sunspot minimum

Periods of time when the relative sunspot number is low.

These periods of time occur approximately every 11 years and represent the minimum in the sunspot cycle. There have been prolonged periods of very low sunspot activity from AD 1100 to 1250, from 1460 to 1550, and from 1645 to 1715. The last period is usually called the "Maunder minimum" after E. W. Maunder, the British astronomer who, along with Gustav Sporer of Germany, first called attention to it. The Maunder minimum coincides very closely with the coldest part of the Little Ice Age.
See also solar cycle.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.