List of solar cycles


The following is a list of solar cycles, tracked since 1755 following the original numbering proposed by Rudolf Wolf in the mid-19th century. The source data are the revised International Sunspot Numbers, 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 still remains not fully confirmed.
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 will be determined restrospectively. Technically, due to the calculation method, this declaration cannot happen less than 7 months after the minimum, although it can often be seen earlier that at least a temporary minimum will occur. Realistically, the declaration must happen some time after this, to discount the possibility of a double trough. When the start of cycle 25 is retrospectively announced, a large number of spotless days that were sitting in cycle 24 will be transferred to cycle 25.
It is more than likely that cycle 25 will begin in December 2019. As of July 24 2020, the way the SIDC algorithm works, even if there are no more spots before the end of January 2021, the averages for the first seven months of 2020 cannot fall below the December 2019 average. If there is no second trough, this December minimum will be 1.81. This would make cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years. The value for January 2020 is 2.2. Here are the minimum possible values for subsequent months :
February 2020: 2.4
March 2020: 2.3
April 2020: 2.3
May 2020: 2.2
June 2020: 2.1
July 2020: 1.8
Aug 2020: 1.5
Solar CycleStart Smoothed minimum ISN MaximumSmoothed maximum ISNAverage spots per dayTime of Rise Duration Spotless days
Solar cycle 11755-0214.01761-061446.311.3
Solar cycle 21766-0618.61769-091933.39.0
Solar cycle 31775-0612.01778-052642.99.3
Solar cycle 41784-0915.91788-022353.413.6
Solar cycle 51798-045.31805-02826.812.3
Solar cycle 61810-080.01816-05815.812.8
Solar cycle 71823-050.21829-111196.510.5
Solar cycle 81833-1112.21837-032453.39.7
Solar cycle 91843-0717.61848-022204.612.4
Solar cycle 101855-126.01860-02186924.211.3561
Solar cycle 111867-039.91870-08234893.411.8942
Solar cycle 121878-123.71883-12124575.011.3872
Solar cycle 131890-038.31894-01147653.811.8782
Solar cycle 141902-014.51906-02107544.111.51007
Solar cycle 151913-072.51917-08176734.110.1640
Solar cycle 161923-089.41928-04130684.710.1514
Solar cycle 171933-095.81937-04199963.610.4384
Solar cycle 181944-0212.91947-052191093.310.2382
Solar cycle 191954-045.11958-032851293.910.5337
Solar cycle 201964-1014.31968-11157864.111.4285
Solar cycle 211976-0317.81979-122331113.810.5283
Solar cycle 221986-0913.51989-112141063.29.9257
Solar cycle 231996-0811.22001-11180825.312.3619
Solar cycle 242008-122.22014-04116495.3In progress1085
Solar cycle 25To be determined1.8 as at December 2019 -
Average9.0178.74.411.04--

The following table is instead divided into cycles starting and ending with a maximum, to give a better feel for the number of spotless days associated with each minimum:
Solar Cycles“Start” Spotless days
Solar cycle 10-111860-02406
Solar cycle 11-121870-081028
Solar cycle 12-131883-12736
Solar cycle 13-141894-01934
Solar cycle 14-151906-021023
Solar cycle 15-161917-08534
Solar cycle 16-171928-04568
Solar cycle 17-181937-04269
Solar cycle 18-191947-05446
Solar cycle 19-201958-03227
Solar cycle 20-211968-11272
Solar cycle 21-221979-12273
Solar cycle 22-231989-10309
Solar cycle 23-242001-11817
Solar cycle 24-252014-04748