List of solar cycles explained

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.

Details of cycles 1 to 25

Solar cycleStart (Min)
(Y-M)		Min SSN
(start of cycle)		Max (Y-M)Max SSNAve spots
per day		Time of Rise
(Y-M)		Length (Y-M)
Solar cycle 11755 – Feb14.01761 – Jun144(70)6-411-4
Solar cycle 21766 – Jun18.61769 – Sep193(99)3-39-0
Solar cycle 31775 – Jun12.01778 – May264(111)2-119-3
Solar cycle 41784 – Sep15.91788 – Feb235(103)3-513-7
Solar cycle 51798 – Apr5.31805 – Feb82(38)6-1012-3
Solar cycle 61810 – Jul0.01816 – May81(31)5-1012-10
Solar cycle 71823 – May0.11829 – Nov119(63)6-610-6
Solar cycle 81833 – Nov12.21837 – Mar245(112)3-49-8
Solar cycle 91843 – Jul17.61848 – Feb220(99)4-712-5
Solar cycle 101855 – Dec6.01860 – Feb186924-211-3
Solar cycle 111867 – Mar9.91870 – Aug234893-511-9
Solar cycle 121878 – Dec3.71883 – Dec124575-011-3
Solar cycle 131890 – Mar8.31894 – Jan147653-1011-10
Solar cycle 141902 – Jan4.51906 – Feb107544-111-6
Solar cycle 151913 – Jul2.51917 – Aug176734-110-1
Solar cycle 161923 – Aug9.31928 – Apr130684-810-1
Solar cycle 171933 – Sep5.81937 – Apr199963-710-5
Solar cycle 181944 – Feb12.91947 – May2191093-310-2
Solar cycle 191954 – Apr5.11958 – Mar2851293-1110-6
Solar cycle 201964 – Oct14.31968 – Nov157864-111-5
Solar cycle 211976 – Mar17.81979 – Dec2331113-910-6
Solar cycle 221986 – Sep13.51989 – Nov2131063-29-11
Solar cycle 231996 – Aug11.22001 – Nov180825-312-4
Solar cycle 242008 – Dec2.22014 – Apr116495-411-0
Solar cycle 252019 – Dec1.8(Jan 2024)
  • Progr:
    131
Progr:
SC25 (73)
SC24 (53)
Average9.0178.74.411.04

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:

Solar CyclesStart (Maximum)Spotless days[10]
Solar cycle 10–111860 – Feb406
Solar cycle 11–121870 – Aug1028
Solar cycle 12–131883 – Dec736
Solar cycle 13–141894 – Jan934
Solar cycle 14–151906 – Feb1023
Solar cycle 15–161917 – Aug534
Solar cycle 16–171928 – Apr568
Solar cycle 17–181937 – Apr269
Solar cycle 18–191947 – May446
Solar cycle 19–201958 – Mar227
Solar cycle 20–211968 – Nov272
Solar cycle 21–221979 – Dec273
Solar cycle 22–231989 – Oct309
Solar cycle 23–242001 – Nov817
Solar cycle 24–252014 – Apr848

Comparison of cycles 24 and 25 by 13-month running averages

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 (Aug 19 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.

Solar Cycle 24Solar Cycle 25
NorthSouthTotalTotalSouthNorth
Dec 20081.40.82.21.80.90.9Dec 2019
Jan 20091.60.92.52.21.30.9Jan 2020
Feb 20091.61.12.72.71.51.2Feb 2020
Mar 20091.71.22.93.01.51.5Mar 2020
Apr 20092.11.23.33.62.11.5Apr 2020
May 20092.31.23.55.63.91.6May 2020
Jun 20092.71.44.17.96.11.8Jun 2020
Jul 20093.61.95.59.07.31.7Jul 2020
Aug 20094.92.57.49.57.61.9Aug 2020
Sep 20096.53.09.5107.92.5Sep 2020
Oct 20097.33.311128.93.0Oct 2020
Nov 20098.13.612149.83.8Nov 2020
Dec 20098.44.31315105.1Dec 2020
Jan 20109.05.01417116.3Jan 2021
Feb 2010105.61619127.2Feb 2021
Mar 2010126.41922148.0Mar 2021
Apr 2010137.42125169.2Apr 2021
May 2010158.423261510May 2021
Jun 2010168.825281612Jun 2021
Jul 2010178.725311813Jul 2021
Aug 2010179.326352016Aug 2021
Sep 2010191029402218Sep 2021
Oct 2010231234452421Oct 2021
Nov 2010261339512724Nov 2021
Dec 2010281442563026Dec 2021
Jan 2011311546603228Jan 2022
Feb 2011341549653530Feb 2022
Mar 2011381654693731Mar 2022
Apr 2011441761733934Apr 2022
May 2011502069773938May 2022
Jun 2011542377814042Jun 2022
Jul 2011582684874146Jul 2022
Aug 2011602686934349Aug 2022
Sep 2011602787974651Sep 2022
Oct 2011592887994851Oct 2022
Nov 20115930891014953Nov 2022
Dec 20116033921075056Dec 2022
Jan 20125936951135459Jan 2023
Feb 20125841981185562Feb 2023
Mar 20125543981215467Mar 2023
Apr 20125144951235469Apr 2023
May 20124744911245668May 2023
Jun 20124542871255868Jun 2023
Jul 20124440841255867Jul 2023
Aug 20124441851245866Aug 2023
Sep 20124441851245965Sep 2023
Oct 20124541861255966Oct 2023
Nov 20124642881286167Nov 2023
Dec 20124543881296565Dec 2023
Jan 20134542871316863Jan 2024
Feb 2013444286(134)Feb 2024
Mar 2013434284(129)Mar 2024
Apr 2013424284(119)Apr 2024
May 2013414687(111)May 2024
Jun 2013395291(102)Jun 2024
Jul 2013375895(92)Jul 2024
Aug 2013356499(81)Aug 2024
Sep 20133570105Sep 2024
Oct 20133473107Oct 2024
Nov 20133274107Nov 2024
Dec 20133276108Dec 2024
Jan 20143376109Jan 2025
Feb 20143576111Feb 2025
Mar 20143678114Mar 2025
Apr 20143680116Apr 2025

Comparison of cycles 24 and 25 by daily spots

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.

Counts
≥100
≥125
≥150
≥175
≥200
≥225
≥250
≥275

As at Aug 19, 2024, solar cycle 25 is averaging 36% more spots per day than solar cycle 24 at the same point in the cycle (Aug 19, 2013).

External links

Notes and References

  1. Web site: Sunspot numbers. WDC-SILSO. Royal Observatory of Belgium. 19 May 2017.
  2. Hathaway. D.. The solar cycle. Living Reviews in Solar Physics. 2015. 12. 1. 4. 10.1007/lrsp-2015-4. 27194958. 4841188. 1502.07020. 2015LRSP...12....4H.
  3. Usoskin. Mursula. Kovaltsov. Heliospheric modulation of cosmic rays and solar activity during the Maunder minimum. J. Geophys. Res.. 106(A8). A8. 16039. 10.1029/2000JA000105. 2001JGR...10616039U. 2001. free.
  4. Usoskin. etal. A solar cycle lost in 1793-1800: Early sunspot observations resolve the old mystery. Astrophys. J. Lett.. 700. 2. L154. 10.1088/0004-637X/700/2/L154. 0907.0063. 2009ApJ...700L.154U. 2009. 14882350.
  5. Usoskin. etal. Solar cyclic activity over the last millennium reconstructed from annual 14C data. Astron. Astrophys.. 649. A141. 10.1051/0004-6361/202140711 . 2103.15112 .
  6. Web site: SIDC smoothing formula. WDC-SILSO. Royal Observatory of Belgium. 19 May 2017.
  7. Meeus. J.. Une formule d'adoucissement pour l'activité solaire. Ciel et Terre. 1958. 74. 445. 1958C&T....74..445M.
  8. Web site: The solar cycle's new clothes. STCE. Royal Observatory of Belgium. 8 July 2015.
  9. Web site: December 2019 confirmed as starting point of the new solar activity cycle. SIDC.
  10. Web site: Spotless Days Page. SILSO. Royal Observatory of Belgium. 3 April 2017.

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