The phenomenon of the moon appearing larger when it’s near the horizon, often referred to as the “moon illusion,” is an optical illusion rather than an actual change in the size of the moon. There are a few factors that contribute to this perception:

1. Visual Cues: When the moon is near the horizon, it is often seen in proximity to familiar objects such as trees, buildings, or mountains. These objects provide visual reference points that our brains use to gauge the size of the moon. In contrast, when the moon is higher in the sky, it appears to be isolated against the vast expanse of the sky, making it seem smaller by comparison.
2. Atmospheric Refraction: When the moon is near the horizon, it is viewed through a thicker layer of Earth’s atmosphere compared to when it’s directly overhead. This atmospheric refraction can cause the moon to appear slightly larger and slightly distorted as its light is bent and scattered by the atmosphere.
3. Psychological Factors: Our perception of the moon’s size can also be influenced by psychological factors, such as the expectation that objects near the horizon should appear larger due to the Ponzo illusion or the Ebbinghaus illusion.

It’s important to note that while the moon illusion makes the moon appear larger near the horizon, it is purely a perceptual phenomenon, and the actual size of the moon remains the same regardless of its position in the sky. This illusion applies not only to the moon but also to other celestial objects and even terrestrial objects such as the sun and buildings.