Appearing as dark blemishes, sunspots are the most noticeable feature on the visible surface of the sun, the photosphere (Fig. 2). Sunspot observations were carried out as early as 300 BC by both the Greeks and the Chinese, and continued throughout the middle ages in Europe [1]. In the early 19^th century it was discovered that the number of sunspots rises and falls in an 11-year cycle, with the period of maximum and minimum activity referred to as the sunspot maximum and minimum. Sunspots usually appear in clusters of two or more spots. Early on in a given cycle, sunspots form in a band located about 30° latitude in both the northern and southern hemispheres. As the cycle progresses, the sunspots groups migrate, forming ever closer to the equator.

Figure 2
Image of the sun in visible light illustrating a number of sunspot groups. Photograph taken by the author through an 8" reflecting telescope at Mt. Wilson Observatory.