The Maunder Minimum

John Flamsteed
Johannes Hevelius
As was mentioned before the number of sunspots observed on the solar surface varies fairly regularly, with an average period of 11-years. However, if we look at the variation of the sunspot number with time, we find that for a period of about 70 years, from A.D. 1645 to 1715, practically no sunspots have been observed. In other words, during this time the solar cycle has been interrupted. This period of time is called the Maunder Minimum (Coincidentally, these dates nearly match those of the birth and death of the English physicist and mathematician Sir Isaac Newton (1642-1727)).
At first people claimed that the main reason for the lack of sunspots during the Maunder minimum was due to the fact that astronomers were not observing the Sun during this time or at least not very systematically. However, this is not the case. Several people, namely:

  1. The polish astronomer Johannes Hevelius (1611-1687).He also compiled one of the earliest detailed maps of the moon.
  2. Jean Picard (1620-1682), a French astronomer. He was the first to measure the size of the earth accurately.
  3. The English astronomer John Flamsteed (1646-1719). He was the first astronomer royal in England and founded the Greenwich Observatory in London.
counted the number of sunspots systematically during that time. They only recorded about 50 sunspots, whereas in any typical 30-year interval during the past hundred years there have been between 40,000 and 50,000 sunspots.

A Frost Fair on the Thames at Temple Stairs (1684)
by Abraham Hondius (1625-1695)

The Maunder minimum also coincided in time with an period of very cold weather in Western Europe (and maybe all over the world). This era is often called the "Little Ice Age". During this time it got so cold that rivers and lakes that were normally ice-free froze over and the snow did not melt all year round even at low latitudes. In the middle of the 17th century temperatures dropped so low that the Baltic sea and the Thames River froze over regularly. The ice on the Thames in London was so thick that people organized winter festivals with skating parties and carnivals on the river. One of these festivals on the Thames was painted by the Dutch painter Abraham Hondius in December 1676. Paintings by other Dutch artists, like Jacob van Ruisdael (1603-1677) and Aert van der Neer (1628-1682), also show how cold the climate must have been, since in these pictures the canals and rivers are always frozen and the landscapes are covered in deep snow.

Winter landscape in a snowstorm (1660) by
Aert van der Neer (1603-1677)
Winter landscape (1680) by
Jacob van Ruisdael (1628-1682)
View of River in Winter (1660) by
Aert can der Neer (1603-1677)

Because of this close correspondence between the Maunder Minimum and the "Little Ice Age" scientists are speculating that there is a connection between the number of sunspots visible on the solar surface and the climate. According to these scientists, we get a cold climate if there are very few sunspots and a warm climate if there are lots of sunspots. Since sunspots are regions of strong magnetic field, we can also say that the climate becomes warmer if the sun produces lots of magnetic fields and that it becomes colder if the Sun produces only small amounts of magnetic field.

Whether this connection between the solar magnetic activity and the climate holds true is actually still not quite clear and it is an area of on-going research. However, scientists were able to discover more of these minima during which the Sun only produced small amounts of magnetic fields. Apart from the Maunder Minimum the most recent are:
  1. The Oort Minimum (1010-1050),
  2. The Wolf Minimum (1280-1340,)
  3. The Spoerer Minimum (1420-1530).
There also seems to exist some evidence that this periods of time were actually very cold as well, which would lend some support to the above theory.