Why does the moon look so much bigger when it is near
the horizon than when it is high up in the sky?
This question has
been pondered for hundreds if not thousands of years. This
problem is commonly referred to as the moon illusion.
Some have speculated that there is some effect that causes
the atmosphere to act like a magnifying glass making the moon
look bigger. It turns out that any distortion caused by the
atmosphere would actually make the moon look a little smaller.
Most scientists agree that the reason the moon looks bigger
is purely in our minds. Our mind interprets the things we see
in interesting ways. For instance, if you look at any door
frame you can see that it is rectangular. But if you were to
sketch the outline of the door frame from the angle that you
are looking at it, most likely you would sketch a trapezoid.
Your mind adjusts the door so that you perceive it as a
rectangle from whatever angle you look at it. That theory is
called shape constancy.
Drag the bottom
picture of the man around and see how big he looks.
Take a look at the picture above. Both images of the person
are the same size, but you perceive the one in back to be
bigger. This is because your mind sees the hallway and decides
that the back person is farther away that the front person, so
your mind adjusts the size of the person to make up for the
increased distance. This phenomenon is called size
Size constancy is happening all the time. If you look down
the street and see a sports car about 50 feet away, and behind
it, about 100 feet away is a big SUV, you know that the SUV is
bigger, even though it produces a smaller image on your eye.
One theory about the moon illusion says that when the moon
is near the horizon we perceive it to be farther away from us
than when it is high in the sky. But since the moon is
actually the same size, our minds make it look bigger when it
is near the horizon to compensate for the increased distance.
One way that you can trick your mind out of the moon
illusion is to bend over at the waist and look at the moon
upside down through your legs.
An alternative explanation holds that the moon illusion is
caused by the way our eyes focus on distant and close objects.
When we focus on the horizon moon, we focus on the moon at a
great distance. The overhead moon lacks visual cues that tell
us how far away the moon is, so we focus on the moon as if it
was a short distance away. See here for the
details of this theory.
So, for now, the right answer is that there is no right
answer. But the one thing people agree on is that the moon
does not physically change its size or distance from Earth as
it moves across the sky. It's all in our heads.