The Moon Illusion
By Grant Ocean
Variations of the Moon Illusion
There are many variations of the
Moon illusion that are not easy to be explained away by previous
theories.
The most important variation is the disappearance of the Moon
illusion when viewed upside-down between legs. This is a phenomenon
that confounded many moon illusion researchers and has perplexed myself
for a long time.
Specifically, the moon illusion disappears when we bend down and look
at the moon upside-down between our legs. At least two hypotheses have been proposed
to account for this phenomenon.
(1) Familiar objects in the field of view may become
useless as distance references because of their unfamiliar appearance when
viewed upside-down.
(2) The illusion may have something to do with our
inner-ear's balance mechanism that tells us whether the head is upright or
upside-down.
The first hypothesis can be dismissed because we have already established the case against the size contrast theory.
As far as the second hypothesis is concerned, we are not quite
sure what exactly the upright or upside-down head has to do with the
disappearance of the Moon illusion.
The reasons why we do not
perceive the Moon illusion when bending down and looking at the horizon
moon through legs are twofold.
First, the legs act like a viewing hole as in The Ames Room Illusion,
and as a result, the converging point cannot pass beyond the legs,
leading to a larger visual field volume for the viewed moon.
More importantly, when we turn our head upside-down, our eyes
are not used to this position and thus the eyes cannot be held
straight; so our eyes will roll downwards due to the gravitational
force.
To look at the horizon moon in this position, we have to roll
our eyes upwards. To roll eyes upwards in the upside-down position is
the same as rolling eyes downwards in the upright position.
Now you can try this: tilt your head upwards when you stand
upright, and roll your eyes downwards to look at an object. The object
appears much smaller while viewed this way.
Hence, the reasons why the Moon illusion disappears when we
look at it upside-down and between our legs are that we roll our eyes
downwards (if in the upright position) to look at it and we look at it
through a fixed viewing frame of the legs.
Similarly,
when you lie on your back and tilt your head up looking at the
horizon
moon, the Moon illusion is gone as well. But, this effect is not just
limited to the Moon illusion. Even if you stand upright and tilt
your head down and roll your eyes upwards to look at an object, it
appears smaller.
It
is the same as when you bend down and look upside-down at the horizon moon
between your legs. In both cases the viewed object appears smaller than before.
In addition, turning your eyes upwards or downwards to look at
objects is similar to looking at objects with crossed eyes, i.e.,
turning your eyes inwards to look at a spot nearer than the target
object, which will result in the so-called oculomotor micropsia.
As I have said earlier, this micropsia is not due to the
oculomotor system, but due to the closer converging point leading to a
larger visual field volume for the target object. I
have done some experiments to determine from how faraway we can
focus on an object when we turn our eyes upwards, downwards and
inwards. The outcome is that we can focus on objects at a distance of
about 10 cm,
meaning that when we turn our eyes upwards, downwards and
inwards, we can see clearly the objects placed at 10 cm from our eyes
and the objects become blurred when they are placed further than this
distance.
As a result, the converging point is fixed at approximately 10
cm from our eyes when we turn our eyes upwards, downwards and inwards
to look at objects. The objects beyond the converging point of 10 cm
look both
smaller and farther.
In other words, turning eyes upwards, downwards and inwards
causes micropsia. So the baffling phenomenon of viewing upside-down to
do away with the Moon illusion is due to micropsia, a familiar concept.
Another variation of the Moon
illusion is that when the visible horizon is perceived to be far away,
e.g., at sea, the difference in size between the horizon moon
and the zenith moon will be relatively large. It is a popular activity
for the travelers on a ship at sea to watch the rising sun in the
morning.
The rising sun over the ocean seems much bigger than that over
the land horizon. The reason is that the ocean is flat, free of
obscuring objects, so its horizon is farther away from the viewer.
To focus on this farther horizon will bring the converging
point closer to the objects on the horizon, i.e., the moon, sun, or
constellation. It is the same reason why the rising or setting sun
viewed from the top of a high mountain appears much bigger than that
viewed on the ground.
We can see a further horizon from a higher location due to the curve of the earth.
One researcher found that the apparent size increase of the
moon over a distant horizon (> 10 km) at an airport was reported by
observers to
be larger than the increase observed over a nearby forest, creating a visible horizon at approximately 2 km from the observer.
This research further confirms that the larger the viewed object appears, the further the converging point is focused on.
Similarly,
the Moon illusion is found to be stronger when there are clouds
in the sky compared with a cloudless sky. The reason is that we can see
clouds that are physically well beyond the surface horizon (perhaps 15
km away) due to their height above the curved earth surface.
Thus, the converging point is brought closer to the moon when
we focus on the clouds which are further away than the surface horizon.
A small percentage of people report that they do not experience the Moon
illusion at all.
The researches and literature on the Moon illusion is by and
large silent on the reasons for this fact. I do not intend to avoid
this issue, but I have to admit that this is a tough issue to deal
with. First of all, I myself do not have this issue;
so I cannot comment on the issue on the basis of my own
experience. I can only guess the reasons according to the concepts of
the visual field volume and the converging point.
These people who do not experience the Moon illusion may have
the ability to focus on the horizon moon only, not paying attention to
the horizon at all. Or they can pay attention to both the horizon and
the horizon moon;
but somehow they have the ability to keep them in the separate
sub-visual field areas in the visual cortex. No matter whether it is
due to the attention separation or the cortical separation, or
something else,
one thing is for sure, that is, they can place the converging
point at approximately one meter from their eyes when they look at the
horizon moon. However, I haven't heard anybody who has not been
impressed by the larger-looking rising and setting sun viewed on a high
mountain top,
or the larger-looking rising and setting moon over the ocean. I
suspect that the conditions for those Moon illusion experiments must
not be ideal for experiencing the illusion.
And some people under those circumstances can effectively separate the horizon and the moon over the horizon.
The last thing I want to comment
on is a theory proposed by Arnold Trehub (1991) in his book The Cognitive Brain.
He
proposed that the moon illusion is consistent with what would be expected from evolutionary
considerations.
We have evolved cognitive processes that provide high quality
visual information from nearby things, and things on our level that we can walk
to and experience from various angles. These are all important to our survival.
Things seen high above, in the sky, or even those seen below, as when looking
over the edge of a cliff, are less important. Therefore distance discrimination
and detailed judgment of other visual properties of overhead objects is
compromised.
It is worth mentioning because this theory is getting a lot of
attention and even acceptance of more and more researchers. In my
opinion, the Darwin's theory of evolution by natural selection has
become a religion for many scientists.
In ancient times people simply said that it was God who did it
when they could not understand a certain phenomenon. In our modern
times some scientists simply say that it is the evolution that has
selected that specific behavior,
perception, and whatsoever for survival and reproduction when
they cannot understand and explain a phenomenon. There is no need to
reason with this kind of theory.
All we need to do is to provide a comprehensive theory to
explain the phenomenon satisfactorily. The theory will go away on its
own.
TOP
The Moon Illusion Index
References
Nanavati, S. (2009). A history and experimental analysis of the moon illusion. The New School Psychology Bulletin, Vol. 6, No. 1.
Trehub, A. (1991). The Cognitive Brain. MIT Press.
Related Information on the Web:
http://facstaff.uww.edu/mccreadd/index.html
http://www.lhup.edu/~dsimanek/3d/moonillu.htm
http://en.wikipedia.org/wiki/Moon_illusion
http://retina.anatomy.upenn.edu/~bart/scriptie.html
http://www.pnas.org/content/97/1/500.full
http://nspb.net/index.php/nspb/article/view/6/3
Appendix A