The number of sunspots observed on the “surface” of the Sun varies from year to year.
The rise and fall in sunspot counts varies in a cyclical way, and this cyclical variation in sunspot counts, discovered in 1843 by the amateur German astronomer Samuel Heinrich Schwabe, is called “the Sunspot Cycle”.
The length of the cycle is around eleven years on average – we’ve just finished Solar Cycle 24.
Visible sunspots are caused by magnetic disturbances in the sun that displace its bright outer layer and reveal the slightly cooler (and darker) interior layers, usually for a few days but sometimes for several weeks. They can vary in size, but are usually vast — often much larger than the entire Earth.
The NOAA/NASA co-chaired, international panel to forecast Solar Cycle 25 released their latest forecast for Solar Cycle 25. The forecast consensus: a peak in July, 2025 (+/- 8 months), with a smoothed sunspot number (SSN) of 115. The panel agreed that Cycle 25 will be average in intensity and similar to Cycle 24.
Additionally, the panel concurred that solar minimum between Cycles 24 and 25 will occur in April, 2020 (+/- 6 months). If the solar minimum prediction is correct, this would make Solar Cycle 24 the 7th longest on record (11.4 years).
The number of sunspots can vary from zero to ca. 200 at the height of the Solar Maximum.
Here’s an historic track and tentative forecast courtesy of the Royal Observatory of Belgium