'Examples of Rapid Formations', 31 July 1865.
© Science Museum / Science & Society Picture Library
Illustration taken from 'Le Soleil' (Paris, 1875) by Secchi, showing a sunspot that had first appeared on 30th July 1865, and by the following day (when this drawing was made) had completely changed form. 'In 24 hours, the dimensions were considerably modified; the length had almost doubled: it was 147 seconds. During the following days the mass which separated the four openings transformed almost into a penumbra on which were dispersed luminous grains.' Secchi was a pioneer of solar astronomy in the 19th century, but although his theories were influential at the time, some of them, including his views on sunspots, are considered obsolete today. Sunspots are relatively cool areas on the Sun's surface, the photosphere.