What is surprising is just how much data we have on the Sun from 400 years ago.
For some aspects of solar activity we barely have a half a solar cycle. For example, on solar spectral changes: UV and Infrared light swing up and down through the solar cycle, but we only got a good grip on these important changes in the last ten yearswith the SORCE mission.
But on other aspects of solar activity there is much more long term data than I expected: 400 years ago quite a few people were carefully recording detailed drawings of sun spots (like Cassini in 1671, right). Others were reporting aurorae — up to 150 a year in parish records, newspaper reports, and scientific observations, which tells us something about the strength of the solar wind. There were also observations of the solar corona during eclipses at the time, which suggest the sun was less active as well.
Lately some (Zolotova et al) have said solar activity was not low during the cold Maunder Minimum period from 1645 – 1715. Usoskin and others have responded by amassing a compedium of historic data demonstrating that something very unusual was going on with the sun during that time. They not only look at sun spots, but aurorae, solar corona observations, Beryllium in ice cores, Carbon 14 in tree trunks, and titanium in meteorites.
We know aurorae were rarer or smaller during the Maunder minimum because for 80 years there were virtually no reports of auroras in Great Britain, though keen observers were looking for them, and recording “clear skies” day after day.
There were reports of aurorae from the late 1500′s in the UK, Denmark, and Prague, but then early in the 1600′s activity fell away. The silence was loud. At the end of the Maunder period, across northern Europe on “Tuesday 17th March 1716″ people all over Northern Europe reported aurorae, including Edmund Halley in Great Britain, who had never seen one before, yet had read about them, and looked for them. He’d begun to despair he might never see one. One Petter Dass in Norway did miss out, he diligently recorded the night sky from 1645 to 1707 when he died, and though he had read many historic reports of aurorae, he never recorded seeing one himself. His bad luck, to be an astronomer and his whole adult life spent during the quietest period for centuries.
The Usoskin paper is an interesting read for people interested in the history of early science as well as for the history of solar activity.
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