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Sunspot cycle
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Sunspot numbers rise and fall with an irregular cycle with a length of approximately 11 years. In addition to this, there are variations over longer periods. These cycles are called Sunspot cycle. The recent trend is upward from 1900 to the 1960s, then somewhat downward.[1] The Sun was last similarly active over 8,000 years ago.
The number of sunspots has been found to correlate with the intensity of solar radiation over the period - since 1979 - when satellite measurements of radiation are available. Since sunspots are dark it might be expected that more sunspots lead to less solar radiation. However, the surrounding areas are brighter and the overall effect is that more sunspots means a brighter sun. The variation is very small (of the order of 0.1%).
During the Maunder Minimum in the 17th Century there were hardly any sunspots at all. This coincides with a period of cooling known as the Little Ice Age.
Wainwright has speculated that there may be a resonant gravitational link between a photospheric tidal force from the [planets], the dominant component by summing gravitational tidal force (75%) being Jupiter's with an 11 year cycle.