List of solar cycles

The following is a list of solar cycles (sometimes called sunspot 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 (ISN v2.0), as available at SILSO.[1] Sunspot counts exist since 1610[2] but the cycle numbering is not well defined during the Maunder minimum.[3] It was proposed that one cycle might have been lost in the late 18th century,[4] but this still remains not fully confirmed.

The smoothing is done using the traditional SIDC smoothing algorithm.[5] 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,[6] with related solar cycles characteristics available in this STCE news item.[7]

The start of solar cycle 25 was declared by SIDC on September 15 2020 as being in December 2019.[8] This makes cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years.

The following table is instead divided into (unofficial) cycles starting and ending with a maximum, to give a better feel for the number of spotless days associated with each minimum:

Following is a comparison of the growth of cycle 25 versus cycle 24, using the 13-month sunspot averages, beginning with the months of the respective minimums.
Numbers in brackets for cycle 25 indicate the minimum possible value for that month, assuming there are no more sunspots between now (2 Feb 2021) and six months after the end of the month in question.

The final 3 months of 2020 averaged 22.8 spots per day, with 11 spotless days. This compares with 10.3 spots per day and 46 spotless days for the same period in SC24.
2020 had 80% more spots than for the same period of SC24.
These figures are in early agreement with a new paper (October 2020) by McIntosh et. al.[11] which projects that solar cycle 25 will almost certainly be stronger than SC24 (ISN max 116), and most likely stronger than SC23 (ISN max 180).