Why does the moon look bigger when it's near the horizon?

The moon doesn't actually change size; the effect is a perceptual illusion known as the moon illusion.

Why does this illusion occur?

The brain interprets the size of objects based on distance and context cues. When the moon is near the horizon, terrestrial objects (trees, buildings) appear that provide a reference for distance, making the brain perceive the moon as larger.

Contributing factors

• Size context: nearby objects make the moon appear larger by contrast.
• Distance perception: the sky appears flatter and closer at the zenith, while the horizon is perceived as farther away.
• Size constancy: the brain adjusts the perceived size by factoring in the estimated distance.

Experiments to verify it

If you take a photograph of the moon at the horizon and another at the zenith with the same aperture and zoom, comparing the sizes in the image will show they are identical. Another way is to look at the moon between your legs or through a tube; the illusion diminishes because the terrestrial context is removed.

In short, the moon appears larger at the horizon due to the way our brain processes visual information about size and distance, not because of any actual change in its apparent size.