The distribution of sunspots, 1904.
© Science Museum / Science & Society Picture Library
'Heliographic Latitude Chart of the Greater Solar Spots, 1881-1903'. Plate detailing the position of sunspots taken from 'Monthly Notices of the Royal Astronomical Society' (page 762, Vol LXIV, 1904). Sunspots are relatively cool areas on the Sun's surface, the photosphere. The number of sunspots is greatest at the point in the cycle of solar activity known as the 'solar maximum', which occurs roughly every 11 years. They form when magnetic field lines below the surface become twisted and protrude through the photosphere. The British astronomer Edward Maunder discovered that during the course of the solar cycle the occurence of sunspots shifts from higher latitudes towards the equator.