Weather Eye with John Maunder

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, while working at The Royal Observatory at Greenwich, 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 was 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 rivers that are 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 to 1250 and 1460 to 1550.
Sunspots generally follow a cycle of about 11 years, but cycles have varied from 8 to 15 years. The connection between solar activity and the earth's climate is an area of on-going 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.
A sunpot 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 4000km, but can exceed 200,000km.

The NASA Solar Physics website includes 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 was September 2002 when the value was 109.6 and the last time the value was above 200 was in August 1990 when the value was 200.3. No sunspots were observed in September 2009.

The next ‘Weather Eye' on February 24 will show a graph of the rainfalls in Tauranga for March from 1898 to 2011.