For millennia, a perplexing sight has materialized in the evening sky: Sometimes, a gigantic moon appears to hover near the horizon, but as that overgrown orb climbs overhead, it shrinks to a fraction of its moonrise size.

This striking phenomenon is known as the moon illusion, and the fact that it happens has been well documented since at least the fourth century B.C. Obviously the moon isn’t actually changing size—but the precise reason it seems to be expanding and contracting continues to puzzle scientists today. (See 11 striking pictures of November’s record-setting supermoon.)

Here are the various explanations for the moon illusion and its enduring mystery.

Back in the fourth century B.C., Greek philosopher Aristotle suggested that Earth’s atmosphere might