Definitive moon size illusion experiment

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 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

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 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.


You can also measure its diameter to the reference point of the width of one
of your fingers held at arm's length and used to cover the lunar disk.
You'll find that even when the moon looks huge on the horizon or smaller
when higher in the sky, your finger still covers its orb.



--
Martin
"Photographs From the Universe of Amateur Astronomy"
http://home.earthlink.net/~martinhowell

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 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.


You can also measure its diameter to the reference point of the width of one
of your fingers held at arm's length and used to cover the lunar disk.
You'll find that even when the moon looks huge on the horizon or smaller
when higher in the sky, your finger still covers its orb.



--
Martin
"Photographs From the Universe of Amateur Astronomy"
http://home.earthlink.net/~martinhowell

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.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

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.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

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 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.

In my opinion, this experiment demonstrates only that you found the
Moon smaller in relation to the nearby branches, not that this is the
only conceivable explanation. (That seemed to be what you meant by
"definitive.")

For instance, any satisfactory theory of the Moon illusion will have
to explain why it seems to work even at sea, when the horizon is often
devoid of *any* points of reference. For me, it even works on my own
planetarium program. The first time I tried out my horizon view, I got
a rather startling impression of increased size at the exact moment my
program drew the horizon line (the very last thing it draws in that
view, by the way).

So although the effect you describe is undoubtedly in play in many
cases, it can't be the whole story.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

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 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.

In my opinion, this experiment demonstrates only that you found the
Moon smaller in relation to the nearby branches, not that this is the
only conceivable explanation. (That seemed to be what you meant by
"definitive.")

For instance, any satisfactory theory of the Moon illusion will have
to explain why it seems to work even at sea, when the horizon is often
devoid of *any* points of reference. For me, it even works on my own
planetarium program. The first time I tried out my horizon view, I got
a rather startling impression of increased size at the exact moment my
program drew the horizon line (the very last thing it draws in that
view, by the way).

So although the effect you describe is undoubtedly in play in many
cases, it can't be the whole story.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On Wed, 07 Jul 2004 13:34:02 GMT, John Steinberg wrote:

I have a pet theory that attempts to explain the moon illusion as a
result of the morphology of the human eye/brain combination. It's
still a work in progress (IOW, we're *all* seeing it for the first time
here) but in brief it states that the human visual field was purpose
built and optimized to look straight ahead from an upright position. I
could go on (and on) about why this is so, but since my own eyes are
already glazing over I'll move on to something else.

Cats provide an example of an animal that clearly has a visual system such as
you describe. The receptor rich foveal region (circular in humans) is quite
elongated horizontally, and the retinal and cortical wiring is optimized for
horizontal movement. Cat eyes apparently evolved to serve an animal that hunted
on the plains. The low level visual and cortical structures in humans are
symmetric (not surprising as we evolved from tree dwellers). So if your theory
is correct, I'd have to think the optimization is fairly subtle.

Many optical illusions are based on experience. Some of the more well know ones
involving interactions between straight lines are seen only weakly, or not at
all, by people raised in primitive environments with few linear structures. It
would be interesting to know if there are any such cultural/environmental
influences on the Moon illusion.

BTW, cats also have high density rods and a reflector behind the retina that
greatly increase the quantum efficiency of their eye. Their limiting visual
magnitude is probably close to -10!

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Wed, 07 Jul 2004 13:34:02 GMT, John Steinberg wrote:

I have a pet theory that attempts to explain the moon illusion as a
result of the morphology of the human eye/brain combination. It's
still a work in progress (IOW, we're *all* seeing it for the first time
here) but in brief it states that the human visual field was purpose
built and optimized to look straight ahead from an upright position. I
could go on (and on) about why this is so, but since my own eyes are
already glazing over I'll move on to something else.

Cats provide an example of an animal that clearly has a visual system such as
you describe. The receptor rich foveal region (circular in humans) is quite
elongated horizontally, and the retinal and cortical wiring is optimized for
horizontal movement. Cat eyes apparently evolved to serve an animal that hunted
on the plains. The low level visual and cortical structures in humans are
symmetric (not surprising as we evolved from tree dwellers). So if your theory
is correct, I'd have to think the optimization is fairly subtle.

Many optical illusions are based on experience. Some of the more well know ones
involving interactions between straight lines are seen only weakly, or not at
all, by people raised in primitive environments with few linear structures. It
would be interesting to know if there are any such cultural/environmental
influences on the Moon illusion.

BTW, cats also have high density rods and a reflector behind the retina that
greatly increase the quantum efficiency of their eye. Their limiting visual
magnitude is probably close to -10!

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

John Steinberg wrote:

If you're not part of the solution, you're part of the precipitate.


Now settle down, John ;o)



--
Martin
"Photographs From the Universe of Amateur Astronomy"
http://home.earthlink.net/~martinhowell