Over two thousand years ago Aristotle would have been surprised to learn that the Sun is not a perfect heavenly sphere, and that its brightness varies with time. Yet had Aristotle heard of the work of Meton, another ancient Greek scientist, he would have known that blemishes on the Sun had been seen at sunrise over a 20-year period.
Today we know that sunspots are closely related to magnetic activity near the surface of the Sun, and that sunspot activity has increased roughly in line with solar magnetic activity. Now Mike Lockwood, Richard Stamper and Matthew Wild at the Rutherford Appleton Laboratory in the UK have found that the magnetic field of the corona - the Sun's outer atmosphere - has more than doubled over the past century (Nature 1999 399 437). Lockwood and colleagues drew their conclusions from recent satellite observations of interplanetary space and measurements of the geomagnetic field dating back to 1868.
In the September issue of Physics World magazine, Neil Arnold from Leicester University in the UK describes the implications for climate change.