The Moon Illusion

As mentioned in the previous post, had you been watching the Lunar Eclipse along a flat, clear horizon, you may have seen the eclipsed Moon at the same time as the eclipsed Sun.  This is phenomenon that rarely occurs.  How is it possible though?  It has to do with the Earth’s air acting as a lens and bending the light of the Moon, to make it look as if the Moon were still over the horizon even after it has physically set.  And how is it that this is possible?  It seems to relate to the perceived inflation of the Moon as it sets in the sky.

The perception that the Moon inflates as it is setting in the sky is known as the Moon Illusion.  I credit Phil Plait for the information I am going to summarize concerning this illusion.  He thoroughly describes the Moon Illusion in one of his many posts concerning space on his blog Bad Astronomy.

The Moon Illusion is closely related to the Ponzo Illusion.  Such an illusion occurs when an object of equal size of another object is perceived to be farther than the second object and is thus assumed to be larger in size.  The “farther” object then actually looks larger in size compared to the second object.  The following images are simple examples of the Ponzo Illusion:

Notice how the top yellow bar in the left image appears to be larger in size than the bottom bar, and how the block furthest to the right in the right image appears to be taller than the other two blocks.  This is due to the human brain’s perception of those objects being farther than the others, and are thus thought and seen to be larger.

The Moon Illusion is in part due to this same effect, but it is also our perception of the sky that creates this illusion.  Though most people describe the sky above their head to be in the shape of a hemisphere, nearly everyone perceives the sky as an inverted bowl that is flat at the top.  Because of this perception, the horizon looks to be farther than any point above our heads.  When the Moon is on the horizon, your brain thinks it is far away, much farther away than when it’s overhead, and so the Ponzo Illusion kicks in and your brain sees the Moon as being huge.

Oddly, the Moon is indeed farther along the horizon than than when it’s overhead.  Such a fact is illustrated by the following image (image credit: Phil Plait):

Evidently, it seems the perception that the Moon is farther along the horizon than when it is overhead is true and not an illusion.  The illusion is that the Moon appears to be larger in size along the horizon than when it is overhead.  I understand what causes this illusion, but the reason of how/why our brains see the Moon “inflating” as it sets in the sky escapes me.  It is our brains’ deceitful perceptions that are the main causes for this illusion.


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