Why Does the Moon Look Larger Near the Horizon?

When a full Moon has just risen above the horizon, it can resemble an enormous disc. Hours later, high in the sky, it seems smaller. The atmosphere is not magnifying it. Photographs taken with the same focal length and camera settings show that the disc remains almost the same width, so its angular diameter has not increased accordingly. The atmosphere can make a low Moon look yellow or red, and refraction may squash it slightly, but neither effect explains the striking difference in apparent size.

What changes is perception. Near the horizon, trees, buildings and distant hills provide cues about distance and scale. High in the sky, the Moon is surrounded by a broad, almost featureless field. The visual system does not read retinal angle as size directly; it interprets that angle together with the surroundings, so images of the same measured size can feel different.

Several accounts remain in contention. Judging the horizon as more distant and comparing the Moon with nearby small objects may each explain part of the effect, but no single model has won general agreement. The safest conclusion is that the Moon illusion is real and measurable, yet chiefly perceptual: the Moon does not grow within a few hours, and the air is not acting as a magnifying glass. Two photographs taken at a fixed focal length offer a simple way to separate physical measurement from perceived size.

https://science.nasa.gov/solar-system/moon/the-moon-illusion-why-does-the-moon-look-so-big-sometimes/
https://pmc.ncbi.nlm.nih.gov/articles/PMC26692/