Autographs of the Sun, 1861-1863.
© National Media Museum / Science & Society Picture Library
Parts a, b and c from a booklet containing '10 Autographs of the Sun, taken with the Heliantograph of the Reverend William Selwyn at Ely by John Titterton, photographer'. The centre and right images, taken on consecutive days in July 1862, show a developing sunspot on the solar equator. 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. Sunspots are closely asociated with solar flares, sudden outbursts of energy which extend far into the Sun's atmosphere.