In article ,
Mark Elkington wrote:

We left a party the other night and drove away up a hill. The moon was
full and low on the horizon, viewable through the branches of trees
and over the rooftops of houses at the top of the hill.

It looked huge, as big as the distant trees and houses.

....as a matter of fact, it's even huger than that really.... :-)

As we drove toward the top of the hill, the moon shrank! By the time
we reached the top, the moon was only the size a soccer ball in the
branches of the now close trees.

So there you have it. Closeness to the horizon was not the cause, but
rather relative distances to terrestrial reference objects.

Mark

When you drove up that hill, towards the top of the hill and also
towards the Moon, your local horizon (the top of the hill) was
significantly above the normal horizon (near 90 deg from the zenith).
I don't know how much above - that depends on how steep the slope up
that hill was. Anyway, on top of that hill I suppose your local
horizon got more normal, since the ground then was less tilted.
Thus, at the top of the hill, the moon appeared significantly higher
above your local horizon than when you drove uphill.

In the moon illusion, it's the moon's altitude above your real,
local, horizon which matters, not its altitude above a theoretical
horizon at 90 degrees from the zenith. And when going uphill, your
local horizon may differ by many degrees from the theoretical horizon,
and it may change quickly when you reach the top of the hill.

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