Which optical illusion of the Moon has no agreed explanation after two millennia?

The Moon illusion: the Moon looks considerably larger near the horizon than overhead, and it is entirely in the head. Photograph both and the discs are identical - marginally smaller at the horizon, in fact, since you are further from it. Hold a coin at arm's length and check.

Ptolemy wrote about it, Alhazen wrote about it, and the argument has not been settled. The leading account is apparent distance: the sky is perceived as a flattened dome rather than a hemisphere, because the horizon comes with terrain and buildings that give it depth cues, so a Moon judged to be further away at the same angular size is inferred to be bigger. Against this sits the fact that people report the horizon Moon as both nearer and larger, which is the wrong way round.

Rival explanations involve the angle of the eyes, the presence of objects to compare against, or the way the brain scales size across the visual field. Experiments have knocked pieces off all of them - the illusion survives when observers lie down, when they look through a tube, and for pilots with no terrain at all. Two thousand years of attention and it remains a live problem in perception.