• Background
  • Instructions
  • Illustration
  • Quiz


The moon illusion is another powerful illusion related to sizedepth relations. What is interesting about this illusion is that we can see it in the night sky several times each month. Find out when the next full moon is, or a day before or after the full moon. Then find out when sunset will occur, and watch the moon rise at sunset and then look at it again a few hours later. When we see the full moon on the horizon (such as when it rises at sunset and sets at sunrise), we perceive it as being larger than when it is higher up in the sky. This perceptual fact is seen in countless romantic movies, in which a love-struck couple stares at an abnormally large full moon.

The explanation for the moon illusion is similar to other size-depth illusions: a misperception of distance causes a change in the perception of size. We see the sky as a giant dome overhead, and objects in the sky as all at the same distance, that is, “painted” on the dome (though intellectually we know this to be untrue). As such, on this dome, the horizon is farther away from you than is the zenith, directly overhead. Because we perceive the horizon as being farther away than the zenith overhead, an object that takes up the same amount of space on our retina must be larger. At zenith, the moon looks smaller because the same-size object is now thought to be closer.

In this activity, you can interact with the perceived flatness of the sky to create a verion of the illusion. The perceived relative flatness of the sky will be created with a texture gradient in the top, or sky, part of the image.


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab


On the Illustration tab, you can adjust these parameters:

You can select from the following pictorial depth cues to illustrate (select them by clicking or pressing the cue to activate the checkbox):

Remove Top Grid: A texture gradient where the sky is is used to flatten the perceived sky. If you remove it, you can see how this flatness contributes to the illusion.
Height of Moon: for comparison purposes, two 'moons' are drown. This slider, moves the right hand 'moon' up and down in the sky.


Pressing this button restores the settings to their default values and allows you to adjust speed and relative size. It also resets the counter before you can indicate if you have the objects arriving at the same time.