The "sky objects" such as the moon, the sun, and constellations always appear larger when they are near the horizon than when they rise high in the sky. However, this is not the case. If you take photos of them or measure them with a telescope, you will find that their size does not change much whether they rise high in the sky or are near the horizon. Strictly speaking, due to the atmospheric refraction near the horizon, the moon or the sun will be slightly flatter and their total area will be slightly smaller than when they rise high in the sky. Therefore, the "moon near the horizon appears larger" should be an illusion.
Why do people have this illusion? One possible reason is that the human brain always estimates objects near the horizon as being farther away than those above the head. Because there are often many familiar objects near the horizon, such as mountains, houses, and trees, while there are no such reference objects for the moon at the zenith. Against the backdrop of these distant objects, people will feel that the moon near the horizon is farther away than the moon at the zenith, and accordingly, they will feel that it is larger. This phenomenon is called the "Ponzo illusion", which means that when estimating the size of an object, the human brain is influenced by the background information around the object. This was first discovered by Italian psychologist Mario Ponzo. If you roll a piece of paper into a tube and observe the moon through it, you will find that the moon near the horizon immediately appears normal.
However, some scientists are not satisfied with this explanation, because pilots on airplanes still have this illusion when observing the moon on a featureless horizon. There must be other reasons for this. Two researchers from the Department of Psychology at Long Island University in New York studied people's subjective perception of the distance to the moon and found that the "moon" near the horizon feels 4.2 times farther away than that in the sky. When estimating the size of an object, people not only compare the angle it occupies in the visual field, but also compare its distance. Under the condition of equal angle, if you already know that one object is farther away than the other, your brain will estimate its size as being larger. Therefore, the moon that feels "farther away" near the horizon is estimated to be larger.
Some scientists have found that the visual convergence point of people when observing objects at different distances in the same direction will slightly change. When people move their gaze from a nearby object to a distant one, they will feel that the angle of the object has slightly increased, which is called the "kinetic magnification effect". When people observe the lone moon in the sky, due to the lack of a sense of distance, they will focus their attention on the near-by area, resulting in the "kinetic microvision effect". These scientists believe that the kinetic effect is the real reason for the moon illusion.
In addition to the above explanations, there are also other explanations. For example, some people believe that this is because the space perceived by human vision is curved; some people believe that this is because human senses pay more attention to visual information near the head; some people believe that this is due to the difference in the sense of direction between looking up and looking straight ahead, etc. Although scientists have done a lot of research, there is currently no universally accepted answer to this question, dear readers. Can you offer your own explanation?