Lunar Illusion

Every once in a while, you may look up into the sky and notice that the moon is, relative to its normal size, enormous.  And typically, these huge moons are orange as well.  For example, consider the one above.  (Or, herehere, or here.) What’s going on?

No, light is not getting trapped in the atmosphere, nor is pollution to blame.  The big orange moon is not caused by an eclipse nor is the moon simply closer to the Earth.  And no, this is not some sort of mass hallucination.

Well, actually, it kind of is some sort of mass hallucination.  It turns out that what you are seeing is not really there.  Cameras don’t see the moon getting larger was it approaches the horizon — only our eyes and brains do.  It’s all in your head — and in all of our heads, for the matter.

What’s causing this global optical illusion?  We aren’t exactly sure.

First, this may be a variant of the Ponzo illusion, which suggests that we perceive the size of objects relative, in part, to the backdrop surrounding the object.  For example, take two identically-sized lines lying across railroad tracks going off into the distance.  The line further away may appear longer, even though the two are the same size, as demonstrated here.  The theory here is that the horizon throws off our sense of perspective, making the generally tiny moon appear enormous.

But there is a problem with that theory: airline pilots.  They see the same enlarged mirage of a moon when aflight, without the ground to give them this (false) sense of perspective we on land experience.  Another theory helps.  A pair of Columbia University researchers suggest that over the course of our lives, we have trained our eyes and brains to create a “flattened dome” view of the sky, allowing us to keep birds and other things in flight in their proper size ratios.  The moon is an outlier here, and our brains simply cannot process the image properly, giving us this fiction in the sky.

And even that may be wrong.  Throughout history, theories have been proposed, rejected, revisited, and rejected again.  The mystery of the illusion is one which has been discussed for over 2500 years, dating back to Aristotle.  We may never know why, a few times each year, the moon makes all of us see something which simply is not happening.


Bonus fact: While the size of the huge orange moon is an illusion, the color is not.  According to Cornell University, the orange-ish hue is because when the moon rising or setting (and therefore near the horizon), “that’s the time when the light has to travel through the most atmosphere to get to you.”

From the Archives: Shadow Boxing: You don’t see what you think you see.

Related: Uncle Milton’s “Moon In My Room.”  It’s a hanging wall ornament which lights up to show different phases of the moon (but only about 5 of them).  Intended for small schoolchildren, it is incredibly popular, averaging 4.5 stars over 279 reviews.

Image copyright Paul Moody (original here) and used under a Creative Commons license.