Sunspot number (monthly mean)

The sunspot number is a count of dark, magnetically intense blotches on the Sun's visible surface, combining the number of individual spots with the number of groups they form. It has been recorded by observatories since the early seventeenth century, making it one of the longest continuous scientific series in existence; today the World Data Centre SILSO in Brussels gathers observations from stations worldwide and issues a monthly mean, distributed alongside space weather products from NOAA. The count rises and falls on a roughly eleven-year cycle, from near zero at solar minimum to monthly averages of a hundred or more at maximum, tracking the Sun's magnetic churn and, with it, the likelihood of flares, geomagnetic storms and auroras. One caveat: the figure is an index built from telescope counts, not a physical measurement, so it depends on observer judgement, instruments and clear skies.

Record high

398 sunspots, May 1778

Record low

0 sunspots, January 1754

First measurement

January 1749

Measurement cadence

monthly

Measurements in this record

3,333