Things everybody “knows” about size illusions
The moon is closer, the air is a magnifying glass, and knowing the trick switches it off. Gently corrected.
MythThe horizon moon is bigger because it’s closer.
FactThe moon’s image on a photograph is the same size whether it is low or high: the change happens in your brain. Exploratorium: Looking at the moon Test it: at arm’s length, about two-thirds of the width of your little finger covers the moon, wherever it is in the sky. Michael Bach: Moon illusion
MythThe air magnifies the moon near the horizon.
FactIf anything, the low moon is a tiny bit squashed from top to bottom, because its light passes through more air near the horizon. Michael Bach: Moon illusion Aristotle wrote that the sun and stars seem bigger when rising and setting; he blamed the air. Michael Bach: Moon illusion
MythGiant moon photos show the moon illusion.
FactPhotos of a giant moon behind a building are taken with a telephoto lens; that is a camera effect, not the moon illusion. Michael Bach: Moon illusion
MythEveryone agrees why the Müller-Lyer illusion works.
FactThe perspective explanation has had critics (Segall and colleagues in the 1960s), and a 2025 paper by Amir and Firestone pushed back on the idea that the illusion is a product of culture. Michael Bach: Müller-Lyer illusion Britannica explains the Müller-Lyer illusion with the Gestalt ideas of convergence and divergence: the fins at the ends seem to lead the eye inwards or outwards, giving a false impression of length. Britannica: Müller-Lyer illusion
MythIf you know it’s a trick, it stops working.
FactAsked to find the middle of the line in the Brentano version, big errors are completely normal: Michael Bach, whose illusion collection this comes from, says he is out by about 10%. Michael Bach: Müller-Lyer illusion
MythIllusions fool everyone the same amount.
FactIt is one of the few illusions with a big age effect: for children under 10 it is weaker. Michael Bach: Ebbinghaus illusion Michael Bach finds the Delboeuf illusion fleeting: some days it hardly seems to be there at all. Michael Bach: Delboeuf illusion
MythPonzo’s famous paper shows the Ponzo picture.
FactMario Ponzo’s 1911 paper turns out not to contain the famous Ponzo picture at all; it is mainly about an illusion of touch. Michael Bach: Size constancy
MythJastrow was first with his illusion.
FactMüller-Lyer actually published the same illusion three years before Jastrow. Michael Bach: Jastrow illusion
MythPeople in an Ames room really change size.
FactAn Ames room looks like an ordinary square room from one peephole, but one side’s ceiling is about twice as high as the other. Exploratorium: Distorted Room Because the brain decides the room is square, people walking across it seem to grow and shrink. Exploratorium: Distorted Room
MythTwo eyes or one, it makes no difference.
FactWith one eye closed, your depth perception is much weaker, so you can’t tell how far away each coin is. Exploratorium: Size and Distance (Science Snack) With both eyes open you get more points of view and judge size and distance much better. Exploratorium: Size and Distance (Science Snack)