The 'ashen light', or AL, is a faint, mysterious glow, or colouration, seen in the night-side hemisphere of the planet Venus. It's often compared to the reflected 'Earthshine' that sometimes illuminates the dark-side of the Moon. First reported in 1643 by Italian astronomer Giovanni Riccioli, AL has been observed many times since, but its faint, transitory and elusive nature has prevented serious study. More problematically, AL has only ever been detected by the human eye and no scientific instrument, either Earth-based or space-based, has ever recorded the phenomenon.
Some authorities have declared the phenomenon illusory, perhaps a contrast effect for the eye, or even 'expectation bias'. Others suggest that instrumental defects may explain the phenomenon; light scattering, optical aberrations, the brightness of the background sky, weather and so on.
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