Secret of the moon: Moon Illusion

Do you have this experience: The horizon moon is bigger than the zenith moon?

What happened? Before answering this question, we have to test if the size of the moon is really inconsistent. One of the easiest ways is to use a ruler and one eye to measure the size of the moon when it is near the horizon (i.e. eye-balling). After 1 to 2 hours when the moon rose, measure the moon again using same method. Oh! The moon has not changed the size! That our eyes had been deceived!

In fact, the above phenomenon is a famous psychological illusion --- Moon Illusion.

According to the scientific theory, it is because when the moon is on the horizon, it is closer to the ground, the surrounding environment such as buildings create a contrast for comparison. Our brain will determine the moon is rising from far-off place and thus we found the moon larger. On the other hand, the zenith moon is surrounded by large expanses of empty sky without specific objects for comparison. Our brain will determine the moon as nearer. By comparing the two, we feel the horizon moon is relatively larger and caused the illusion. This Apparent Distance Theory proposes that the horizon moon looks larger than the zenith moon because it looks farther away.

We have to do some calculation in another explanation. Our "impression size" depends on both the "size of retinal image" and the "perceived distance".

Let's apply the above calculation to explain the moon illusion. First, the "size of retinal image" of both the horizon moon and zenith moon is the same (as the true size of the moon is fixed). The "perceived distance" of the horizon moon is farther away (as compared with the surrounding objects) while the "perceived distance" of the zenith moon is closer (as without surrounding objects for comparison). Therefore, the longer the "perceived distance" of the moon, the larger is the "impression size" of it.


The horizon moon:

Same "size of retinal image" of the moon X Longer "perceived distance"

 = Larger "impression size" of the moon. 

The zenith moon:

Same "size of retinal image" of the moon X Shorter "perceived distance"

 = Smaller "impression size" of the moon.


In fact, the above are evidence-based theories which are commonly used. Many psychologists continue the research of Moon Illusion and actually it has not yet been completely resolved. When you are enjoying your moon cakes next time, try to tell your friends this secret of the moon.