heinrich Schwabe, was the first to note this cycle, in 1843. The part of the cycle with low sunspot activity is referred to as "solar minimum" while the portion of the cycle with high activity is known as "solar maximum."

By studying the sun's magnetic field, modern astronomers have discovered that the cycle covers twenty-two years, with an eleven-year cycle of sunspots above the equator followed by an equal cycle below the equator. According to Fisher, "the overall magnetic field structure changes in a way that is very interesting: It turns out that the magnetic fields primarily point from west to east in the Northern Hemisphere (of the sun), and from west to east in the Southern Hemisphere. In the next eleven-year cycle, the fields are reversed. So the cycle is really twenty-two years."

Sunspots most often appear in the low latitudes near the solar equator, and they almost never appear below 5 or above 40 degrees north and south latitude. As the sunspot cycle progresses, the visible sunspots move gradually towards the equator.

From 1645 to 1715, there was a drastically reduced number of sunspots. This period of reduced solar activity, which was first noticed by G. Sporer and was later investigated by E.W. Maunder, is now called the Maunder Minimum. This period of time was also unusually cold on earth, and it has been referred to as the "Little Ice Age." This has led to some speculation that sunspot activity may affect the earth's climate. Similar periods of low solar activity seem to have occurred during the Spoerer Minimum (1420-1530), the Wolf Minimum (1280-1340), and the Oort minimum (1010-1050). Solar astronomers label solar cycles from one minimum to the next, and assign them numbers, starting at one, with the 1755-1766 cycle.

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Observatory  1998 The Exploratorium.