The Following Moon
Q:
When you move, the moon seems to follow you. Why does it do that?.
A:
The sun does the same thing, and for that matter, so does any object that is far
away, like a distant mountain. The related phenomena of perception, perspective
and parallax are responsible for this illusion.
It is a simple fact of perspective that objects appear to be smaller when they are
further away and parallel lines appear to converge as they become more distant. Although
the moon appears to be as small as a thumbnail, in reality it is as big as a continent, but very far away.
Our brains use perspective to help create the illusion of depth. We see depth in a
flat photograph because we have prior experience with perspective and the relative
sizes of objects. Through one eye or in a photograph, an apple at arm's length may
appear larger than a basketball across the room , but because we know their true relative
sizes, our brains can figure out their relative distance from us.
Normally we judge depth by a combination of parallax and by comparing the relative
sizes of familiar objects.
To see the effect of parallax, close one eye and point at a distant object. Now switch
eyes and you will see your finger appear to jump, so that it is no longer pointing
at the same place. One result of the geometry of parallax is that distant objects
appear to move less than those in front of them. If you point and move your head around,
you will see that distant objects appear to follow you. This happens because the
background is much larger and much further away than your finger.
Now do you see why the moon appears to follow you? It is much further away and much
bigger than anything we have experienced here on earth, and we have no clues as to
its true size. There are no converging lines to give us perspective, and no other
objects with which to compare its true size. It doesn't really follow you, it is just very
large and appears to stay in the same place as you move past objects which are in
front of it.
Richard Brill is associate professor of science at Honolulu Community College . Send
questions to Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817
or email to rickb@hcc.hawaii.edu. Visit our web page at http://www.hcc.hawaii.edu/~rickb/SciDoc.html
The Following Moon 1996 RCBrill