<p>In addition to James K's perception methods, one can make a stereo pair of images by taking photos at different times in the Moon's libration as explained in <a href="http://physics.kenyon.edu/EarlyApparatus/Astronomy/Moon_Stereo/Moon_Stereo.html" rel="noreferrer">Stereoscopic Pictures of the Moon</a>. </p>

<p><a href="http://www.stereocamera.ca/StereoMoon.html" rel="noreferrer">Newportts</a> has a nice modern example:</p>

<p><a href="https://i.sstatic.net/8yh3G.gif" rel="noreferrer"><img src="https://i.sstatic.net/8yh3G.gif" alt="enter image description here"></a>
If you can do the eye-crossing trick you can see a 3D Moon. It is more impressive if you open the <a href="http://www.stereocamera.ca/Graphics/StereoFullmoonS.gif" rel="noreferrer">image</a> in a browser window and fullscreen it.</p>

<p>(The pairs in "Stereoscopic Pictures of the Moon" are meant to be viewed left image with left eye and right image with right eye, so if you use the eye-crossing trick you will see a concave Moon.)</p>

CC BY-SA 4.0
2019-03-16T11:40:08.443Z
I can do the eye-crossing trick. Good one Keith.
John Duffield
CC BY-SA 4.0
2019-03-16T11:53:48.833Z
Wow the magic eye on this is spectacular. It’s literally a ball
Autodidact
CC BY-SA 4.0
2019-03-16T13:02:05.653Z
Why I do interprete each single image as 3D? Just because I know that the moon is a sphere or bacuse the features it displays? If I try the cross eyes I see three inmges.
Alchimista
CC BY-SA 4.0
2019-03-16T16:05:16.347Z
@Alchimista Try holding your hands a few cm from your face so the L eye can only see the R image and vice versa.
Keith McClary
CC BY-SA 4.0
2019-03-17T11:50:34.910Z
@alchimista - just concentrate on the middle one of those three images. Once you have it right your brain will tube the others out a bit
Rory Alsop
CC BY-SA 4.0
2019-03-17T14:54:12.463Z
Thanks all. I am still puzzled by the fact  that if not really 3d also each single image drives me think of a ball and not a disk.  Not astronomy at all, rather brain function/knowledge. I know it is the moon and that the moon is round.  Obviously a gray homogeneous disk does not.
Alchimista
CC BY-SA 4.0
2019-03-16T04:41:43.073Z
2019-03-16T15:52:55.510Z
2019-03-16T15:52:55.510Z
Keith McClary
<p>Our brains rely on a variety of clues to determine depth. The most powerful clue is binocular vision but that only works at short distances, it's irrelevent here.</p>

<p>Other clues include the relative brightness of different parts of the image, the size and shape of shadows and the curving of linear features, the brain puts these clues together and sees a ball.</p>

<p>When you look at the moon with the naked eye at night it's difficult to see much detail because of it's small size and high brightness (compared to the rest of the night sky). So your brain can't see the clues that tell it the moon is not flat.</p>

CC BY-SA 4.0
2019-03-16T15:20:57.733Z
2019-03-16T15:20:57.733Z
Peter Green
<p>&quot;There's no way around it: when I look at the full moon at night it looks like a mostly flat disc, with at most a bit convexity in the middle. Phases of moon look like a full moon seen through a cut-out.&quot;</p>
<p>Moon looks flat because its surface is <em>not</em> matte (strangely also called 'flat'), in other words it is not Lambertian. Perceived surafce brightness of a matte surface (as opposed to glossy surface, for example) is given only by angle between sunlight and normal to the surface - it doesn't depend on position of observer (it's angle of view). Such a ball has graduation of surface brightness even on day side of Moon - which is not the case.</p>
<p><a href="https://i.sstatic.net/QMCFn.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QMCFn.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2024-01-21T19:58:03.380Z
Not sure I agree with the "not matte" statement.  If it were glossy, there would be a specular highlight.  Rather it seems it is matte, but the surface is noisy enough to make the shading difficult to perceive.
Greg Miller
CC BY-SA 4.0
2024-01-21T20:18:35.807Z
Its surface brightness depends on observers angle (so it Is not matte) in such a way (the other way than glossy surface) that it looks flat.
Leos Ondra
CC BY-SA 4.0
2024-02-27T20:16:35.043Z
This is the best answer IMHO. Lack of binocular vision and movement seem obvious, but ***this*** is what makes it feel flat. But more than that, not only is a full moon not darker at the edges as expected, it's actually brighter there, which is counter intuitive.
ispiro
CC BY-SA 4.0
2024-01-21T19:26:01.437Z
2024-01-21T19:26:01.437Z
Leos Ondra
<p>It is an optical illusion.</p>

<p>We perceive nearby objects in 3d because we have two eyes. As we see objects from two different viewpoints, our brain can put the images together to make a 3d image.</p>

<p>Objects that are more distant perceived as 3d if they are moving fast enough for us to see them change appearance. Again it is not in the eyes but in the brain that the perception of depth occurs</p>

<p>The moon is a long way away, far too far for any 3d from our two eyes, and doesn't change fast enough for to see any motion. It is too small for us to see shadows or any other clues that we could interpret as 3d structure, so it looks like a small  flat shape.</p>

<p>Telescopes can make shadows visible, and time-lapse can make the motion of the moon apparent, and so add to the perception of a solid.</p>

CC BY-SA 4.0
2019-03-16T01:12:57.053Z
Phases of the moon, where it has a distinct horned shape, reveal its not just some flat disk.
Wayfaring Stranger
CC BY-SA 4.0
2019-03-16T05:22:04.683Z
Why would shadows be more visible through a telescope?
VienLa
CC BY-SA 4.0
2019-03-16T06:53:45.327Z
The shadows are too small to be visible with the naked eye.
James K
CC BY-SA 4.0
2019-03-15T20:59:18.247Z
2019-03-15T20:59:18.247Z
James K
<p>There's no way around it: when I look at the full moon at night it looks like a mostly flat disc, with at most a bit convexity in the middle. Phases of moon look like a full moon seen through a cut-out.</p>

<p>Why is that the case? I recall, when I was really young, looking at it through what at the time seemed to be a very strong telescope, and it appeared not only a lot more textured but also a lot rounder. A similar thing can be seen in a video I saw a couple months ago, at 2:05: <a href="https://www.youtube.com/watch?v=XCrJ3NflOpE" rel="noreferrer">https://www.youtube.com/watch?v=XCrJ3NflOpE</a> . So it's clearly a naked eye thing.</p>

<p>I presume it's related to the optics, which made me hesitant to post it on this stack exchange (thinking it might be more suited to the physics one), but chose against it presuming people here would have more experience with celestial bodies and "large" distances.</p>

<p>I have a hunch that the answer is very simple, but I really don't know what that might be, especially since it's done over what I view as large distances.</p>

CC BY-SA 4.0
2019-03-15T20:30:40.233Z
A bit of motion can help a lot. Eg, https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif
PM 2Ring
CC BY-SA 4.0
2019-03-16T02:53:53.127Z
Also see [Moon Phase and Libration 2019](https://svs.gsfc.nasa.gov/4442) and the videos on that page.
Mike G
<the-moon><telescope><optics>
Why does the Moon appear to be flat?
VienLa
# Why does the Moon appear to be flat?

(Asked by: username_0 on 2019-03-15)

There's no way around it: when I look at the full moon at night it looks like a mostly flat disc, with at most a bit convexity in the middle. Phases of moon look like a full moon seen through a cut-out.

Why is that the case? I recall, when I was really young, looking at it through what at the time seemed to be a very strong telescope, and it appeared not only a lot more textured but also a lot rounder. A similar thing can be seen in a video I saw a couple months ago, at 2:05: <https://www.youtube.com/watch?v=XCrJ3NflOpE> . So it's clearly a naked eye thing.

I presume it's related to the optics, which made me hesitant to post it on this stack exchange (thinking it might be more suited to the physics one), but chose against it presuming people here would have more experience with celestial bodies and "large" distances.

I have a hunch that the answer is very simple, but I really don't know what that might be, especially since it's done over what I view as large distances.

**Tags:** the-moon, telescope, optics

> **Comments:**
> **username_1**: A bit of motion can help a lot. Eg, https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif  
> **username_2**: Also see [Moon Phase and Libration 2019](https://svs.gsfc.nasa.gov/4442) and the videos on that page.  

---

## 4 answers

### Answer by username_12 on 2019-03-15. Score: 13 | Accepted answer

It is an optical illusion.

We perceive nearby objects in 3d because we have two eyes. As we see objects from two different viewpoints, our brain can put the images together to make a 3d image.

Objects that are more distant perceived as 3d if they are moving fast enough for us to see them change appearance. Again it is not in the eyes but in the brain that the perception of depth occurs

The moon is a long way away, far too far for any 3d from our two eyes, and doesn't change fast enough for to see any motion. It is too small for us to see shadows or any other clues that we could interpret as 3d structure, so it looks like a small flat shape.

Telescopes can make shadows visible, and time-lapse can make the motion of the moon apparent, and so add to the perception of a solid.

> **Comments:**
> **username_13**: Phases of the moon, where it has a distinct horned shape, reveal its not just some flat disk.  
> **username_0**: Why would shadows be more visible through a telescope?  
> **username_12**: The shadows are too small to be visible with the naked eye.  

---

### Answer by username_3 on 2019-03-16. Score: 8

In addition to username_12's perception methods, one can make a stereo pair of images by taking photos at different times in the Moon's libration as explained in [Stereoscopic Pictures of the Moon](http://physics.kenyon.edu/EarlyApparatus/Astronomy/Moon_Stereo/Moon_Stereo.html).

[Newportts](http://www.stereocamera.ca/StereoMoon.html) has a nice modern example:

[![enter image description here](https://i.sstatic.net/8yh3G.gif)](https://i.sstatic.net/8yh3G.gif)
If you can do the eye-crossing trick you can see a 3D Moon. It is more impressive if you open the [image](http://www.stereocamera.ca/Graphics/StereoFullmoonS.gif) in a browser window and fullscreen it.

(The pairs in "Stereoscopic Pictures of the Moon" are meant to be viewed left image with left eye and right image with right eye, so if you use the eye-crossing trick you will see a concave Moon.)

> **Comments:**
> **username_4**: I can do the eye-crossing trick. Good one username_3.  
> **username_5**: Wow the magic eye on this is spectacular. It’s literally a ball  
> **username_6**: Why I do interprete each single image as 3D? Just because I know that the moon is a sphere or bacuse the features it displays? If I try the cross eyes I see three inmges.  
> **username_3**: @username_6 Try holding your hands a few cm from your face so the L eye can only see the R image and vice versa.  
> **username_7**: @username_6 - just concentrate on the middle one of those three images. Once you have it right your brain will tube the others out a bit  
> **username_6**: Thanks all. I am still puzzled by the fact  that if not really 3d also each single image drives me think of a ball and not a disk.  Not astronomy at all, rather brain function/knowledge. I know it is the moon and that the moon is round.  Obviously a gray homogeneous disk does not.  

---

### Answer by username_8 on 2019-03-16. Score: 5

Our brains rely on a variety of clues to determine depth. The most powerful clue is binocular vision but that only works at short distances, it's irrelevent here.

Other clues include the relative brightness of different parts of the image, the size and shape of shadows and the curving of linear features, the brain puts these clues together and sees a ball.

When you look at the moon with the naked eye at night it's difficult to see much detail because of it's small size and high brightness (compared to the rest of the night sky). So your brain can't see the clues that tell it the moon is not flat.


---

### Answer by username_9 on 2024-01-21. Score: 4

"There's no way around it: when I look at the full moon at night it looks like a mostly flat disc, with at most a bit convexity in the middle. Phases of moon look like a full moon seen through a cut-out."

Moon looks flat because its surface is *not* matte (strangely also called 'flat'), in other words it is not Lambertian. Perceived surafce brightness of a matte surface (as opposed to glossy surface, for example) is given only by angle between sunlight and normal to the surface - it doesn't depend on position of observer (it's angle of view). Such a ball has graduation of surface brightness even on day side of Moon - which is not the case.

[![enter image description here](https://i.sstatic.net/QMCFn.jpg)](https://i.sstatic.net/QMCFn.jpg)

> **Comments:**
> **username_10**: Not sure I agree with the "not matte" statement.  If it were glossy, there would be a specular highlight.  Rather it seems it is matte, but the surface is noisy enough to make the shading difficult to perceive.  
> **username_9**: Its surface brightness depends on observers angle (so it Is not matte) in such a way (the other way than glossy surface) that it looks flat.  
> **username_11**: This is the best answer IMHO. Lack of binocular vision and movement seem obvious, but ***this*** is what makes it feel flat. But more than that, not only is a full moon not darker at the edges as expected, it's actually brighter there, which is counter intuitive.  

---

