The ‘Maunder Minimum' is the name given to the period from 1645 to 1715 when the number of sunspots – ‘storms' on the sun – became almost zero.
The period is named after the solar astronomer Edward Walter Maunder (1851-1928), who was working at The Royal Observatory at Greenwich when he discovered the dearth of sunspots during this period.
During one 30-year period within the Maunder Minimum there were only about 50 sunspots compared with a more typical 40,000.
Maunder was a driving force in the foundation of the British Astronomical Association and a Fellow of the Royal Astronomical Society.
The sun was well observed during the period of the Maunder Minimum and this lack of sunspots is well documented.
This period of solar inactivity corresponded to a climatic period called the ‘Little Ice Age' when in Europe rivers that were normally ice-free, froze and snow fields remained at low altitudes throughout the year.
There is evidence the sun had similar periods of inactivity during the years 1100-1250 and 1460-1550.
Sunspots generally follow a cycle of about 11 years, but cycles have varied from eight-15 years.
The connection between solar activity and the earth's climate is an area of ongoing and sometimes controversial research.
Time will tell whether the sun will once again go into another ‘Maunder Minimum' within the lifetime of the present generation, but if this happens we're likely have a much colder climate for a few decades.
The sunspot number for May 1 2018 is 0, with the monthly values in 2018 being January 7, February 11, March 3, and April 9.
A sunspot is a relatively dark, sharply defined region on the solar disc – marked by an umbra, dark area, which is 2000 degrees cooler than the effective photospheric temperature.
The average diameter of a sunspot is 4000 km, but they can exceed 200,000 km.
The NASA solar physics website, and other websites such as the Royal Observatory of Belgium, include information on sunspot numbers, the ‘Maunder Minimum' and sunspot cycle predictions.
The sunspot index is updated monthly and available from 1749. The last time the monthly sunspot number was above 100 during the last 14 years was in September 2002, when the value was 110. February 2014 had a sunspot value of 102.
The last time the value was above 200 was in August 1990, when the value was 200.3 – and there were no sunspots observed in September 2009.
For further information see the following textbooks books (all available at specialised libraries)
"The Value of the Weather" (1970), "The Uncertainty Business - Risks and Opportunities in Weather and Climate" (1986), "The Human Impact of Climate Uncertainty - Weather Information, Economic Planning, and Business Management " (1989), all by W.J.Maunder