This conundrum has puzzled scientists and philosophers for thousands of years. Ptolemy's idea has long since been disproved. In fact, the refractive nature of the earth's atmosphere makes the moon appear slightly smaller at the horizon. Alhazan's theory doesn't quite hold water, either. For starters, the moon illusion still affects viewers at sea, where there are no objects to influence our perception.
Airplane pilots flying above the clouds also experience the moon illusion.
For most people, the illusion goes away, even though the horizon cues are still in place. But Alhazan's suggestion does get at the essence of the puzzle. The moon illusion is a perception problem that occurs because the human brain isn't quite able to resolve all of the visual clues it receives about size and distance into an accurate picture. Such visual illusions are quite common. Two identical, solid black rectangles are drawn between the tracks, one closer to the bottom of the page and one closer to the top of the page.