Maunder's 'butterfly diagram' illustrating the distribution of sunspots, 1904.
© Science Museum / Science & Society Picture Library
'Distribution of spot centres in latitude, rotation by rotation, 1877-1902', a plate concerning the distribution of sunspots taken from 'Monthly Notices of the Royal Astronomical Society' (page 760, 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. This 'butterfly diagram' was first plotted by the British astronomer Edward Maunder in 1904. It shows how over the course of a solar cycle the occurence of sunspots shifts from higher latitudes towards the equator.