Almost all important phenomena on the Sun show periodic behaviour. From the historical point of view the best-known is the periodical occurrence of sunspots.

The most obvious manifestation of the solar periodic activity is the eleven-year cycle. Evidence of it can be seen in a number of different characteristics of the solar activity, the most striking of which is the number of sunspots. This eleven-year solar cycle was discovered by Heinrich Samuel Schwabe (1789 - 1875). He succeeded in presenting reliable proof of this cycle by studying historical documents going back in history to about 1716. The following picture shows the average values of the relative number of sunspots (N) since 1749. An uninterrupted series of sunspots observations exists from this year onwards until today. The relative number of sunspots was introduced by Rudolf Wolf (1816 - 1893), head of the astronomical observatory in Zⁿrich. The number is determined from the number of sunspot groups G and the total number of sunspots S visible at a given moment on the Sun using the formula

N = 10G + S.

The attempts to extend the series of observations of the eleven-year cycle way back before 1716 were initially unsuccessful. Although there were historical resources available their study resulted in very low values of the relative number. At first, the resources were considered unreliable. The first person to come up with the idea that in the second half of the 17th century the eleven-year cycle almost did not exist was the English physicist Edward Walter Maunder (1851 - 1928). Recent research provides a number of indirect, but very plausible, proofs that he was right. The existence of what is called Maunder's minimum is supported, for example, by the study of tree rings, historical documents confirming that, within the given period, neither a single sunspot nor the aurora were observed, and that there were striking climatic changes. The period of Maunder's minimum is matched closely by the exceptionally cold weather in Europe, sometimes referred to as the Little Glacial Epoch. The following diagram shows the estimated relative numbers of the sunspots derived from historical resources going back to about 1650. Only individual observations are plotted before 1650. They do not create a continuous series.

Sunspots do not occur throughout the whole of the solar photosphere. Most frequently, they can be found in the area between 35░ northern and 35░ southern heliographic latitude. Only rarely will a sunspot occur farther than 40░ from the Sun's equator. No sunspots occur in the area around the solar poles. In the first half of the 19th century the amateur astronomer Richard Christopher Carrington (1826 - 1875) discovered, by carefully plotting the positions of the sunspots, that the heliographic latitude where the sunspots occur most frequently changes remarkably within the eleven-year cycle. After a period of minimum solar activity sunspots begin to appear about 30░ away from the equator, both on the southern and northern hemisphere. As they grow in number over time, the area of the most frequent occurrences shifts to the lower heliographic latitudes. During maximum activity the sunspots occur most frequently about 15░ away from the equator. Then their number decreases and the occurrences move towards the equator. The cycle ends when the area with sunspots reaches the equator. At that time, sunspots belonging to a new cycle may already be present in higher heliographic latitudes. If we plot the heliographic latitude of each spot in relation to time we will obtain a diagram that is aptly called "the butterfly diagram".

In 1913, another interesting phenomenon concerning the occurrence of sunspots was revealed. The discovery was mostly contributed by the American astronomer George Ellery Hale (1868-1938). The sunspots, or sunspot groups, usually occur in pairs oriented in the west û east direction. The western sunspot is called leading, the eastern one is called following. The leading spot has always the opposite polarity of the magnetic field to the following sunspot. All leading spots in the northern hemisphere have the same polarity. The same applies to the leading sunspots in the southern hemisphere. However, the polarity of the leading spots in different hemispheres is different (of course, the same is valid for the following sunspots). If we investigate the behaviour of the polarity of the magnetic field of the sunspots in two solar cycles following on from each other we find that a reversal of poles has occurred, i.e. within the same hemisphere, the leading and the following spots have exchanged their polarity. Consequently, in terms of the development of sunspots, it would seem reasonable to speak of a twenty-two year cycle, rather than an eleven-year cycle.

The magnetogram pictured on the left was made just a few hours after the solar eclipse of August 11, 1999. The areas with different magnetic field polarity are distinguished by different colors û orange and blue. The picture demonstrates rather well the differences between the northern (upper) and southern (lower) hemisphere.