Kamis, 07 November 2013

Sir William Herschel, Variable Stars, Sunspots and the Price of Wheat

Today we're going to start something new on Simostronomy. I've invited a guest blogger, Gael Mariani, to add content to the stellar astronomy blog. Gael recently joined the AAVSO and has already contributed an excellent article to the AAVSO website on Henrietta Leavitt. This essay tells a story about one of my favorite astronomers, William Herschel, that I had never heard before. I am pleased to share it with you here.

Sir William Herschel, Variable Stars, Sunspots and the Price of Wheat

By Gael Mariani

Students and scholars of astronomy need little introduction to the life and work of the German-born English astronomer William Herschel (1738-1822). A true polymath, Herschel was a pioneer of the study of binary stars and nebulae, the discoverer of infrared radiation in sunlight, a skilled mathematician, optical lens grinder and telescope maker, a ground-breaking naturalist and a prolific classical composer. His discovery of the planet Uranus in 1781, as well as two of its moons and two more moons of Saturn, garnered him fame, acclaim and a place in astronomical history. However, not all of Herschel’s scientific work was equally well received, and not all his discoveries are as well known today.

One of Herschel’s key areas of study, and a subject of great fascination for him, was those stars that seemed to change their brightness: what we now call variable stars; and he was responsible for much of the progress made in the understanding of these distant suns. His son John Frederick W. Herschel wrote in the 1833 A Treatise on Astronomy that, thanks to his father’s catalogue of brightness of the stars in each constellation, ‘amateurs of the science with only good eyes, or moderate equipment, might employ their time to excellent advantage.’

In today’s science, we know why variable stars vary in brightness. But in Herschel’s time, this was still a source of some mystery. As he sought to understand why these stars appeared to change, he attempted to correlate the phenomenon with another that he had studied extensively, namely the existence of sunspots on our own planet’s nearest star. Herschel posed the hypothesis that these more distant suns might also possess spots, which perhaps were the cause of their vacillation from brightness to dimness. Just two centuries after Galileo had proposed that sunspots were dark clouds floating about in the solar atmosphere, Herschel shared the contemporary scientific view that the greater the number of spots on the sun, the more these would block out the light energy radiated to earth: hence, the ‘spottier’ a variable star, the less bright it would appear from Earth.

Spurred on by the fact that he had perfected a telescope that gave him a view of the sun whose clarity was unprecedented at the time, Herschel deepened his study of sunspots, and this led him to form a new and radical notion: the possibility of a correlation between the number of sunspots and Earth’s climate.

He had noticed that, between July 1795 and February 1800, there had been a number of days when there had been no sunspot activity at all. Then, they had suddenly returned in abundance. He wrote: ‘It appears to me . . . that our Sun has for some time past been labouring under a disposition, from which it is now in a fair way of recovering’. In 1801 he presented a paper to the Royal Society entitled ‘The Nature of the Sun’, in which he wrote: ‘I am now much inclined to believe that openings [sunspots] with great shallows, ridges, nodules and corrugations, instead of small indentation, may lead us to expect a copious emission of heat, and therefore mild seasons . . . A constant observation of the sun with this view, and a proper information respecting the general mildness or severity of the seasons, in all parts of the world, may bring this theory to perfection or refute it if it be not well founded.’

But how was Herschel to back up his hypothesis? Hampered by the lack of precise meteorological records by which to test his theory, he persevered by lateral thinking. Given the effects of lesser or greater quantities of sunshine on vegetation, it struck him that records of good or bad harvests might provide him with the data he needed. Any correlation between these and periods of many or few sunspots would theoretically support his argument. Using Adam Smith’s The Wealth of Nations as his source, he was able to single out five periods when, due to poor harvests, the price of wheat in England had been particularly high. Comparing these records to those of sunspot activity during those periods, he discovered to his surprise a clear correlation between poorer wheat harvests and a relative lack of sunspot activity. Contrary to what had been thought until then, the presence of sunspots did not reduce the amount of heat from the sun, the opposite was true: greater sunspot activity corresponded to good weather and lower wheat prices, while a lack of sunspots corresponded to high wheat prices, which implied less favourable weather. ‘It seems probable,’ he wrote, ‘that some temporary scarcity or defect of the vegetation has taken place when the sun has been without those appearances which we surmise to be the symptoms of a copious emission of light and heat’. As we now know, the sun emits greater ultraviolet radiation, causing more heating of the Earth’s atmosphere, during periods of greater sunspot activity, or solar maximum. But in Herschel’s time this was a revolutionary idea – and the apparent correlation with Earth’s climate made it more revolutionary still.

Excited by his findings, Herschel urged his scientific colleagues to examine solar activity in more detail. Sadly, far from praising his discovery, his peers responded with scepticism and even ridicule. A piece in The Edinburgh Review lambasted his ‘erroneous theory concerning the influence of the solar spots and the price of grain’ as a ‘grand absurdity’. Clearly, the world was not ready to accept such stuff. For once in his illustrious career, the great William Herschel had fallen flat and his attempt to wake the scientific community to his radical idea had failed.

And even to this day, the prevailing views remain largely unchanged. While nobody would now dispute the correlation between solar activity and geometric disturbances on Earth – one only has to think of the SOHO Satellite and the data it sends back, containing potential warnings of increases in solar activity which could have a detrimental effect on such things as telecommunications systems – scientists have generally remained deeply sceptical of claims that there may be a correlation between solar activity and weather on Earth. One respected meteorologist in the 1960s warned that climate researchers risked branding themselves as cranks if they entertained any notion of sun-weather relationships. And in the modern era of sensitive political debate over climate change and global warming, pointing at possible links between earthly weather and cycles of solar activity has become more charged and contentious than ever.

But the time may come when scientists will be forced to revise the orthodox view. Two hundred years after William Herschel urged the Fellows of the Royal Society to investigate the links between sunspots and Earth’s climate, Israeli scientists Dr Lev A Pustilnik and Dr Gregory Yom Din used modern statistical methods to re-examine Herschel’s ideas and concluded that the great astronomer had been right after all. The modern findings confirmed that wheat prices in England during that period did indeed fluctuate in line with solar activity, being higher at solar minimum than at solar maximum, suggesting that the crop was more difficult to grow when sunspot activity was at its lowest.

The implications of this finding go far deeper. In August 2012, scientists studying climate patterns in Central Europe, specifically the winter freezing patterns of the Rhine, revealed a striking correlation between unusually cold Central European winters and periods of low solar activity. The studies, headed by Frank Sirocko, Professor of Sedimentology and Paleoclimatology at the Institute of Geosciences of Johannes Gutenberg University, Mainz, Germany, suggest that the extremely cold European winters of 2010/11 were the result of the North Atlantic Oscillation which Sirocko and his team now link to the low solar activity during that time. Furthermore, the researchers found that out of fourteen episodes between 1780 and 1963 when, according to historical records, the Rhine is known to have frozen over, ten corresponded to periods of minimal sunspot activity – establishing for the first time a possible common link between very cold European winters of the last 230 years. The known 11-year cycle of solar activity makes it possible, according to these results, to predict to some degree how the number of sunspots at any given period could affect our climate on Earth. What first drew Professor Sirocko’s attention to this possibility was the fact that the 125-mile skating race he once attended in the Netherlands can only be held every 11 years, when the rivers freeze up. ‘There must be a reason for this,’ Sirocko remembers thinking, ‘and it turns out there is.’

The Sun Kings’, Stuart Clark, Princeton University Press, 2007
The Herschel Chronicle: The Life Story of William Herschel and his Sister Caroline Herschel, Constance Ann Lubbock, 1933
Understanding Variable Stars, Professor John R Percy, Cambridge University Press, 2007
Daily Science online article, August 2012

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