>From: email@example.com (Rev Dr Phil Herring BMath DD (Ret))
Subject: Re: moon size
Date: 12 Aug 91 06:58:16 GMT
Organization: Dept of Computer Science, Wollongong University, Australia
In article <firstname.lastname@example.org> email@example.com (Terry Chan)
> In article <1991Aug8.firstname.lastname@example.org>
> utoronto.ca (Joel Goldberg) writes:
> +As a.f.u is the repository of all truth and wisdom [Siman Rishum]
> +can anyone enlighten me definitively :-) as to why the moon appears
> +larger when it is near the horizon than when it is high up in the
> +sky? As Martin Gardner points out in one of his books, the effect
> +persists at sea, so it isn't because objects on the horizon provide
> +a reference scale.
> Well, in _MotSD_, Cece maintains that this is a purely optical illusion.
> The primary variable is that objects near the horizon provide visual
> cues which leads one to think that the moon is larger when it is near
> the horizon.
This may not be quite the best way to put it. The moon being larger near
the horizon is actually the nonillusory part of it; rather, the illusion
is that the moon is smaller when it's high up. After all, if you can see
buildings and trees next to the moon, and you're getting the size of the
moon wrong, then one would think that you'd be getting them *all* wrong,
which is probably unreasonable. In fact you're getting them all *right*.
With no visual cues, however, you get the size of the moon wrong when it's
The same thing affects constellations as well; basically, any extended
object in the sky is subject to it.
> Finally, Cecil notes that the
> illusion should disappear if you look at the moon through a tube
> (which would eliminate the visual cues).
My personal contribution to the issue (and my only published contribution
to the science of astronomy, as printed in Sky & Telescope, May, 1985,
letters) is the following observation: use a tube over one eye to eliminate
the foreground (close the other eye); note how small the moon suddenly
appears. Now open the other eye. The eye with the unobstructed view sees
a larger moon, and you have the slightly unnerving experience of seeing
two different-sized moons at once. This observation proves that the illusion
occurs independently in each eye, and places limits on any mechanism
proposed to eplain it.
In article <email@example.com> firstname.lastname@example.org (Werjun) writes
(and in article <1991Aug9.email@example.com>
barnick@sunshine.Kodak.COM (William M Barnick x73638) something
+firstname.lastname@example.org (Terry Chan) writes:
+> utoronto.ca (Joel Goldberg) writes:
+ [Story about why moon appears larger at horizon than high in the sky
+> Well, in _MotSD_, Cece maintains that this is a purely optical illusion.
+> The primary variable is that objects near the horizon provide visual
+> cues which leads one to think that the moon is larger when it is near
+> the horizon. While Cecil does not specifically address Gardner's point
+> about the illusion persisting even at sea, in his list of visual cues,
+> Cecil does mention "waves at sea" also.
+It was explained to me many many moons ago in an amateur astronomy class that
+the moon looks larger and closer because of the prescence of the earth's
+atmosphere---diffuses the light or some such thing. I have no idea whether
+this is a valid argument or not, but that's the answer I gave on the test.
Well, the explanation of the Earth's atmosphere acting as a lens was
exactly the next question in _MotSD_ that someone posed to Cecil (or
in the writer's words to Cecil "sorry to disappoint you, but you're
completely wrong.") The writer goes on to note that sufficient water
vapor in the atmosphere coupleted with the air itself forms a large
Cecil begins by saying "Years of dealing with you kind has taught me
patience". He notes that the refraction effect of the Earth's atmosphere
and the right density of water vapor is noticeable only at the point
of rising and setting and accounts for a sort of pear shape to both
the moon and the sun, and lasts for only a couple of minutes. This
is not the effect that people refer to when the moon appears to grow
larger in the sky because that illusion is apparent even when the moon
is a couple of degress of arc above the horizon.
Terry "Moonshine" Chan