Is the solar cycle drifting, and is the Sun going quiet?

Two claims circulate about the sunspot cycle. The textbook one: it runs about 11 years, like a metronome. The current one, floated whenever a cycle underperforms (Cycle 24 drew a wave of it): the Sun is quieting down, headed for a new grand minimum. Both are checkable against the same table — SIDC/SILSO and NOAA SWPC's own numbering of every solar cycle since telescopic sunspot counting began, 25 cycles, 1755–2019, each with an official start (minimum), a peak (maximum), and a peak sunspot number. Cycle 25, which started December 2019, is still running — its length is not yet known and is left blank on the source table and here, not guessed at; its peak has already passed (October 2024) and is included wherever only the peak matters.

The ~11-year figure holds, almost exactly, and shows no drift. Across the 24 complete cycles, mean length is 11.03 years (95% CI [10.55, 11.52]) — the schoolbook number is not a rounding of something else, it is close to the honest average. Regressing length on the cycle's own start year finds no trend at all: -0.00165 years per year, 95% CI [-0.00813, +0.00483], R²=0.013, p=0.60. That interval contains zero, and a 4,000-draw bootstrap agrees it's not close (68.5% of resamples land negative, far short of the 95% a real drift would need in either direction). Two hundred seventy years of cycles have not gotten meaningfully longer or shorter.

Peak strength shows no long-run decline either — regressing peak sunspot number (max SSN) on start year: +0.0336 SSN/yr, 95% CI [-0.266, +0.334], R²=0.0023, p=0.82. Splitting the 25 cycles at their own median start year (1890, not chosen to flatter either side): the first 12 cycles average 177.2 SSN at peak, the most recent 13 average 178.7 — a gap of +1.4, Welch t=0.06, p=0.95. The specific claim in circulation right now is narrower than a full-history median split: the last 5 cycles (21–25, since 1976) against everything before them. That comparison also returns nothing — 180.6 SSN vs. 177.3, gap +3.2, p=0.90 — and the point estimate runs the wrong direction for the "going quiet" story (recent cycles average slightly higher, not lower).

That null is not a wave of the hand over a real, named weak cycle. Cycle 24 (started 2008) peaked at SSN 116 — the 4th-weakest of all 25 on record, a fact this run states rather than smooths over. But Cycle 25 that followed it peaked at 161, back up near the middle of the pack, and neither cycle moves the 5-cycle or full-history trend to a number that clears zero. One weak cycle, even a real one, is not yet a trend; the desk needs more than a single data point to call a direction, and does not have it here.

One relationship in this data is not a near-miss: the Waldmeier effect. Solar physicist Max Waldmeier noted in 1935 that cycles which climb from minimum to maximum faster tend to reach higher peaks — and it is still there in the full 270-year record. Regressing peak SSN on rise time (years from minimum to maximum): -38.3 SSN per additional year of rise, 95% CI [-52.8, -23.8], R²=0.57, p=1.5e-05. That interval sits nowhere near zero; a 4,000-draw bootstrap puts 100% of resamples negative, and the rank-based Spearman check agrees without the linear assumption (ρ=-0.77, p=8.3e-06). Concretely: Cycle 3 rose in 2.9 years and peaked at 264; Cycle 5 took 6.8 years to rise and topped out at 82. R²=0.57 means rise time alone accounts for more than half the variance in how strong a cycle turns out to be — a genuinely useful early read on a cycle's eventual strength, years before it peaks.