The solar cycle is a roughly 11-year periodic change in the Sun's sunspot activity, measured by the variation in the number of sunspots observed. Humans used telescopes to observe sunspots and solar cycles as early as the 17th Century; however, NOAA and NASA satellites are now major ways scientists use to study the Sun.
Sunspots are small areas of particularly strong magnetic forces on the Sun's surface that appear as darker spots because they are cooler. During solar maximum, there is a high number of sunspots, and during solar minimum, there is a low number. Sunspots appear in a wide variety of shapes and forms. They can also change size and shape and may last for only a few hours to days and even months.