Moon illusion

Moon illusion

. We make judgements about size because of our perceptions of distance. The two black disks in this figure are the same size, but we see the bottom one as smaller because we think it is closer. The top disk seems larger because it appears to be farther away. The Moon on the horizon is similarly thought to be huge because comparisons with objects on the ground make us think it is far away - see Fig. 5.6. When people look straight up at the Moon, in an otherwise empty sky, they no longer have land clues to compute the Moon's distance and it is perceived as being closer and smaller.

Copyright 2010, Professor Kenneth R. Lang, Tufts University