The cyclic occurrence of spots that travel across the surface of the sun as it rotates. The number and area of these spots waxes and wanes. They were first observed by Galileo and much later by Heinrich Samuel Schwabe in 1826. By 1843, Schwabe had detected an average 10-year cycle, which he later corrected to 11 years. In 1908, G. E. Hale showed that the magnetic polarities of the sunspots in one cycle were reversed in the next. The average total cycle is thus 22 years.
Several terrestrial effects have been attributed to this particular cycle. Throughout the 1920s and 1930s, it was proclaimed to be the cause of economic boom and bust and of climatic variations. That the number and extent of sunspots do affect Earth indirectly is very likely. The dark areas on the Sun’s surface turned toward Earth reduces the intensity of solar cosmic rays that reach our atmosphere, and this variation in the bombardment of the upper atmosphere undoubtedly has effects near and at ground level. But belief in them as being the cause of so many of our ills has diminished, and it is seen as pseudo¬ scientific rather than scientific.