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 (as with any other cycle) will be determined restrospectively. Mathematically, this declaration cannot happen less than 7 months after the minimum. 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 June 14 2020, the way the SIDC algorithm works, even if there are no more spots before the end of August the January and February averages cannot fall below the December average. This December minimum will likely be 1.8 or 1.9.

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: