The Moon Illusion refers, roughly speaking, to the common experience that the moon appears larger (and closer to us) when it is near the horizon than when it is at its zenith. Wishing to investigate t

The Moon Illusion refers, roughly speaking, to the common experience that the moon appears larger (and closer to us) when it is near the horizon than when it is at its zenith. Wishing to investigate the causes of the illusion (which are still debated), we have designed an experiment in which participants must adjust the diameter of a variable circular light placed at eye level to match the diameter of a fixed circular light placed well above eye level. (See, for example, Kaufman, L. & Rock, I. (1989), "The moon illusion thirty years later," chapter 8 in M. Hershenson (Ed.) The Moon Illusion, Hillsdale, NJ: L. Earlbaum.) We have tested 25 participants and recorded for each participant her experimental "adjustable/ fixed ratio." Our data are summarized in the following histogram: