'Spoerer's Curves of Sun-spot Latitude', 1910.
© Science Museum / Science & Society Picture Library
Plate taken from 'The Sun' (London, 1910) by C A Young, showing a graph charting the latitude of sunspots observed between 1855 and 1880. German astronomer Gustav Spoerer (1822-1895) discovered that the distribution of sunspots progresively moves towards the equator from higher latitudes over the course of the 11 year cycle of solar activity. In Britain, Richard Carrington made the same discovery independently at about the same time. Sunspots are relatively cool areas on the Sun's surface, the photosphere. They form when magnetic field lines below the surface become twisted and protrude through the photosphere. The point in the solar cycle at which their number is greatest is known as the 'solar maximum'.