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

Solar cycle Start (Min)(Y-M) Min SSN(start of cycle) Max (Y-M) Max SSN Ave spotsper day Time of Rise(Y-M) Length (Y-M)Solar cycle 11755 – Feb14.01761 – Jun144(70)6-411-4Solar cycle 21766 – Jun18.61769 – Sep193(99)3-39-0Solar cycle 31775 – Jun12.01778 – May264(111)2-119-3Solar cycle 41784 – Sep15.91788 – Feb235(103)3-513-7Solar cycle 51798 – Apr5.31805 – Feb82(38)6-1012-3Solar cycle 61810 – Jul0.01816 – May81(31)5-1012-10Solar cycle 71823 – May0.11829 – Nov119(63)6-610-6Solar cycle 81833 – Nov12.21837 – Mar245(112)3-49-8Solar cycle 91843 – Jul17.61848 – Feb220(99)4-712-5Solar cycle 101855 – Dec6.01860 – Feb186924-211-3Solar cycle 111867 – Mar9.91870 – Aug234893-511-9Solar cycle 121878 – Dec3.71883 – Dec124575-011-3Solar cycle 131890 – Mar8.31894 – Jan147653-1011-10Solar cycle 141902 – Jan4.51906 – Feb107544-111-6Solar cycle 151913 – Jul2.51917 – Aug176734-110-1Solar cycle 161923 – Aug9.31928 – Apr130684-810-1Solar cycle 171933 – Sep5.81937 – Apr199963-710-5Solar cycle 181944 – Feb12.91947 – May2191093-310-2Solar cycle 191954 – Apr5.11958 – Mar2851293-1110-6Solar cycle 201964 – Oct14.31968 – Nov157864-111-5Solar cycle 211976 – Mar17.81979 – Dec2331113-910-6Solar cycle 221986 – Sep13.51989 – Nov2131063-29-11Solar cycle 231996 – Aug11.22001 – Nov180825-312-4Solar cycle 242008 – Dec2.22014 – Apr116495-411-0Solar cycle 252019 – Dec1.82024 – Oct161Progr:SC25 (87)SC24 (66)4–10Median9.31804–111–3

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.

Solar Cycles Start (Maximum) Spotless daysSolar cycle 10–111860 – Feb406Solar cycle 11–121870 – Aug1028Solar cycle 12–131883 – Dec736Solar cycle 13–141894 – Jan934Solar cycle 14–151906 – Feb1023Solar cycle 15–161917 – Aug534Solar cycle 16–171928 – Apr568Solar cycle 17–181937 – Apr269Solar cycle 18–191947 – May446Solar cycle 19–201958 – Mar227Solar cycle 20–211968 – Nov272Solar cycle 21–221979 – Dec273Solar cycle 22–231989 – Oct309Solar cycle 23–242001 – Nov817Solar cycle 24–252014 – Apr849Solar cycle 25–262024 – Oct4

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