Crater Illusion

The crater illusion, also called the dome illusion, is an optical effect that occurs in photographs of astronomical bodies and of the Earth taken from orbit, in which impact craters and other ground-level depressions appear to viewers as though they were raised domes or mountains rather than depressions sunk into the surface. The illusion stems from the brain's learned assumption that light normally comes from overhead; orbital images are typically taken under conditions of nearly horizontal sunlight, since that is the lighting condition under which shadows within a crater actually become visible at all, and under such lighting the brain misinterprets the shadow pattern inside the crater as indicating raised terrain rather than a depression below the surrounding surface. The effect has been observed on multiple celestial bodies, including the Moon, the planet Mercury and the dwarf planet Ceres, with craters such as Occator and Goclenius used as examples of how simply rotating an image can eliminate the illusion entirely; a reverse version of the illusion can also occur with genuinely elevated features, such as Algeria's Tin Bider crater structure, which can instead appear sunken due to the particular positioning of its shadows.