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In the years since, by tabulating sunspot records going back to the early 18th century and using improved telescopes, satellites, advanced instruments and modern theory, scientists have become ever more familiar with the bizarre dance of the sunspots. Each cycle begins when spots show up in both the northern and southern hemispheres about 35 degrees away from the solar equator. As the cycle matures and the older sunspots fade away (some last only a few hours, others for weeks and even months), new and more numerous spots appear at lower latitudes. Toward the end of the cycle, diminished in number, they appear at latitudes some 5 degrees from the equator.
Sunspots tend to travel in pairs or groups of opposite polarity, like the ends of horseshoe magnets poking through the solar surface. During one eleven- year cycle, as the blemishes traverse the face of the sun in an east-west direction, the leading spots of each group in the northern hemisphere will generally have positive polarity, the trailing spots negative. In the southern hemisphere, the leading spots will be negative. During the next cycle, the hemisphere polarities will reverse. On average, then, 22 years will pass between solar maximums of the same sunspot polarity. This suggests to many astronomers that the fundamental solar cycle is 22 years rather than eleven.
Since the sun in myriad ways governs the very existence of all terrestrial life, the cyclic changes in the sunspot population have, ever since Schwabe, inspired speculation about their effect on solar radiation and, consequently, on the earth. Though the sun is a rather ordinary star, its vital statistics are breathtaking by earthly standards. Some 865,000 miles in diameter, it consists largely of hydrogen (72%) and helium (27%) and is 333,000 times as massive as the earth. Solar temperatures range from about 27 million degrees F* in the core, where 600 million tons of hydrogen are fused into helium every second, to 10,000 degrees F on the photosphere, or surface.
Like a giant nuclear-fusion furnace in the sky, the sun radiates stupendous amounts of energy. Some of it departs in the form of speeding particles, mostly electrons and protons, that form a solar wind blowing from the sun in all directions. It is this continuously flowing wind that feeds particles into the earth's Van Allen radiation belts and distorts the terrestrial magnetic field into a teardrop shape. It also sets off the frequent minor auroral displays visible at higher latitudes.
Also radiating from the solar surface is energy in the form of visible light, ultraviolet and X rays. Enough of this energy penetrates the atmosphere to deliver some 100 trillion kW of power to the earth. Reduced to more comprehensible terms, solar radiation amounts to 1.35 kW falling on every square meter of earth, a number that scientists call the solar constant.