Solar cycles are nearly periodic 11-year changes in the Sun's activity that are based on the number of sunspots present on the Sun's surface. The first solar cycle conventionally is said to have started in 1755. The source data are the revised International Sunspot Numbers (ISN v2.0), as available at SILSO. Sunspot counts exist since 1610 but the cycle numbering is not well defined during the Maunder minimum. It was proposed that one cycle might have been lost in the late 18th century, but this remains not fully confirmed. Solar cycles can be reconstructed indirectly, using the radiocarbon 14C proxy, for the last millennium. The smoothing is done using the traditional SIDC smoothing algorithm. Using this algorithm, if the month in question is notated month 0, a weighted average is formed of months −6 to 6, where months −5 to 5 are given weightings of 1, and months −6 and 6 are given weightings of 0.5. Other smoothing formulas exist, and they usually give slightly different values for the amplitude and timings of the solar cycles. An example is the Meeus smoothing formula, with related solar cycles characteristics available in this STCE news item. The start of solar cycle 25 was declared by SIDC on September 15, 2020 as being in December 2019. This makes cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years.
Details of cycles 1 to 25
Notes on Solar cycle 25 The maximum SSN of 160.9 for October 2024 is 73% above the maximum SSN predicted by Zharkova for SC25. SILSO has announced October 2024 as the peak of the cycle. It says: "For the coming years, another maximum remains a possibility, but it is unlikely it will be higher than the one in October last year because the Sun has completed its polar field reversal in 2023, and because the ongoing solar cycle is already 5.5 years in progress." As at April 30, 2026, SC 25 is averaging 30% more spots per day that SC24 at the same stage in the cycle.
Unofficial cycles starting with a maximum The following table is instead divided into (unofficial) cycles starting and ending with a maximum, for the purpose of indicating the number of spotless days associated with each minimum. It begins with cycle 10-11 due to the significant amount of missing daily data before this time, which allows estimated averages to be calculated, but does not permit totals to be counted.
References
External links Solar Influences Data Analysis Center (SIDC) Graphics of historic solar cycles at the SIDC page Near realtime solar images from SOHO SORCE Total Solar Irradiance (TSI) Data