The ash light of Venus , often referred to by its English literal translation ashen light (which implicitly refers most often to Venus, contrary to the French term ashen light that almost always refers to the Moon), is an evanescent luminous phenomenon, to the reality discussed, which would be in the form of a diffuse glow to discernible grief illuminating the dark part of Venus' disc when this planet appears from Earth as a crescent end. It seems to be easier to perceive near the vesperal terminator - the one visible from Earth at sunset, Venus having a retrograde rotation.
This light was observed for the first time by the Jesuit priest and Italian astronomer Giovanni Riccioli in 1643, then by the German-British composer and astronomer William Herschel in the XVIII century, and in the XX century, by Anglo-Saxon astronomers and extensionists such as Patrick Moore and William Hartmann; other astronomers, on the other hand, notably the American Edward Emerson Barnard, known for the acuity of his observations, never managed to see it. The observation of Venus in this phase is, in fact, not easy because, by definition, the planet is then angularly little removed from the Sun in the sky.
Several attempts to quantify this phenomenon and confirm or deny its real existence have yielded partial and contradictory results; it is also possible that, if it really exists, the ashy light of Venus only manifests itself intermittently. Research to detect it factually is based on two major assumptions.
The oldest of these hypotheses is based on the presence of many lightning that would illuminate the Venusian night, lightning detected especially by the Soviet probe Venera 9 > and sensed since the beginning of the exploration of Venus>; this hypothesis suffers from difficulties that have earned it from the outset some solid refutations>, but the existence of lightning in the atmosphere of Venus was confirmed in 2007 by the European spacecraft Venus Express>.