ashen light

The ashen light is a faint glow occasionally observed on the dark side of Venus. First reported by Giovanni Riccioli in 1643, it has been explained over the years in terms of various effects from fires lit by the inhabitants (see Gruithuisen, Franz von Paula) to electrical phenomena in the upper atmosphere. While some astronomers dismiss the ashen light as nothing more than an optical illusion, there remains the possibility that it is a genuine physical effect, perhaps similar to the airglow seen in the Earth's atmosphere.