Giovanni Riccioli discovers a faint glow on the "night" side of Venus exactly 366 years ago on January 9, 1643. Other people over the ensuing centuries also observed the Ashen Light, making it one of the longest-running mysteries that still defies a conclusive explanation today.
The Ashen Light on Venus have mostly been seen unexpectedly and cannot be seen for those who are actively searching for it. Those who've seen the Ashen Light of Venus report it looks a lot like the reflected "Earthshine" that sometimes casts a dull glow on the moon. It's most easily sighted when the dusk edge of the sunlight on Venus faces Earth.
One person who have seen the Ashen Light said "I have seen the Venusian phenomena and believe in it, though I have not seen it since that one time back in May of 1980 that lasted over a month." The phenomenon was also spotted after 1980 that spanned to 2001 but much more difficult to see and more obscure.
One possible explaination for the luminescent glow is multiple lightning flashes. Lightning was observed like AM radio static during terrestrial thunderstorms. On the other hand, "observations of Venus' ionosphere reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz that have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds."
It's also possible the Ashen Light of Venus is caused by solar particles energizing the atmosphere like the terrestrial Aurorae Borealis and Australis hence its evanescence.
Or it's some previously unknown combination of things we understand.
Or something we don't understand at all.
Source: Eastbay Astronomical Society
Sorry, it probably doesn't relate to Venus, but wasn't she born in 1980?
Hopefully that phenonmenon is back this year