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.^[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 remains not fully confirmed.

Solar cycles can be reconstructed indirectly, using the radiocarbon ¹⁴C proxy, for the last millennium.^[5]

The smoothing is done using the traditional SIDC smoothing algorithm.^[6] 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,^[7] with related solar cycles characteristics available in this STCE news item.^[8]

The start of solar cycle 25 was declared by SIDC on September 15, 2020 as being in December 2019.^[9] This makes cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years.

• Notes:

(1) The SSN of 125.14 for June 2023 is 34% above the maximum SSN predicted by Zharkova for SC25.
(2) The SSN fell in July 2023 for the first time since the start of the cycle. This makes 125.14 a first maximum for SC25, compared to the maximum of 116.43 for SC24. There is still the possibility of a second 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:

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 (Apr 17 2024) and six months after the end of the month in question.
The table shows averages for each hemisphere and the average for the entire Sun.

The following table gives the number of days so far in cycle 25 against the number up to the same point in cycle 24, which have passed various thresholds for the numbers of sunspots.

As at Apr 17 2024, solar cycle 25 is averaging 27% more spots per day than solar cycle 24 at the same point in the cycle (Apr 17 2013).

Year 1 of SC25 (Dec 2019 to Nov 2020) averaged 101% more spots per day than year 1 of SC24.
Year 2 of SC25 (Dec 2020 to Nov 2021) averaged 7% more spots per day than year 2 of SC24.
Year 3 of SC25 (Dec 2021 to Nov 2022) averaged 8% more spots per day than year 3 of SC24.
Year 4 of SC25 (Dec 2022 to Nov 2023) averaged 41% more spots per day than year 4 of SC24.
Year 5 of SC25 (Dec 1 2023 to Apr 17 2024) is currently averaging 46% more spots per day than the corresponding period of SC24.