September is the month during which the greatest number of sunspots are observed. Sun's rotation axis is tilted 7.25 degrees from perpendicular to the plane of Earth's orbit, so during September Sun's north pole is tilted toward us. For reasons which astronomers do not understand, there are more sunspots in Sun's northern hemisphere than southern. Consequently, more of them are observed in September than in other months.

        This annual variation -- more sunspots always being observed in September -- modulates the approximately eleven-year increase and decrease in the number of sunspots. This eleven-year solar cycle is not well understood by astronomers, either. In fact, understanding the physical mechanism for the production of sunspots is a major challenge in astronomy today.

        Records exist of naked eye observations of sunspots by the Greeks as early as the fourth century B.C. The early observers sometimes saw tiny black dots or specks on Sun's surface as they viewed it through clouds or early morning fog. Since Sun was believed to be a perfect, unblemished sphere, however, the explanation was always of something in the clouds or fog, or in the observer's eye. Not until Galileo used his telescope to project a solar image on a screen, around 1610, was it certain that these spots really were on Sun. Galileo's observation of their motion across the solar disk was correctly interpreted by him as being due to Sun's rotation, and he correctly deduced its rotation period of approximately 25 days from these observations.

        Sunspots are regions of Sun's outer layers -- its atmosphere -- which are at a temperature of only about 7200 F. This is, of course, very high, but low compared to the nearly 11,000 F temperature of most of the sun's surface. They thus appear dark compared with the surrounding region. The average diameter of a sunspot is about 6000 miles, but some have been observed that are more than 90,000 miles across. They seem to be "insulated" from the surrounding, hotter regions by magnetic fields very much stronger than in the surrounding regions, but the exact mechanism has not been satisfactorily explained.

        The eleven year cycle, another discovery of Galileo's, from one minimum in the number of sunspots to the next minimum is not a definite eleven years, but rather is an average. In some cases the period has been as short as seven years and in other cases it has been as long as 17 years. Sunspots currently seem to be on the increase following a minimum in 1997. This minimum marked the end of a cycle that began with a minimum in 1986 and had its maximum in 1991.

        Sunspots often appear in pairs, aligned approximately in an east-west direction. They have opposite magnetic polarities, with all eastern spots, for example, being "north" magnetic poles and all western spots being "south" magnetic poles in the northern hemisphere and with the opposite senses of polarities in the southern hemisphere. The magnetic field emerges from the sun's atmosphere at one of the spots in the pair and reenters at the other. This east-west polarity holds for all pairs during any one sunspot cycle and is then reversed during the next, suggesting that the cycle should really be thought of as a 22-year cycle.

        Increases in the number of sunspots are the most visible indicators of an increase in solar activity. Increased activity is also detected in radio emission by the solar atmosphere, in the rate of emission of particles in the solar wind, in the occurrence of solar flares, in the frequency of aurorae and their observation farther from the poles, and other phenomena that are correlated with violent events in Sun's outer layers. All of these phenomena and their evident association with varying magnetic fields remain a puzzle being studied intently by solar physicists.