<P> The solar cycle or solar magnetic activity cycle is the nearly periodic 11 - year change in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number and size of sunspots, flares, and other manifestations). </P> <P> They have been observed (by changes in the sun's appearance and by changes seen on Earth, such as auroras) for centuries . </P>

What is the length of the sunspot cycle