This one is a real puzzler, that perplexed the ancient Greek and Chinese scientists. The moon appears to be larger in size when it is near the horizon than when it is higher in the sky. The moon does not increase its size, nor does it get closer nor is there any atmospheric effect generating this, such a refraction. One can take a photo of the full moon on one’s mobile phone when the moon is full, and see that the object which seems so large and close that one could touch it, is really the same size as before. Thus, the effect is an illusion, but what causes it?
This puzzle has resisted explanation from scientists for over 2,500 years, and according to one frequently cited review, is still unexplained: F. Egan, “The Moon Illusion,” Philosophy of Science, vol. 65, 1998, pp. 604-623; S. Nanavati, “A History and Experimental Analysis of the Moon Illusion,” The New School Psychology Bulletin, vol. 6, 2009, pp. 15-25. The later paper gives a good historical overview of proposed solutions, but my article will focus upon modern attempts since it is generally agreed that the ancients failed to solve the problem.