A sunspot is a relatively dark area on the Sun, at temperatures of only about 3,000 Kelvin, occupying up to 0.2 per cent of the Sun's visible area (Figure 2.7). Each spot appears to drift eastwards (showing that the Sun rotates once every 27 days) and towards the Sun's equator, before disappearing.

Sunspots alter the solar constant by less than 0.3 per cent, but are associated with very slight increases of ultra-violet radiation and of solar wind, which consists of high-speed electrons and nuclear particles. The annual number of sunspots varies (Chapter 10), with a maximum about every eleven years (e.g. in 1906, 1917,

Figure 2.8 Variation of the number of sunspots observed each year.

1928, 1938, 1947, 1958, 1969, 1980, 1988), as reported by Samuel Schwabe in 1843. The maxima are especially marked every eighty-five years or so (Figure 2.8). When the cycle of sunspot numbers is less than eleven years, there tend to be more sunspots and Earth temperatures show some warming. Also, there were hardly any sunspots between 1650-1710, a period called the Maunder Minimum, named after Annie Russell Maunder (1868-1947). It coincided with the Little Ice Age (Chapter 15). Later there was the Dalton Minimum, between 1795-1823 (Figure 2.8).

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