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The solar cycle, explained

What the roughly 11-year solar cycle is, how sunspot numbers are counted, where Solar Cycle 25 stands using live NOAA data, and why forecasting cycles is hard.

What the cycle is

The number of sunspots rises and falls over roughly 11 years. The peak is called solar maximum and the low point solar minimum. Cycles are numbered from Cycle 1, which began in 1755. [1]

A new cycle is considered to have started when sunspot groups begin appearing at high solar latitudes with leading-spot magnetic polarity opposite to the previous cycle's. [1] The Sun's overall magnetic poles flip near each maximum, so a full magnetic cycle takes about 22 years. [2] [3]

How sunspots are counted

The official international sunspot number comes from the World Data Center SILSO at the Royal Observatory of Belgium. The classic formula counts each group as 10 plus 1 for each individual spot, so a Sun with any spots at all scores at least 11. [1] [4]

On 1 July 2015 SILSO replaced the historical series with a recalibrated Version 2.0, which corrects several inconsistencies in the past record and uses a new reference scale. Clette et al. (2014) review the problems behind the revision. Figures quoted from before and after 2015 may therefore not be directly comparable. [7] [5]

Where Solar Cycle 25 stands

Cycle 24 started in December 2008, peaked in April 2014 and ended in December 2019, which was also the start of Cycle 25. [1] NASA and NOAA announced in October 2024 that the Sun had entered the maximum period of Cycle 25. [3]

In NOAA's smoothed series, Cycle 25 has reached 160.9 (2024-10), compared with Cycle 24's peak of 116.4 (2014-04). The latest monthly value is 60.3 for 2026-09 (preliminary). Smoothed values lag by about six months, so the true peak is only confirmed later. [6]

For the forecasting debate, including how the 2019 panel consensus compared with what happened, see our claim page on the Solar Cycle 25 prediction debate.

Why it matters

Active regions around sunspots produce most significant flares and CMEs, so space weather events, and auroras at lower latitudes, are more frequent around maximum. [1] [3] Hathaway's review covers the cycle's observed regularities and the open problems in predicting it. [2]

Sources

1. NOAA SWPC: Sunspots/Solar Cycle https://www.swpc.noaa.gov/phenomena/sunspotssolar-cycle
2. Hathaway, D. H. (2015). The Solar Cycle. Living Reviews in Solar Physics. doi:10.1007/lrsp-2015-4
3. NASA (2024): NASA, NOAA: Sun Reaches Maximum Phase in 11-Year Solar Cycle https://science.nasa.gov/science-research/heliophysics/nasa-noaa-sun-reaches-maximum-phase-in-11-year-solar-cycle/
4. SILSO, Royal Observatory of Belgium: World Data Center for the sunspot index https://www.sidc.be/SILSO/
5. Clette, F., Svalgaard, L., Vaquero, J. M. & Cliver, E. W. (2014). Revisiting the Sunspot Number. Space Science Reviews. doi:10.1007/s11214-014-0074-2
6. NOAA SWPC: Solar Cycle Progression https://www.swpc.noaa.gov/products/solar-cycle-progression
7. SILSO: Sunspot Number Version 2.0, new data and conventions https://www.sidc.be/SILSO/newdataset

Sources were checked on 2026-10-21. Text on this page is original. Quoted figures are attributed and linked to the source. General education only, not for safety or operational decisions. Disclaimer.