VENUS in a new light

Has the Parker Solar Probe solved the mystery of the Ashen Light? Paul G. Abel assesses the evidence

- Paul G. Abel

On the evening of 9 January 1643, the Italian astronomer Giovanni Riccioli turned his telescope on to the planet Venus and, in doing so, ignited a mystery that has persisted until the present day. Riccioli was able to observe a dull glow coming from the night side of Venus - a phenomenon called 'the Ashen Light'.

The Ashen Light is only visible when Venus is in the crescent stage and observed in a dark sky. When present, some (or sometimes all) of the night side of Venus is seen to be glowing with a greyish light. Its appearance is unpredictable, and it seems to be sighted more when Venus is at eastern elongations and visible as an evening star. Over the years, the Ashen Light has been recorded by a number reliable observers - Patrick Moore observed the phenomenon many times and described the effect as looking similar to earthshine on the Moon.