# Astronomy: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 July 9 2020, the way the SIDC algorithm works, even if there are no more spots before the end of November 2020, the January, February, March, April and May 2020 averages cannot fall below the December 2019 average. If there is no second trough, this December minimum will be 1.81.

Solar Cycle Start (Minimum) Smoothed minimum ISN (start of cycle) Maximum Smoothed maximum ISN Time of Rise (years) Duration (years) Spotless days[8][9]
Solar cycle 1 1755-02 14.0 1761-06 144 6.3 11.3
Solar cycle 2 1766-06 18.6 1769-09 193 3.3 9.0
Solar cycle 3 1775-06 12.0 1778-05 264 2.9 9.3
Solar cycle 4 1784-09 15.9 1788-02 235 3.4 13.6
Solar cycle 5 1798-04 5.3 1805-02 82 6.8 12.3
Solar cycle 6 1810-08 0.0 1816-05 81 5.8 12.8
Solar cycle 7 1823-05 0.2 1829-11 119 6.5 10.5
Solar cycle 8 1833-11 12.2 1837-03 245 3.3 9.7
Solar cycle 9 1843-07 17.6 1848-02 220 4.6 12.4
Solar cycle 10 1855-12 6.0 1860-02 186 4.2 11.3 561
Solar cycle 11 1867-03 9.9 1870-08 234 3.4 11.8 942
Solar cycle 12 1878-12 3.7 1883-12 124 5.0 11.3 872
Solar cycle 13 1890-03 8.3 1894-01 147 3.8 11.8 782
Solar cycle 14 1902-01 4.5 1906-02 107 4.1 11.5 1007
Solar cycle 15 1913-07 2.5 1917-08 176 4.1 10.1 640
Solar cycle 16 1923-08 9.4 1928-04 130 4.7 10.1 514
Solar cycle 17 1933-09 5.8 1937-04 199 3.6 10.4 384
Solar cycle 18 1944-02 12.9 1947-05 219 3.3 10.2 382
Solar cycle 19 1954-04 5.1 1958-03 285 3.9 10.5 337
Solar cycle 20 1964-10 14.3 1968-11 157 4.1 11.4 285
Solar cycle 21 1976-03 17.8 1979-12 233 3.8 10.5 283
Solar cycle 22 1986-09 13.5 1989-11 214 3.2 9.9 257
Solar cycle 23 1996-08 11.2 2001-11 180 5.3 12.3 619
Solar cycle 24 2008-12 2.2 2014-04 116 5.3 In progress 1074 (4 July 2020)
Solar cycle 25 To be determined (1.8 as at December 2019 - likely minimum)
Average 9.0 178.7 4.4 11.04

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:

Solar Cycles “Start” (Maximum) Spotless days
Solar cycle 10-11 1860-02 406
Solar cycle 11-12 1870-08 1028
Solar cycle 12-13 1883-12 736
Solar cycle 13-14 1894-01 934
Solar cycle 14-15 1906-02 1023
Solar cycle 15-16 1917-08 534
Solar cycle 16-17 1928-04 568
Solar cycle 17-18 1937-04 269
Solar cycle 18-19 1947-05 446
Solar cycle 19-20 1958-03 227
Solar cycle 20-21 1968-11 272
Solar cycle 21-22 1979-12 273
Solar cycle 22-23 1989-10 309
Solar cycle 23-24 2001-11 817
Solar cycle 24-25 2014-04 737 (4 July 2020)

## References

1. "Sunspot numbers". Royal Observatory of Belgium. Retrieved 19 May 2017.
2. Hathaway, D. (2015). "The solar cycle". Living Reviews in Solar Physics 12 (1): 4. doi:10.1007/lrsp-2015-4. PMID 27194958. Bibcode2015LRSP...12....4H.
3. Usoskin; Mursula; Kovaltsov (2001). "Heliospheric modulation of cosmic rays and solar activity during the Maunder minimum". J. Geophys. Res. 106(A8) (A8): 16039. doi:10.1029/2000JA000105. Bibcode2001JGR...10616039U.
4. Usoskin (2009). "A solar cycle lost in 1793-1800: Early sunspot observations resolve the old mystery". Astrophys. J. Lett. 700 (2): L154. doi:10.1088/0004-637X/700/2/L154. Bibcode2009ApJ...700L.154U.
5. "SIDC smoothing formula". Royal Observatory of Belgium. Retrieved 19 May 2017.
6. Meeus, J. (1958). "Une formule d'adoucissement pour l'activité solaire". Ciel et Terre 74: 445. Bibcode1958C&T....74..445M.
7. "The solar cycle's new clothes". Royal Observatory of Belgium. Retrieved 8 July 2015.
8. "What's Wrong with the Sun?". NASA. Retrieved 29 July 2014.
9. "Spotless Days Page". Royal Observatory of Belgium. Retrieved 3 April 2017.