Divination by staring into a reflective surface — water, a black mirror, a crystal — is traditionally done by moonlight rather than candlelight or day.

The practical reason is that moonlight is dim enough for the eye to lose its grip: stare at a low-contrast surface long enough and the visual system starts filling in shapes. That is a real perceptual effect, the same one behind the Ganzfeld experiments. Whether the shapes mean anything is the part the tradition and the psychology disagree about.