Solaemon's Spotless Days Page

The Spotless Days Page has been moved to the SILSO website of the Royal Observatory of Belgium.

The old SC23-24 Spotless Days page remains available underneath.

ContentsLast update
Periods with spotless days (>20 days) since 184905 September 11
Number of periods with spotless days (P0) per solar cycle05 September 11
Main characteristics per solar cycle05 September 11
Evolution of spotless days in SC23-24 and comparison with other cycle transits05 September 11
Monthly number of spotless days05 September 11
Smoothed monthly Wolfnumber and smoothed monthly number of spotless days05 September 11
Top 25 of years with most spotless days since 184905 September 11

Source: Daily Wolfnumbers from SILSO, World Data Center for the Sunspot Index, Royal Observatory of Belgium, 1849-present. (Data source: SILSO)

Periods with spotless days (>20 days) since 1849

Perioden met vlekkenloze dagen (>20 dagen) sinds 1849

The table above shows all periods with 20 or more consecutive spotless days. The period from 31 July 2009 till 31 August 2009 is one of the longer ones since the beginning of daily solar observations in 1849. It is interesting to note that the longest spotless stretch during the SC23-SC24 transit was also the last to make it into the table above. An even longer period in recent history occured during the SC22-SC23 transition, from 13 September 1996 till 24 October 1996, when the Sun was spotless for 42 consecutive days. One of the longest spotless periods (since 1818) is probably from 24 October 1822 till 12 March 1823 (140 days!), but unfortunately, the series are broken on 29 December 1822 (no observation available for that day). With 9 entries, the current solar cycle transit ranks 4th in transits with the most of 20 or more spotless days stretches. Only the transits to cycles 14 (10), 12 (12) and 15 (17) had even more such periods.

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Number of periods with spotless days (P0) per solar cycle

Aantal perioden met vlekkenloze dagen (P0) per zonnecyclus

SC10 < P0 < 1515 < P0 < 2020 < P0 < 30P0 > 30Total
10 784322
11 723214
12 1297533
13 1343020
14 9104629
15 15713439
16 861116
17 1101315
18 01214
19 562114
20 21003
21 42107
22 21104
23 80019

Above table and graph clearly show 2 groups. SC10-15 had many long periods with spotless days. SC16-23 do hardly have any such periods. Although the first spotless day for SC24 dates already back to January 2004, it is only since mid 2007 that SC24 has started producing longer stretches of spotless days. Now, the distinction between SC24 and SC16-23 has become obvious, both in number of spotless periods as in number of periods with 20 or more spotless days.

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Main characteristics per solar cycle

Hoofdkenmerken per zonnecyclus

SCMonth 1st Rd=0Month SC-minDeltaTotal Rd=0
10April 1850January 185669654
11October 1861April 186766406
12May 1873December 1878671028
13January 1885February 189061736
14November 1895September 190170938
15March 1908June 1913631019
16April 1920April 192336534
17September 1930September 193336568
18November 1941April 194429269
19December 1950April 195440446
20November 1961August 196433227
21July 1973March 197632272
22November 1983September 198634273
23April 1994May 199625309
24January 2004December 200866 +/- 4797 +/- 245
33 +/- 5362 +/- 134

Table above gives for each new solar cycle the month in which the first spotless day appeared (after solar maximum...), the month in which the (Meeus smoothed) solar cycle minimum took place, the delay in months between those 2 timings, and the total number of spotless days that occured during the cycle transition. As can be seen, there is a big difference between solar cycles 10 to 15 and 16 to 23. The first group has a long delay (66 months) and many spotless days (almost 800 on the average), whereas the other group, containing the more recent cycles, has only 33 months between first spotless day and cycle minimum and not even half of the total number of spotless day of the first group (less than 400 days!). The SC23-24 cycle transit clearly belongs to the first group, though with 59 months it falls just outside the standard deviation range (66 +/- 4 months). With 151 months, SC23 is the longest solar cycle in over 180 years (SC06: 151 months, SC04: 169 months).

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Evolution of spotless days in SC23-24 and comparison with other cycle transits

Evolutie van de vlekkenloze dagen in SC23-24 en vergelijking met voorgaande cyclusovergangen

The thick green line in the graph above shows the accumulated (total number of) spotless days so far during the transit from solar cycle 23 to 24. The thick blue and red line show the same parameter, but respectively for the average of solar cycles 10 to 15 (SCo) and 16 to 23 (SCn). The thin threaded lines show the respective standard deviations (+sd and -sd). The number of spotless days of the current solar cycle transit follows very well that of the average of the group of old solar cycles. Nonetheless, it must be noted that the variation between the cycles is quite large, as can be seen from this raw graph. The pink and purple vertical lines show the average timing of the cycle minimum (and standard deviations) compared to the month with first spotless day, and this for the respective cycle groups (SCo min, SCn min). Based on SIDC-data, the SC-minimum was reached in December 2008, only 3 months shy of the lower standard deviation for the SC10-15 group. The current solar cycle transit has accumulated 817 spotless days so far. Notice the flattening, indicating that few spotless days are to be expected and that the SC23-24 transit is over.

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Monthly number of spotless days

Maandelijks aantal vlekkenloze dagen

The two graphs on top show the monthly number of spotless days during this cycle transit (red) compared to respectively the average number during SC10 to 15 and during SC16 to 23. This cycle transit mimicks much better the series of SC10-15. Since November 2009, the number of spotless days has fallen sharply, but as can be seen from the top graph, it is perfectly possible that an isolated spotless day can be produced during the maximum period of a moderate to low amplitude SC. In this respect, the spotless day of 14 August 2011 is nothing extraordinary.

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Smoothed monthly Wolfnumber and smoothed monthly number of spotless days

Afgeglad maandelijks Wolfgetal en afgeglad maandelijks aantal vlekkenloze dagen

This graph shows the smoothed monthly Wolfnumber (Rs) and smoothed monthly number of spotless days (SDs) since 1849. For obvious reasons, the instances of minimum Rs and maximum SDs happen for most cycles in the same month. Exceptions are SC14 (3 months later) and SC23 (4 months later). There is a strong relationship between the two parameters during solar cycle minimum: SDsmax = -1,4755 Rsmin + 27,076 (r2=0,92). The smoothed Rs and SDs both reached their respective extrema in December 2008 (resp. values of 1,71 and 24,2). This is comparable with the low activity transits in the early 20th-century, and certainly contrasts with the previous 4 cycle transits.

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Top 25 of years with most number of spotless days since 1849

Top 25 van jaren met het meeste aantal vlekkenloze dagen sinds 1849

This graph shows the number of spotless days per year since 1849. There are 109 years (including 2011) with at least 1 spotless day, of which 24 years with 150 or more spotless days. 1913 is recordholder with a staggering 311 days, while 2008 ranks fourth in years with a spotless sun (265 days). With 262 days (Data-file 03 September 10!), 2009 falls just short of 2008 but it still makes it in the top 5 of years with most blanc suns. Only 1878, 1901 and 1913 have even more such days.

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CLOSED: 07 January 2017

Last update: 05 September 2011

Correction: 23 May 10 - Karl Wiedemann spotted a small formatting error in the graph "Top 25 of years with most spotless days". Corrected with no further changes required. Thanks!

Correction: 02 January 2009 - Both Kim Mackey and Chuck Beason spotted a missing 22-day stretch of spotless days (18 Nov 08 - 09 Dec 08). First two tables and related graph were corrected. Thanks everybody for keeping a watchful eye on this page.

Correction: 12 June 2008 - Jeff Lackey (ARRL) spotted an error in the top table (June-July 1976). As a result, the entire database was double-checked. The first two tables and graph were adapted. There is no change in the overall conclusions on the evolution of spotless days in the solar cycles considered.

Original: 07 October 2007