Final answer:
The moon illusion is the perception that the moon appears larger when it's on the horizon compared to when it's overhead. It is an optical illusion involving human perception, not a physical change in the moon's size.
Step-by-step explanation:
The moon illusion refers to the optical illusion that the moon appears larger when it is on the horizon than when it is directly overhead. This phenomenon occurs despite the fact that the moon's size doesn't physically change as its position in the sky shifts. The reasons for the moon illusion are not fully understood but are thought to involve human perception and the way the brain processes visual cues in the presence of surrounding objects on the horizon, such as trees or buildings, compared to the vast open sky when the moon is overhead.