The phenomenon you are referring to is known as the "moon illusion" or "lunar illusion." It is an optical illusion that makes the Moon appear larger, closer, and sometimes to jiggle when it is near the horizon compared to when it is higher in the sky.
The exact cause of the moon illusion is still not fully understood, and there are several theories that attempt to explain it. One common explanation is the relative size hypothesis, which suggests that when the Moon is near the horizon, our brain compares it to objects on the ground, such as buildings or trees, and perceives it as larger than when it is high in the sky with no reference points. This comparison with familiar objects on the horizon creates an illusion of increased size.
Another theory, known as the apparent distance hypothesis, proposes that our brain judges the Moon's distance based on its perceived proximity to the horizon. When the Moon is on the horizon, it appears to be farther away because of obstacles such as trees and buildings. As a result, our brain compensates by perceiving it as larger.
The atmosphere can also play a role in the moon illusion. When the Moon is near the horizon, we are looking at it through a thicker portion of the Earth's atmosphere. This atmospheric refraction can cause the Moon to appear slightly distorted or to have a "jiggling" effect due to the varying density of the air.
It's important to note that the moon illusion is a perceptual phenomenon and not an actual physical change in the Moon's size or position. The Moon's size remains relatively constant as it orbits the Earth. The illusion is a fascinating aspect of human perception and can vary from person to person based on individual visual experiences and environmental factors.