The Ashen Light

The Ashen Light first reported in 1643 is one of the oldest unsolved mysteries of the solar system. It consists of a faint luminosity on the nightside of the planet, similar in appearance to ``earthshine'' on the Moon but generally not as bright. Ashen Light is most often reported when Venus is in the evening sky at which time the evening terminator of Venus is toward the Earth. While some of this local time asymmetry may be associated with a tendency of observers to view Venus more in the evening hours than in the morning hours, we believe the asymmetry is too great to be explained by terrestrial effects alone. Thus, it appears that Ashen Light is principally a dusk side phenomenon on Venus. To our knowledge there is only one phenomenon that shows a marked dawn-dusk asymmetry on Venus and that could also explain the occurrence of Ashen Light. That phenomenon is Venus lightning whose local time of occurrence has been inferred from impulsive VLF waves observed by the Pioneer Venus Orbiter in the night ionosphere. The following factors appear to affect the visibility of Ashen Light from Earth: the distance of Venus from the Earth: the length of time Venus is above the horizon and the local time distribution of the Ashen Light source on Venus. Presently an Ashen Light campaign undertaken by amateur observers worldwide is underway. This campaign should help resolve whether the reported local time asymmetry is indeed intrinsic to the planet.