What is the Moon Illusion? It is often hard for an observer on earth to judge the true size of the Moon. The Moon may look bigger or smaller depending on where in the sky we see it in the sky. Most observers will say the Moon looks bigger when it is close to the horizon, and smaller or normal when it is higher in the sky.

Why? A universally excepted explanation of the phenomenon is still being hotly debated but most agree that the Moon Illusion exists. The distance between the Earth and the Moon changes very little—not enough to cause the effects we see. The Moon does not change size. It’s physical size is constant at 3,476 km. There is not enough atmosphere around the Earth to cause a lens effect, and the illusion is probably not caused by magic. Nevertheless, something does cause the Moon to appear bigger, or smaller depending upon where it is in the sky. The phenomenon is often called the Moon Illusion.

Start the Moon IllusionHere is an Interactive Demonstration of the Moon Illusion. Please note that this is not the actual moon illusion. However it is an accurate representation of the actual effect. Our minimum and maximum moon sizes are based on the apparent size differences (± 1.5) as reported by most observers. Here is what the Moon might look like to an observer on the ground.

Read/See More About It To learn more about the Moon Illusion we suggest a visit to Donald E. Simanek’s web page, The Moon Illusion, An Unsolved Mystery. You’ll find an informative article complete with references and links. Here at Sandlot we’ve reprinted a short article from ScienceNews called, The Moon Moon Also Rises—And Assumes New Sizes; Feb 2000.