Plate taken from The Illustrated London News, Vol 32, 1858. illustrating the Sun's corona and 'Baily's Beads' . The corona is the very hot outer part of the solar atmosphere which consists of highly ionised gas superheated to temperatures in exces of 1 million degrees Celsius. It can normally only be seen visually during a total solar eclipse, when it appears as a halo or cloud of pearly white light. The Moon does not have a perfectly smooth surface, so when it covers the Sun during an eclipse, the Sun's light can be seen pasing along valleys on the lunar surface. The short-lived effect, which resembles beads or diamonds on a ring, occurs just before or after totality, and is named after Francis Baily, the British astronomer who first recorded the phenomenon in 1836.
© Science Museum / Science & Society Picture Library