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

(Mark Elkington) wrote in
om:

So there you have it. Closeness to the horizon was not the cause, but
rather relative distances to terrestrial reference objects.

I think this is rather simplistic myself. It doesn't explain why the
phenomenon occurs at sea, or in the Sahara Desert. It doesn't explain why
the Sun is also affected, or the constellations.

I watched Pegasus rise last night, and it's a constellation that is hugely
affected. When it's low in the NE, it appears to cover a huge amount of the
visible sky.

I was wondering if passengers riding in a jetliner at 35,000 feet see the
same effect when the moon rises. Does anybody know?

(Mark Elkington) wrote in
om:

So there you have it. Closeness to the horizon was not the cause, but
rather relative distances to terrestrial reference objects.

I think this is rather simplistic myself. It doesn't explain why the
phenomenon occurs at sea, or in the Sahara Desert. It doesn't explain why
the Sun is also affected, or the constellations.

I watched Pegasus rise last night, and it's a constellation that is hugely
affected. When it's low in the NE, it appears to cover a huge amount of the
visible sky.

I was wondering if passengers riding in a jetliner at 35,000 feet see the
same effect when the moon rises. Does anybody know?

On 6 Jul 2004 16:24:02 -0700, 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.

Sorry, but wrong. The illusion is alive and well when out on the ocean,
with no terrestrial objects anywhere in sight.


--
- Mike

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