Someone asked in sci.astro why the moon appears larger on the horizon
than at the zenith.  The following responses appeared:

Actually the moon appears so much larger because it is almost
twice as close to you when on the horizon as it is when it is
overhead.

When the moon is on the horizon, it is attracted by all the mass
of the Earth you see running from where you are standing to the
point on the horizon where you see the moon.  Up above you,
there is no mass of Earth between you and the moon, so the force
is less.

The massive force brings the moon much closer when it rises and
sets.  As it gets higher, the force is less, and it moves
further away.  Then it comes back in again.

The moon is actually closest to the Earth when below the
horizon, only you can't see it then.  Those of us on the other
side of the Earth actually get a really good view.  Apollo
reached the moon by leaving from the other side of the Earth
when the moon was close.


The real reason is the effect of gravity on light rays.  When
the moon is directly overhead, the effect of gravity is to speed
up the light rays as they come from the moon to you, causing the
image to appear to be contracted in all directions.

When the moon is just above the horizon, gravity bends the light
rays, having the effect of a convex lens, but has little effect
on their speed.

The net effect is as if you were looking at the moon through a
magnifying glass when it is near the horizon, but looking at it
through the wrong end of a telescope when it is directly
overhead.


The real truth of the moon illusion is that it is all due to the
same effect as "the aberration of starlight".  When you are
moving, distant objects appear to move forwards, towards the
direction you're travelling.  If you were going at .9999c,
almost the whole sky would appear to be in front of you, only
points almost directly behind you are in your rearward
hemisphere, and so they appear distended, as a small patch of
sky has to cover the entire hemisphere.

When you see the full moon setting, you are travelling directly
away from it due to the rotation of Earth.  This explains why
the setting full moon appears larger than the moon at the
zenith, and larger still than the rising full moon.


What really happens is a physiological effect. As one
tilts his head back to observe the moon at the zenith, the lens
of the eye is flattened by gravity and its magnifying power is
decreased.