Why Is the Moon Bigger on the Horizon?

weirdly accurate.

Why Is the Moon Bigger on the Horizon?

    What is the moon illusion test?

    This moon illusion test lets you measure the famous effect on yourself instead of reading about it. You watch the full moon rise over a landscape, look straight up, and set the moon overhead to the size you remember. Only after the third moon does the page tell you how far off you were, in percent, and why the horizon moon looks the way it does. Researchers have measured the illusion this way since Kaufman and Rock’s 1962 experiments with moons projected onto the real sky.

    This test measures your own moon illusion by size matching from memory, the approach Kaufman and Rock used. The full moon rises over the sea, a plain and a city skyline; you watch it for three seconds, the view tilts straight up, and you resize the moon overhead until it matches what you remember. Each round’s result is your moon’s width divided by the real one, and your score is the average of the three, shown in percent.

    Nobody has published an average for the moon illusion on a screen, so the result is not ranked; the result sheet sets it next to what studies measured under a real sky. Matching from memory is noisier than comparing side by side, so your number will move a few points between runs. It is an illusion on a screen, not a vision test.

    What the test measures

    One number: how far the horizon changes the size of the moon you see, in percent.

    The task

    • Three moonrises: by the sea, on a plain and over a city
    • Watch the moon on the horizon for three seconds, then make the moon overhead the same size: drag out from it, pinch, scroll or use the adjust bar
    • Size matching from memory, as in Kaufman and Rock’s projected-moon experiments
    • The overhead moon starts small, then big, then small, so where it starts cannot push every answer the same way

    Your result

    • Your moon illusion in percent: the average of three size ratios, taken in logs because they are ratios
    • Each moon’s result, which landscape moved it most, and how consistent your three settings were
    • Reference points from studies under a real sky and outdoors, set next to your number once you have finished
    • Why it happens, and three ways to check it yourself the next time the full moon rises
    • A share card and a challenge link; the link carries your result in the part after the #

    Before you start

    • About two minutes
    • Works on a phone or a computer; sound is optional
    • Free, no account; your browser needs 3D (WebGL) switched on

    Moon illusion FAQ

    Is this test free? Do I need to sign up to see my result?

    Yes, it’s free, and there’s nothing to sign up for: no account, no email, no payment. Your result appears on this page the moment you finish, and you can share it or play again straight away.

    Why does the moon look so big when it rises?

    The result sheet explains it after your third moon, with the studies behind the explanation. Knowing the reason first would change how you set the moon overhead, so the page waits until you have committed to all three. The explanation draws on Kaufman and Rock’s 1962 projected-moon experiments and on Kaufman and Kaufman’s 2000 study in PNAS, in which five observers placed the two moons in depth.

    Is the moon actually bigger on the horizon?

    The result sheet answers that once you have set all three moons, using NASA’s figures for the Moon’s distance and diameter and for Earth’s radius. It also gives three ways to check for yourself the next time the full moon rises, so you don’t have to take the page’s word for it. Until then, the test only asks what the two moons look like to you.

    How big is the moon illusion?

    The result sheet sets your number next to the published ones after the third moon: Kaufman and Rock’s 1962 experiments with projected moons, Ross and Plug’s 2002 book on the illusion, and Kaufman and Kaufman’s 2000 study. Those figures come from a real sky, so they are reference points rather than a ranking, and the page keeps them back until you have finished so they can’t steer your settings.

    How does the moon illusion test measure my result?

    For each of three moons, the moon illusion test divides the width you set for the moon overhead by the real moon’s width; 1.10 means the moon you set was 10% wider than the real one, and 0.90 means 10% narrower. The three ratios are averaged in logs, and anything within ±5% counts as a match, about as finely as people can compare two sizes at all.

    Why is the moon illusion test not ranked?

    Nobody has published an average for the moon illusion on a screen, so there is no population to rank you against. The studies on the result sheet were run under a real sky, with real landscapes, which is a different situation from a phone or a monitor. Matching from memory also adds a few points of noise each run, so your number is yours, on this screen, today.

    What do I need for the moon illusion test?

    The moon illusion test needs about two minutes and a phone or computer whose browser has 3D (WebGL) switched on. Sound is optional, and there is no account and nothing to install. Each moon stays on the horizon for three seconds before the view tilts up, so watch it closely while it is there: you set the moon overhead from memory.

    Where the numbers come from

    There is no population data for the moon illusion on a screen, so results are never ranked; the studies are reference points only. The projected-moon range and the outdoor figure are taken from second-hand summaries and still need checking against the originals. The size and distance figures on the result sheet are calculated from NASA’s distance and diameter. The site does not publish its own players’ data.