Important If True 25: The Fresno Experiment

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I think I've got the best way to think about the Moon illusion.

Imagine a plane directly overhead.  Put your hand over your head at arm's length and pretend to measure the plane between your fingers.  Now imagine the plane flying to the horizon and keeping it framed between your fingers.  Your fingers will get closer together as it gets further away, right?


Now imagine the moon overhead.  Do the same thing, frame it between your fingers and imagine following it as it moves across the sky to the horizon.  It doesn't get any smaller!  So what's going on?  It must not be going away like the plane did.  It must be "closer" at the horizon.


Since we expect clouds or planes to look smaller at the horizon because they are further away, when the moon doesn't get smaller on the horizon it seems too big or too close.


Another way of thinking about it is that clouds and birds and planes in the sky are on a hemisphere the center of which is far below us at the center of the earth.  Looking straight up we are right up against the edge of the circle and things at the horizon are far away.  But the hemisphere of the moon and the stars is so far away we're basically at the center of the hemisphere and things at the horizon are essentially the same distance as things overhead.  But if our brains imagine the moon and stars on the same hemisphere as the clouds, everything at the horizon ought to be much farther away and therefore smaller.  But it's not smaller, so it seems either too big or too close.

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