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Sunspots, climate and wheat yields

Herschel examined the correlation of solar variation and solar cycle and climate. Over a period of 40 years (1779–1818), Herschel regularly observed sunspots and their variations in number, form and size. Most of his observations took place in a period of low solar activity, the Dalton Minimum, when sunspots were relatively few in number. This was one of the reasons why Herschel was not able to identify the standard 11-year period in solar activity. Herschel compared his observations with the series of wheat prices published by Adam Smith in The Wealth of Nations.

In 1801, Herschel reported his findings to the Royal Society and indicated five prolonged periods of few sunspots correlated with the price of wheat. Herschel's study was ridiculed by some of his contemporaries but did initiate further attempts to find a correlation. Later in the 19th century, William Stanley Jevons proposed the 11-year cycle with Herschel's basic idea of a correlation between the low number of sunspots and lower yields explaining recurring booms and slumps in the economy.

Herschel's speculation on a connection between sunspots and regional climate, using the market price of wheat as a proxy, continues to be cited.According to one study, the influence of solar activity can actually be seen on the historical wheat market in England over ten solar cycles between 1600 and 1700. The evaluation is controversial and the significance of the correlation is doubted by some scientists.