Why does the full moon look larger on the horizon than overhead?
Flash Quiz answer:
The optical illusion arises from the lack of reference points in the sky. Basically, when near the horizon, there are terrestrial objects (trees, hills/mountains, houses, etc.) whose size we recognize as large. The moon, looking larger than these objects, appears even bigger by comparison. When high in the sky, however, the only thing to compare the moon's apparent size to is the sky itself, which is much larger than the moon. The result is that the moon appears smaller.