Ken Tapping, May 19 2010

In the 17th Century, Galileo, Scheiner and others reported that the Sun has spots on it. Making observations from day to day they saw them moving slowly across the solar disc and concluded that this is because the Sun is a rotating sphere, turning once on its axis every 27 days or so. More scientists got interested, and sunspot counting became a regular activity. This led to the discovery that the numbers rise and fall with a 10-13 year rhythm, which is now called the solar activity cycle.

They also noticed that the beginning of the cycle is marked by the appearance of sunspots at high solar latitudes. As time passes, they increase in number and move towards the equator. The maximum number occurs 5-6 years after the beginning of the cycle. Then the number starts to fall, but the spots continue to appear closer and closer to the equator. Finally they disappear, and new ones appear at the high latitudes, starting the whole process over again.

When we dig back into the historical counts of sunspots, we find regular reports back to about 1700 AD. In the decades before then the reports are very infrequent. In 1976, astronomer John Eddy concluded that during that few decades before the beginning of the 18th Century, solar activity almost ceased, with just a very occasional spot turning up. He named this period the "Maunder Minimum", after the 19th Century astronomer who collected and analyzed sunspot observations he used.

What is just as intriguing is that around the time of the Maunder Minimum the Earth's climate cooled, to the point where the River Thames in London froze over. This does not normally happen. At the beginning of the 19^th Century the Sun went through another period of low activity, although not as low as happened in the Maunder Minimum. At that time there was another cooling of the climate. We now know that when the Sun is more active, producing more sunspots and other forms of activity, it also gets brighter, and when that activity falls, it gets dimmer. This is the Sun's contribution to changing climate, and one reason we keep a close eye on it.

The last solar maximum was in 2000-2002. Then the activity declined to the next minimum, and then stayed there. Some suggested another Maunder or Dalton Minimum was starting. However, early this year the next activity cycle started. On the other hand it's not clear we're out of the woods yet, because this new cycle is different. The relationships between activities in different parts of the Sun have changed, and the sunspots have weaker magnetic fields than they used to.

Many other stars show activity cycles, just like the Sun's; others do not. Maybe these stars are taking some time off, the way the Sun does. Calculations by a friend at the University of Montréal show activity cycles can stop and restart, and another at the NRC Institute for Information Technology has found that subtle changes in the rhythm of solar activity are fairly common. The cycles are not simple oscillations, like the movement of a swing. Today, with our technologies and environment under increased pressure, nderstanding and monitoring solar behaviour is critically important.