how we discovered the sun went quiet for 70 years

The Mechanism

The sun rotates through an 11-year cycle of activity, marked by the rising and falling number of dark spots on its surface. By the 1970s solar astronomers had observed this cycle for two and a half centuries and treated it as a fixed feature of the sun. But John A. Eddy, an astronomer at the High Altitude Observatory in Boulder, Colorado, set out in 1974 to test whether the cycle had really been steady or whether changes in the sun might be linked to long-term shifts in Earth's climate. He went back to the original telescopic notebooks of Galileo, Scheiner, Hevelius, Flamsteed and Cassini, to the rare visual sunspot records in Chinese and Korean dynastic histories, to historical catalogues of aurora-borealis sightings, and to tree-ring records of atmospheric ¹⁴C. All four lines of evidence pointed to the same conclusion. Between 1645 and 1715, professional astronomers across Europe had recorded only a handful of sunspots in seven decades; aurorae, which usually appear several times a year in northern latitudes, had nearly vanished; and ¹⁴C in tree rings (which rises when the solar wind is weak) was abnormally high through the same window. The 11-year cycle had not just been weak — it had effectively stopped. The phenomenon had been noticed by Edward Walter Maunder of the Royal Greenwich Observatory in two papers in 1890 and 1894, and by Gustav Spörer before him, but had been dismissed as gaps in observation. Eddy showed that the gaps were the data: the sun itself had gone quiet, in a long minimum that coincided with the coldest decades of the Little Ice Age in Europe. He named it the Maunder Minimum. The paper appeared in *Science* on 18 June 1976.