Sunspot variation

Have you ever seen those old prints of happy Londoners holding “frost fairs” on the frozen Thames? The fairs were held when the Thames flowing through London froze hard enough for people to move onto the ice for weeks at a time. The earliest seems to have been in 1434. In the 1660s, ice skating was in fashion on the river. In 1683-84, the ice on the river was 28cm thick (known in those days as 11 inches). And in 1715-16, the ice was so thick that a storm surge up the river lifted it by 5m but it did not break. The first fair with properly organized socializing was in 1564-65, and there was an epic as late as 1814 with eating and donkey rides on the ice, and whole streets of shops appearing. For more information see:

H2G2 Frost Fairs Guide bbc.co.uk/dna/h2g2/A970733

Well, I have lived in London since 1973 and have yet to spot pack ice. There seem to be two reasons why the fairs became possible and why they died out. The first is old London Bridge, removed and replaced in the 1830s. It was a massive structure with shops and houses, and ponded water behind it, slowing the flow of the river. The new bridge is much sleeker and lets the water flow more freely so it is less likely to freeze. However, the main reason the river no longer freezes is that things were a lot cooler back then. Indeed, it was not just the Thames that froze. There are accounts of people walking from Staten Island to Manhattan in 1780.

Part of the reason seems to be that the Sun was a lot less lively in that period. From about 1400 to 1800, it seems to have had fewer spots on its surface than we see today.

This view is problematic, since systematic telescope observation of the Sun only dates from the seventeenth century. For information before that period we are dependent upon patchy, naked-eye observations. Between 1645 and 1715, a period known as the Maunder Minimum, sunspot activity seems to have been at its lowest, but at this time the practice of sunspot-spotting was in its infancy.

However, sunspot dearths later in the period, such as the Dalton Minimum, from 1790 to 1820, are more solidly established. And it is certainly true that temperatures were significantly cooler at this time, earning it the name the 'Little Ice Age”.

The Little Ice Age was preceded by the “Medieval Warm Period' which lasted from around 1000 to 1300. The end of the Medieval Warm Period seems to have been a factor in the disappearance of the Viking colony in Greenland, which vanished from history at this time. It was also responsible for relegating England from the first division of wine-making nations to the lower leagues, closing down most of the vineyards of southern England – although the current period of global warming may|see more vines returning to England's slopes.