The number of sunspots increases and decreases over time in a regular, approximately 11-year cycle, called the sunspot cycle. The exact length of the cycle can vary. It has been as short as eight years and as long as fourteen, but the number of sunspots always increases over time, and then returns to low again.
We are now (July 2020) exiting Solar Cycle 24 and entering Solar Cycle 25.
History of the study of sunspots.
Samuel Heinrich Schwabe (1789-1875) was a German astronomer who is best remembered for his work on sunspots.
Schwabe was originally an apothecary (dispensing of drugs to physicians & surgeons), who turned his attention to astronomy. In 1826 he commenced his observations on sunspots. Schwabe was looking for a possible planet inside inside the orbit of Mercury. Due to the proximity to the Sun, it would have been very difficult to observe such a planet, and Schwabe believed one possibility to detect a new planet might be to see it as a dark spot when passing in front of the Sun.
For a period of 17 years, from 1826 to 1843, on every clear day, Schwabe would scan the Sun and record its spots trying to detect any new planet among them. He did not find any planet but noticed the regular variation in the number of sunspots and published his findings in a short article entitled "Solar Observations during 1843".
In this document, Schwabe made the suggestion of a probable ten-year period of the sun (that at every tenth year the number of spots reached a maximum).
This paper at first attracted little attention, but Rudolf Wolf (1816-1893), a Swiss astronomer and mathematician, was impressed so he began regular observations of sunspots.
The eleven year sunspot cycle is now well recognised, and Schwabe's observations are one of the most important discoveries in astronomy.
In 1857 Schwabe was awarded the Gold Medal of the Royal Astronomical Society.
Rudolf Wolf was born in Fällanden, near Zurich. He studied at the universities of Zurich, Vienna, and Berlin. Encke was one of his teachers. Wolf became professor of astronomy at the University of Bern in 1844 and director of the Bern Observatory in 1847. In 1855 he accepted a chair of astronomy at both the University of Zurich and the Federal Institute of Technology in Zurich.
Wolf was greatly impressed by the discovery of the sunspot cycle by Heinrich Schwabe and he not only carried out his own observations, but he collected all the available data on sunspot activity back as far as 1610 and calculated a period for the cycle of 11.1 years.
In 1848 he devised a way of quantifying sunspot activity. The Wolf number, as it is now called, remains in use. In 1852 Wolf was one of four people who discovered the link between the cycle and geomagnetic activity on Earth.
More information can be found on the NASA website at......