A poster in sci.astro asked 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.
_________

        > 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.

        This is an exaggeration; it is only slightly closer, certainly
        much less than 10%.

        A more important effect is that 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.

        I hope this clears up all your questions.
_________

        Of course, no one who lacks even a cursory knowledge of
        elementary physics can doubt that the above explanations have
        merit, but the primary effect is a physiological one.  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.