Written by Chris Duffy, PhD Candidate AOP

William Herschel, who died 200 years ago this year, is famous for a number of things we take for granted in modern astronomy, not least his discovery of Uranus in 1781 the first planet discovered in the modern era of astronomy. Less known in his work which helped lay the groundwork, inadvertently, for establishing the now well known solar cycle — the 11 year cycle the Sun undergoes where the number of sunspots we see on the surface of the Sun varies dramatically.

The Sun is at the centre of the Solar System. Its gravity dictates the orbits of the planets. The Sun is 6000 degrees Celsius on its surface and 14,000,000,000 degrees Celsius at its core. Credit NASA/SDO

By Herschel’s time sunspots had been known for well over 2000 years however their effect on the the activity level of the Sun was unknown. Herschel believed that they were correlated with the amount of energy given out by the Sun which he believed impacted the climate here on Earth. Unlike today, however, Herschel didn’t have access to the years of temperature data he’d need (even in Armagh where we’ve been recording the temperature daily since 1795 he’d only have had a few years to go by!). What he did have access to though was the price of wheat in previous years and he believed that years that the price was higher indicated less wheat had been grown and that it had been colder and that in years where it was warmer and there was more what then it must have been warmer.

William Herschel (Image credit :via Wikimedia.org)

Armed with this data all there was left to do was look at the number of sunspots that he and others had seen in the preceding years. Herschel’s analysis found that high wheat prices coincided with an absence of sunspots. This meant that the more sunspots present on the Sun meant it was releasing more heat out into space. But was he right?

Wheat Field. Credit: WikiCommons

Like all nice stories of discovery in science, the answer is yes and no. At the time his fellow scientists were sceptical that you could link wheat prices to the activity of the Sun and later a number of others tried to replicate his results but no one has been able to link wheat prices and sunspots. Despite this his idea that the number of sun spots seen occurred in a cycle stuck and others built on it observing the Sun over many, many years before landing on the 11 year cycle that we know exists today; and confirming that the greater the number of sunspots on the Sun the the more energy we on Earth receive.

In the end Herschel wasn’t quite right; but his idea lead us to one of the key pieces of knowledge we have about our Sun. It goes to show that sometimes for astronomy all you need is a field of wheat.


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