Illuminated or lit up by the light of the moon.
From 'moon' (Old English 'mona,' from Germanic roots) + 'lit' (past participle of light). This is a compound adjective combining a celestial body with the state of being illuminated.
Moonlit appears constantly in romantic and gothic literature because moonlight is special—it's strong enough to see by but creates shadows and mystery that sunlight erases, so writers have used 'moonlit' for centuries to signal that something magical or dangerous is about to happen.
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