<p>The apparent anomaly, known as the Lunar Terminator or Moon Tilt Illusion, is indeed a matter of perspective. A brief explanation can be found here: <a href="http://chrisjones.id.au/MoonIllusion/" rel="noreferrer">http://chrisjones.id.au/MoonIllusion/</a></p>

<p>The essence: 'The illusion occurs when the moon and sun are separated by a wide angle, so that they are perceived relative to the horizon, as if in a panorama. A panoramic photograph is a cylindrical projection. In this projection, most straight lines project as sinusoidal curves. The moon-sun line is curved, unless the moon and sun are on the horizon or directly above one another.'</p>

<p>A long, technical explananation is in the PDF <a href="https://www.seas.upenn.edu/~amyers/MoonPaper20June.pdf" rel="noreferrer">The moon tilt illusion</a>. Quoting from page 21:</p>

<blockquote>
  <p>Modern cameras use lenses whose properties are designed to deliver a rectilinear
  or curvilinear image.  Rectilinear lenses reduce barrel or pincushion distortion from
  the image but such lenses are difficult to manufacture for the wide angles (90°
  and above)  needed  to  record  both  the  sun  and  the  moon  on  a  single  photograph.   A
  photograph [3]  of  the  moon  and  sun  at  an  azimuth  difference  of  80°
  containing  a
  leaning  tower  and  unnaturally  leaning  trees  illustrates  the  difficulty  of  eliminating
  distortion in a wide-angle photograph</p>
</blockquote>

<p>That reference [3] is to <a href="http://www.astropix.com/html/l_story/moonill.html" rel="noreferrer">A Different Moon Illusion</a> on Jerry Lodriguss astrophotography site <em>Catching the light</em> where he shows and explains the exact same situation (picture taken from there):</p>

<p><a href="https://i.sstatic.net/NOym6.jpg" rel="noreferrer"><img src="https://i.sstatic.net/NOym6.jpg" alt="enter image description here"></a></p>

CC BY-SA 4.0
2018-08-08T16:47:49.953Z
Since the phenomena can be clearly seen by the naked eye it is therefore a real phenomena and that appeals to photographic optical illusions are not necessary. If you were to draw on a blackboard what was observed then optical illusions are ruled out.
Jackamus
CC BY-SA 4.0
2018-08-28T21:53:40.893Z
A phenomenon that you observe, incorrectly, with the naked eye is the definition of an optical illusion. This answer explains how the illusion occurs, in spite of what you think you are seeing.
haresfur
CC BY-SA 4.0
2018-10-06T17:15:48.780Z
If a torch were placed on a pole coincident with the sun and a football also on a pole and coincident with the moon, the torch would not illuminate the football. I think the reason it is called an illusion is because it doesn't fit the standard paradigm.
Jackamus
CC BY-SA 4.0
2021-10-28T20:03:41.853Z
The idea that this is being called an illusion is because it doesn't fit the official narrative because everyone and his dog KNOWS the Sun illuminates the Moon. It is impossible for this accepted Moon-Sun system to be viewed in any other way than the way I have been maintaining. A bright light illuminating a distant smaller spherical object can only behave in the way that a smaller light (torch) and smaller closer spherical object (football).
Jackamus
CC BY-SA 4.0
2021-10-28T20:04:24.623Z
Out there in space there are no curved planes or celestial spheres on which to build an explanation to try and compensate for the obvious anomaly. In space there is no up or down or left or right. Whatever you observe is what is actually there. Viewing it from a position here on Earth will make no difference to what is observed - a light shining on a sphere.
Jackamus
CC BY-SA 4.0
2021-10-28T20:05:10.013Z
If only the illuminated sphere is observed then it is a simple extrapolation to determine from where it is being illuminated. You have to make a choice between using common sense and basic physics or denying them in order to protect the status quo by appealing to optical illusion. There was one animation on this thread that showed a Moon and Sun projected onto a vertical curved plane and a curved line connecting the Moon and Sun.
Jackamus
CC BY-SA 4.0
2021-10-28T20:06:32.413Z
It then animated the curved plane into a flat vertical plane showing a straight line between the Moon and Sun. This part of the animation was showing reality whilst the first part (curved plane) was showing a false way to explain the anomaly. Is it possible to add jpg graphic to a Comment?
Jackamus
CC BY-SA 4.0
2018-08-08T08:53:05.233Z
2018-08-08T13:51:58.183Z
2018-08-08T13:51:58.183Z
Esquared
<p><a href="https://i.sstatic.net/WvOM5.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WvOM5.jpg" alt="enter image description here" /></a>Here my wife holds a globe with the moon in view.  I carefully processed in a bit-mode to find the curve of light on the globe and drew perpendicular lines.  Plainly we can see that the moon-lite side is facing the sun in the same direction of my globe.</p>
<p>The illusion is due to the sun being so far that the difference in angles shown in the drawing would not be noticeable. Imagine two planes where both of them contain the sun and moon, the level plane would be how an observer would see the moon when both the moon and the sun were at the horizon. The tilted plane is how an observer would normally see the moon high in the sky even though the sun was at the horizon during sunset. The observers on both planes would see the same half-moon shape even though the relative heights in the sky vary greatly.  This construct might only work for quarter moon illuminated.<a href="https://i.sstatic.net/S2QrE.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/S2QrE.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-10-18T16:19:51.877Z
You seem to agree that one would expect the moon to be tilted more towards the sun but it isn't. The moon's illumination seems to be coming from another direction. I realise that what I am saying goes against what everyone believes but that may be the problem - it is a 'belief' that the sun illuminates the moon and not a fact.
Jackamus
CC BY-SA 4.0
2021-10-20T07:33:29.953Z
@Jackamus It is very much fact that the moon is illuminated by the sun. https://www.livescience.com/45979-why-does-the-moon-shine.html
Chris Rogers
CC BY-SA 4.0
2021-10-26T23:14:55.523Z
Humans can't judge linear metrics beyond 8km. So, when a person visualizes the location of a light source illuminating the moon, the problem is 2D only, involving ORTHOGONAL ANGULAR DISPLACEMENTS (e.g, right ascension/declination, altitude/azimuth, etc.). 

One traces a "line" from the moon's day/night terminator, orthogonal to its midpoint. This "line" is actually a 2-D "HALF-INFINITE" TRIANGULAR PLANAR SECTION which continues from moon to the light source, with the observer at the vertex.  In this case, the moon is on the left edge, and the section continues rightward UNTIL a light source!
Bruzote
CC BY-SA 4.0
2021-10-26T23:22:58.840Z
@Jackamus, per my previous comment, one's eyes cannot perceive distance. Thus, it is only an optical illusion that leads to one to think one perceives that an imaginary "line" from the moon is receding into a point in space. One can only see in angles at astronomical distances. So, where would one stop when sweeping from the terminator in the direction of the light source? Well, one stops at the light source! One must not stop before then, because literally there is no justification to stop. Any perception of a "line" in 3-D space that has terminated is a self-imposed illusion.
Bruzote
CC BY-SA 4.0
2021-10-27T19:52:16.007Z
Looking at the link offered by Chris Rogers above I noticed that the intensity of the light on the moon's surface is even right across its face. If the Moon is a sphere shouldn't the intensity of the reflected light fall-off towards the edges?
Jackamus
CC BY-SA 4.0
2021-10-27T20:00:26.317Z
Comment to Bruzote - Creating a triangle plane between me, the Moon and the Sun will not actually change anything. All you are doing is simply joining-up the three points with a line which means that at my position I will be observing a straight line between the Moon and Sun as I have been saying all along. Claiming it to be an illusion is a false way to try and explain why what is observed is different from what is believed - I use the word 'believed' deliberately.
Jackamus
CC BY-SA 4.0
2021-10-27T20:17:16.943Z
A further comment to Bruzote - You are trying to complicate a very simple property of physics with complicated geometry which finishes up, not demonstrating anything but stating it is an illusion. You are trying to use scale as a 'Get out of jail card'. Supposing I showed you a photo of a half illuminated sphere without any reference to its size (football or planet). How would you go about determining the line that would lead you to the light source?
Jackamus
CC BY-SA 4.0
2023-01-27T05:39:16.343Z
I would determine the line as I said by tracing an arc that subtends that line. Start by pointing your arm and finger at the lit object (e.g. moon). Now imagine a vector extending perpendicular to that lit hemisphere. Start sweeping your finger and arm in an arc to trace that vector. **Note: Because the line you are tracing is in a plane involving you, the object, and the light source, tracing that line with your finger can only be done by continuing to sweep in that plane.** If you keep sweeping in that plane, you follow the line and eventually point at the light source. It works every time.
Bruzote
CC BY-SA 4.0
2021-10-14T02:08:51.423Z
2021-10-28T18:52:01.947Z
2021-10-28T18:52:01.947Z
Eric Reiter
<p>You can see this effect on the powerline above the railroad tracks in the picture you posted too, it doesn't look straight, even though it likely was in reality! The straight line between the sun and the moon is just even <em>more</em> curved. Just like what you see at <a href="http://chrisjones.id.au/MoonIllusion/" rel="nofollow noreferrer">http://chrisjones.id.au/MoonIllusion/</a> , like the top answer already posted. If you accept that the curvature of the powerline on this picture is just an optical illusion/distortion introduced by the camera, then so is the sun-moon illumation!</p>
<p><a href="https://i.sstatic.net/TqHHU.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/TqHHU.png" alt="the picture curves straight lines" /></a></p>
<p>The error in your diagram is that you assume a rectangular and linear (0, 1 or 2 point foreshortening) perspective. As you can see <a href="https://youtu.be/Y2gTSjoEExc?t=190" rel="nofollow noreferrer">in this VSauce video</a>, straight lines in our surroundings actually become curved lines from a single eye (or photocamera) perspective. After you've seen the VSauce video, you can check that this holds true for you too: if you rotate your head, you'll notice that the place where your wall and ceiling meet will curve when you're moving from side to side. Straight lines, when viewed from below from left to right cease to be straight! A 5-point curved perspective better captures the way straight lines appear to us.</p>
<p><a href="https://i.sstatic.net/X9lKd.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/X9lKd.jpg" alt="5 point perspective" /></a></p>
<p>Note, by the way, that even from a &quot;torch&quot; perspective, as you insist, and without any optical illusion, we can get the moon shining &quot;upward&quot;. Consider the following overview picture: the moon (left object) is illuminated by the sun (right object) in exactly the way you expect. Now, however, consider that you are an observer (where the two red lines converge), in particular, that you are standing <em>below</em> the sun and moon. As you know, if you stand below things, you cannot see what is on top -- and similarly, you can see more of what is on the bottom of that object. That is what's drawn in the dotted lines from the observer view: they tangentially hit the moon on the dark side on the bottom, and cannot observe the top of the moon. Just because we're below that object.</p>
<p><a href="https://i.sstatic.net/VaU9B.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/VaU9B.png" alt="Overview picture of moon and sun" /></a></p>
<p>Thus, if you place yourself in the position of the observer, the objects will appear as if they have both rotated away from the horizon, because we can see <strong>both</strong> bottoms of the objects!</p>
<p>If you put the two effects together (one, the strange effects of panning from left to right to lines above us; and the effect of being located below the moon), we end up with the following effect, where it appears that the &quot;moon&quot; is facing upwards to the right in the first few frames when we're looking to the left, but once we zoom out you can see that the sun is actually level with the moon!</p>
<p><a href="https://i.sstatic.net/EhVnt.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/EhVnt.gif" alt="sun-moon system" /></a></p>
<p>The effect nearly disappears entirely once I add a line connecting the two bodies:</p>
<p><a href="https://i.sstatic.net/2DM0E.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2DM0E.gif" alt="sun-moon system with added lines" /></a></p>
<p>Now the effect looks like it isn't even there anymore, because somehow our brains are able to make sense of the perspective. Note how in every single picture, the lines are all straight (so, no trickery with wide-angle or fish-eye lenses), but when we rotate from left to right, they seem to &quot;change direction&quot; from pointing northeast to northwest and back.</p>

CC BY-SA 4.0
2021-10-22T20:13:47.413Z
This answer is in response to Eric as a 'Comment' was rejected for being too long.

Eric I think your animation is flawed. What you do not take into account is the observing eye level (horizon) is always coincident with a straight (perpendicular line) between the centre of the Moon and the Sun. You cannot be looking straight ahead with the Moon and Sun above your eye level (possibly out of your area of vision) and observe the same thing as when you tilt your eyes upwards to see the underside of the Moon and Sun (now in the centre of your area of vision). To be continued in another comment.
Jackamus
CC BY-SA 4.0
2021-10-22T20:15:14.513Z
Your animation is not showing what you will actually observe when you move your eyes to keep the Moon and Sun always in the centre of your vision. This is why wide angle lens cameras show curved lines at their peripherals

Imagine that the centre of the Moon (football) has a rigid bar connected to the centre of the Sun (torch) no matter where your position of observation is that rigid bar will always be straight and not curved. It will only appear curved when you move your head to observe them with your peripheral vision just as you would with a wide angle lens at its edges. to be continued.
Jackamus
CC BY-SA 4.0
2021-10-22T20:17:01.227Z
Therefore your animation is showing what would be observed through a wide angle lens that moves up and down. If you kept them in the centre of your vision as you move up and down then the rigid bar would always be straight and not curved.

This can be confirmed by looking at some NASA footage that shows the Earth's curve changing from concave to horizontal to convex as the camera moves up and down just like your animation. End of comment.
Jackamus
CC BY-SA 4.0
2021-10-23T07:41:31.227Z
@Jackamus: Okay, so I have replaced the two animations with two new ones, which hopefully show the full effect. You can see a moon (on the left) that appears to "point upwards", but when zooming out the effect disappears. Even if the sun/moon are level with the camera (so, we're still looking up, but they are positioned in the center line of vision) as in https://imgur.com/a/W1aVBEy, you can see that the effect persists.
Erik
CC BY-SA 4.0
2021-10-23T10:19:19.350Z
I think those new animations are the most intuitive answer I've seen on here (and they match the answer I never got around to posting 3 years ago).  Note you can easily reproduce this effect IRL by looking up at a wall/ceiling join through a directors viewfinder formed by your hands and then swivelling.
Deditos
CC BY-SA 4.0
2021-10-24T08:09:40.583Z
Your argument still doesn't stand up. If you zoom out so that you can see the sun and the moon at the same time they will be joined by the purple line between their respective centres thus confirming what I have been saying. It doesn't matter where in space you view the system there will always be a direct line between their contres if one illuminates the other.
Jackamus
CC BY-SA 4.0
2021-10-24T08:16:48.247Z
@Jackamus It'd be helpful to make sure you agree (or not) on two points first. (1) do you agree that the powerline in your picture appears more curved than it would be in real life? (2) do you agree that in the zoomed-in perspective (without lines present) it kind of looks like the "moon" is illuminated from above-right rather than from just from the right? These are relevant points for me!
Erik
CC BY-SA 4.0
2021-10-25T11:56:15.230Z
Yes I agree that the power lines look curved because of the wide angle lens effect. The zoomed-in image of the Moon shows that it is lit from slightly above. It is that angle that is the problem. It is not possible to observe a straight light ray between the centre of the Sun and the centre of the Moon because of the Moon's upward facing lit area. Can please confirm that must be a straight line light ray that is perpendicular from the centre of the two bodies and which causes normal illumination as would a torch and a football. As yet nobody has acknowledged this to be a given truth.
Jackamus
CC BY-SA 4.0
2021-10-25T13:54:36.560Z
@Jackamus I agree that the moon should be (and is) illuminated by straight rays following the shortest path from the sun to the moon. Just like a torch and a football. But doesn't the animation show that the moon (or football) might *seem* to be lit from above (from a zoomed in perspective below the object, looking to the side), even when it really is lit from the side only (as visible from a zoomed out perspective)? To me, the animation [and curved power line] confirms that your picture is an optical illusion: a stitched-together picture of the zoomed-in perspective on the moon.
Erik
CC BY-SA 4.0
2021-10-25T17:03:07.543Z
@Jackamus Next time when you see the sun and the moon together, don't take a picture but take a piece of string between your left and right hands. Hold your left hand at any point between your eyes and the moon, and your right hand at a point between your eyes and the sun. Following that piece of string, it should be a straight line from the moon's illumination towards the sun. Just like the first frame in the animation shows a straight line [https://imgur.com/FsPoa2g], perpendicular to the moon's terminator, which goes straight to the sun. That should prove the whole issue.
Erik
CC BY-SA 4.0
2021-10-25T17:12:55.237Z
@Jackamus from https://www.seas.upenn.edu/~amyers/MoonPaperOnline.pdf, “...the line connecting the horns of the moon, between its first quarter and full moon, for instance, does not appear to be at all perpendicular to the direction from sun to moon; we apparently think of this direction as being a curved line. Fix this direction by stretching a piece of string taut in front of your eye; however unlikely it may have seemed to you at first you will now perceive that the condition of perpendicularity is satisfied”
Erik
CC BY-SA 4.0
2021-10-26T16:22:59.627Z
I haven't tried the string idea because trying to sight one end of the string on Moon whilst trying to sight the other end on the Sun would be very difficult to do with any accuracy. This is because I cannot view the Moon and the Sun at the same time and the chance of my hands moving are very great. You accept the torch-football idea so why not Moon - Sun?
Jackamus
CC BY-SA 4.0
2021-10-26T16:24:47.177Z
I know why appeals are being made to it being an illusion is because observation does not fit the official narrative. If some suspended a huge light in the sky say over a football stadium, that was coincident with the Sun and the same with a huge ball coincident with the moon and viewed from a seat in the stands, what you would observe is the ball being directly illuminated by the light but it would not look the same as the Moon-Sun observation. Can someone explain why the football - torch arrangement is different from the Moon - Sun arrangement?
Jackamus
CC BY-SA 4.0
2021-10-26T18:44:55.967Z
@Jackamus Try harder, you only have to move your head while keeping your hands (nearly) at the same spot. Or get a friend to do it. There's no way you'll find an effect as large as what your picture suggests. The other questions are amusing me, because they are exactly the point: the effect exists *because* you cannot see the sun and moon at the same time. If you could, you would realize that the moon isn't illuminated from seemingly "above" at all. It would be the same with a torch/football setup, if you would set them up s.t. you couldn't see both at the same time, you'd see the same effect!
Erik
CC BY-SA 4.0
2021-10-20T11:15:55.200Z
2021-10-23T08:27:10.510Z
2021-10-23T08:27:10.510Z
Erik
<p>When looking at the sky, spherical coordinates are used as a convenience. We turn our head and body in azimuth and altitude to compare the position of objects. In this regards, the sky looks like a sphere. (At least I visualize it as a sphere.) A Cartesian XY (or XYZ) grid cannot be easily used.</p>
<p>The question then becomes: what is the path of a light beam in spherical coordinates? Realizing that light takes the shortest distance from point A to point B, another way to phrase the question is <strong>what is the shortest distance between two points in the sky? The answer to the re-phrase question is a great circle</strong>.</p>
<p>As Bruzote's Oct 25 2021 answer was indicating, the problem is it is difficult to visualize a great circle on the sky, and the path created by a great circle is not intuitive to most people (including me). Fortunately, you can easily observe where one particular great circle exists and solve the “Moon illumination anomaly” at the same time: go out and observe the Moon! The Moon orbits the Earth in a plane, and the intersection of the orbital plane and the sky is a great circle.</p>
<p>For example, observe the position of the Moon for several days starting a few days after New Moon. Dec 5 through the 13 2022 is a good opportunity. Make the first observation just before or at sunset and observe 4 minutes earlier on each succeeding day. (The 4 minute difference compensates for the Earth's motion around the Sun. Unless your plot of the Moon's position in the sky is precise to a fraction of a degree, you do not need to be so precise with your timing.) If you plot the position of the Moon, it will look something like the following plot: (created from <a href="https://skyandtelescope.org/interactive-sky-chart/" rel="nofollow noreferrer">Sky and Telescope's Interactive Sky Chart</a>)</p>
<p><a href="https://i.sstatic.net/T9XdJ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/T9XdJ.png" alt="Moon's changing position Dec 5 to Dec 13 2021" /></a></p>
<p>Note that:</p>
<ol>
<li>The “straight line” of the great circle is a curve going across the sky. It does not “feel” straight. (The black line in the figure is the ecliptic, which is the great circle created by the Sun’s apparent path for 1 year. The Moon’s orbit is inclined approximately 6 degrees to the ecliptic -- close enough for reference.)</li>
<li>The illuminated face of the Moon points toward the position of the Moon on the previous day.</li>
<li>When connected together, the illuminated directions make a “line” (technically a great circle) that connects to the Sun. That is, the illuminated direction is in the plane of the Moon’s orbit and always faces the Sun. (The Moon's great circle crosses the Sun when there is an eclipse. At other times, the great circle may pass above or below the Sun by 6 degrees.)</li>
</ol>
<p>On May 16 2016, the all-sky plot looks like the following:</p>
<p><a href="https://i.sstatic.net/li4OZ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/li4OZ.png" alt="May 16 2016 sky" /></a></p>
<p>When shown in a horizon projection, the image looks like the following. The illumination on the Moon appears to be coming from the upper right because that is the shortest direction to the Sun!</p>
<p><a href="https://i.sstatic.net/iskmJ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/iskmJ.png" alt="May 16 2016 horizon" /></a></p>
<p>All images are approximations, just like the original photograph, due to projections from 3D onto 2D. The last image is stitched from several images which introduces more distortions.</p>

CC BY-SA 4.0
2021-12-04T09:42:03.727Z
With the greatest respect you start from a position that the Moon IS illuminated by the Sun whereas I start from the simple position of what I observe. Because of your presupposition you have to construct a very complicated method to support your belief. What I observe does not confirm that the Moon is illuminated by the Sun. I then compared the Sun- Moon relationship with a football and torch which is an accurate model its only the scale that is different.
Jackamus
CC BY-SA 4.0
2021-12-04T09:42:20.563Z
Continued from above:
Jackamus
CC BY-SA 4.0
2021-12-04T09:42:55.547Z
In order to try and accommodate your great-circle idea, which doesn't actually exist i.e. when looking at the sky a straight line can be drawn between any two points in the sky but why is then necessary to call it a 'great-circle'. I suggest that this idea only applies to the Sun and Moon. This being the case why doesn't this apply to any object i.e.mountains or tall buildings, attached to the Earth. Even though the earth be curved it is still a straight line.
Jackamus
CC BY-SA 4.0
2021-12-04T09:43:43.693Z
Concluded from above. A good example of this is the way lasers are used. We are told that a laser was used to reflect off a mirror placed on the Moon. Would this mean that an observer outside the Moon-Earth connection would see it as a curve?
There are two points I keep making that nobody has addressed directly and that is comparing the Moon-Sun with a football and torch and that it is only a question of scale.
Jackamus
CC BY-SA 4.0
2021-12-04T14:18:59.957Z
Have you tried your torch example? I predict that when you do, you will find the illumination is different, and your theory is not supported by observation. Secondly you need a theory for how the moon is lit if not by the Sun.
JohnHoltz
CC BY-SA 4.0
2021-12-05T16:48:39.983Z
Common sense tells me that if I set-up a torch and a football it would be possible to see/detect a straight line between them. Its just plain simple optics/physics. It applies to everything - all illuminated objects except the Moon.
Jackamus
CC BY-SA 4.0
2021-12-05T16:53:49.650Z
Continued - Just because I have stated that the Moon does not appear to be illuminated by the Sun does not mean that I have to, by default, offer an alternative explanation. How the Moon is actually illuminated, if not by the Sun, is for others to hypothesize. Also you have not commented on my two unanswered point in my previous comment.
Jackamus
CC BY-SA 4.0
2021-12-05T21:42:37.717Z
Sorry, but I have no idea what your "two unanswered points" are.
JohnHoltz
CC BY-SA 4.0
2021-12-07T08:28:32.593Z
The last sentence of the 5th paragraph above -'There are two points...'
Jackamus
CC BY-SA 4.0
2023-01-27T06:06:40.313Z
Jackamus, per your comment on 12/5/2021 @16:48, you should not have said common sense. You should have "misperception". Perceiving a straight line involves perceiving the 3-D angle to the line and the *distance* (radius) to the line. It is commonly known and thus common sense that you can NOT properly judge the distance to a line at celestial distances. Thus, you may misperceive how far away each segment of the line is. Then you end up thinking the line of illumination is curved - WHEN IT IS NOT. That is you perceiving it incorrectly, just like everyone else. Welcome to being human.
Bruzote
CC BY-SA 4.0
2021-12-02T21:45:11.120Z
2021-12-02T21:45:11.120Z
JohnHoltz
<p>Here is what you do. Look at the moon. See which way the sunlit face of the moon is pointing, not in a picture but in a real-life view of the moon. Using your own perception, trace a line INCREMENTALLY through space that goes out in the direction &quot;pointed to&quot; by the lit side is pointing. Keep tracing that line until your line hits an object bright enough to light the moon OR until the line extends past the horizon. This process ill eventually lead you across the sky to the sun or in the direction of the sun at the horizon. Every. Single. Time.</p>
<p>If you find otherwise, then you have an insufficient sense of three-dimensional direction and that requires either some kind of skills therapy or else just acceptance that you can't accurately imagine 3-D line tracing in the real world.</p>

CC BY-SA 4.0
2021-10-25T16:38:54.827Z
The issue is a "straight" line in the sky is not straight; for example, a line from the Moon to the Sun. A straight line in the sky is a great circle, and a great circle "arcs across the sky". This is the phenomenon that needs to be explained. The light from the Sun to the Moon is in the plane of the ecliptic (within 6 degrees of the ecliptic), and the ecliptic projected onto the sky is curve. After removing the distortions due to photographic lenses, the curving line of the ecliptic is exactly like the original photo. The illuminated part of the Moon points in the ecliptic plane to the Sun.
JohnHoltz
CC BY-SA 4.0
2021-10-25T16:59:34.723Z
@JphnHoltz - You are right. I realized that after my comment. Still, Jackamus should follow a great circle across the sky. Unfortunately, she doesn't seem to intuit why she should do that. Instead, she uses fallacies to discredit explanations that she will not understand. She clearly demonstrates frustration with explanations that are not convenient for her.  The reality is she simply doesn't "get" how she is confusing 2-D and 3-D geometry and line tracing. Further, she seems to want only validation of her emotions, not a logical explanation of her misperception.
Bruzote
CC BY-SA 4.0
2021-10-26T16:27:04.700Z
With a name like 'Jackamus' I would have thought that my gender was obvious - Male! Regarding the 'Great circle' it doesn't exist - there is no celestial sphere. I said this in earlier comments when someone mentioned it. The celestial sphere is simply a convenient way to plot stars with respect to each other. BTW I was a technical illustrator for over 30 years and part training was understanding perspective geometry and light ray tracing. So I do know the difference.
Jackamus
CC BY-SA 4.0
2021-10-26T22:50:07.897Z
Jackamus,- You have a good handle on perspective? Fine. Then see my explanatory comment under Eric Reiter's answer. Hopefully it puts to rest your confusion.
Bruzote
CC BY-SA 4.0
2021-10-27T20:25:34.950Z
Would someone care to explain to me why appeals to the 'celestial sphere' and the 'great circle' are constantly made when we all know it doesn't exist. It only enables you to plot on star relative to another star using spherical coordinates for the sake of convenience. It cannot be used to answer my Moon-Sun anomaly. Would someone please confirm this?
Jackamus
CC BY-SA 4.0
2023-01-27T05:59:28.203Z
Jackamus, they are not appeals but explanations limited by how the brain perceives. The brain can't see 3-dimensionally at large distances. PERIOD. That means you CAN'T judge 3D lines accurately since the points will be at varying, unperceived distances. Your binocular acuity is only good for a short distance, other visual cues like atmospheric attentuation help to extend that a bit. Your brain ONLY collects angular information about celestial objects. Thus, following straight lines between celestial objects means your eyes sweep across a "celestial sphere" of undefined ("infinite") radius.
Bruzote
CC BY-SA 4.0
2021-10-25T07:11:19.670Z
2021-10-25T07:11:19.670Z
Bruzote
<p>EDIT: I am making two simplifications which still display the &quot;anomaly&quot;.</p>
<ol>
<li>I consider the 2 dimensional case.</li>
<li>Sunbeams (blue lines) are nearly parallel, since the Sun is so far away.</li>
</ol>
<p>At sunset the observer can see the unlit part of the Moon at the bottom, so it appears illuminated from above.
<a href="https://i.sstatic.net/mz7U1.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mz7U1.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-10-17T10:07:30.717Z
Sorry but no. The observer will see exactly what you have drawn. The light coming from the right is perpendicular to  the illumination of the moon. The place of observation will make no difference to what is observed. In your drawing a second observer is observing the first observer and what that first observer sees.
Jackamus
CC BY-SA 4.0
2021-10-17T15:43:12.643Z
The observer will see the shaded portion of the moon at the bottom. The new moon always appears illuminated at the bottom at sunrise or sunset, for the same reason.
Keith McClary
CC BY-SA 4.0
2021-10-18T16:11:49.430Z
You still seem to be missing the point. It doesn't make any difference where I view the moon from. If it is being illuminated by the sun then there will be a perpendicular between the moon and the sun. I cannot understand why this is not obvious to you as indeed you would have to agree that a football and a torch would look just the same. The problem is, with the greatest respect, 'cognitive dissonance'. You are being shown an observable fact that conflicts with what you believe.
Jackamus
CC BY-SA 4.0
2021-10-18T16:45:59.797Z
"there will be a perpendicular between the moon and the sun" Usually we speak of a line perpendicular to another line (or perhaps a plane). What do you mean by this?
Keith McClary
CC BY-SA 4.0
2021-10-19T18:29:58.157Z
By 'perpendicular I mean a straight line drawn between the centre of the Moon and the centre of the Sun.
Jackamus
CC BY-SA 4.0
2021-10-15T20:05:54.900Z
2021-10-20T02:19:57.757Z
2021-10-20T02:19:57.757Z
Keith McClary
<p>I agree that this problem is very counterintuitive! However, the answer is very simple: <strong>the horizon is curved</strong> (it is actually a circle that passes behind us as well as in front of us).</p>

<p>Although it is curved, we generally perceive it as straight unless we specifically look for the curvature. This "correction" in our brains is what makes the angles seem wrong. You can see the curvature of the Earth anywhere there is a wide angle of open water (greater than 90 degrees is best) reaching the horizon. If you were standing on the moon and the moon's surface was smooth instead of mountainous, the curvature would be more apparent.</p>

<p>The apparent direction the lit side of the moon is facing will always be higher than the apparent position of the sun due to the convex nature of the horizon. The greater the angular distance between the sun and the moon, the greater the effect.</p>

<p>Another optical illusion explained the same way is that the apparent angles make it seem that the sun is as close as, or closer than the moon in your photo since the moon is at least half illuminated by the sun.</p>

CC BY-SA 4.0
2019-01-24T12:34:52.253Z
I think you are trying to make the facts fit the theory. Why can't you compare the anomaly with a torch shining on a football? Surely it is only a matter of scale.
Jackamus
CC BY-SA 4.0
2021-10-16T00:10:11.140Z
@Jackamus It's more like a nuclear blast shining on a grain of sand 1000 miles away.
f.thorpe
CC BY-SA 4.0
2021-10-17T10:00:08.463Z
Light travels in a straight line and the grain of sand will still be illuminated on a perpendicular line from the nuclear blast. The moon does not appear to be illuminated in this way.
Jackamus
CC BY-SA 4.0
2019-01-23T03:02:51.657Z
2019-01-23T03:02:51.657Z
CJ Dennis
<p>Below is a photo that my son took in Scotland showing the sun and moon at the same time. I immediately noticed this anomaly that the light illuminating the moon could not possibly come from the sun. I sent the photo to 4 University astronomy departments and only one responded and that was Cambridge University which is near where I live. The response came from the department librarian (not an astronomer) who said he had never heard of this before. He gave me two possible solutions, one was was from an engineer (not an astronomer) in which he got confused between perspective and light ray tracing and the other was referring to Einstein's theory of light bending by gravity. I check out Einstein and the effect was so small as to be almost immeasurable.</p>

<p><a href="https://i.sstatic.net/elFma.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/elFma.jpg" alt="enter image description here"></a></p>

<p>I have looked at the various 'complex' explanations for what to me is a very simple model.  What need is there to introduce 'curved planes' and 'starry sky domes' all of which do not exist in reality?
It is only referred to as an 'illusion' because observation doesn't fit the conventional model hence the complex explanations to try and make it work.
The anomaly is acknowledged to exist with or without photos. Since everyone believes that the moon is illuminated by the sun then simple normal physics do not seem to work. Either the physics is wrong or the sun does not illuminate the moon. I realise that is a heavy statement!</p>

<p>Therefore I state once again:</p>

<ol>
<li>The sun and the moon are two objects (like a torch and a football) that are suspended in a 3 dimensional space and size should not matter.</li>
<li>The moon/football are illuminated by the sun/torch and a perpendicular line or light ray can be drawn between them.</li>
<li>It doesn't matter where in space you choose to view them, a perpendicular line or light ray can still be drawn between them.</li>
</ol>

<p>This drawing explains my doubts:</p>

<p><a href="https://i.sstatic.net/zwQgm.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zwQgm.jpg" alt="enter image description here"></a></p>

<p>I'm very surprised that some of you have never noticed it before hence the suggestion asking me to post a video. This is a very common occurrence and I have seen it many many times as I go for my morning walk at about 8.00am every morning. I have never thought of actually tabulating my observations. </p>

CC BY-SA 4.0
2018-08-08T13:36:22.320Z
One thing that just stood out to me... is that the moon seems more lit than I'd expect from being as close to the sun as the picture hints visually. I did find in further investigation that the moon on 5/16/16 should've been roughly 135 degrees from the sun in the sky ([suncalc path](http://suncalc.net/#/56.4907,-4.2026,6/2016.05.16/15:58), [mooncalc path](https://www.mooncalc.org/#/56.4907,-4.2026,6/2016.05.16/15:58/1/0)).
JeopardyTempest
CC BY-SA 4.0
2018-08-08T13:37:00.390Z
Given the picture has some vertical tilt (up)... I would think the image should have a curved ground if a panorama, or is using some sort of lens to adjust the curvature maybe?  I wonder if that might be a key factor?
JeopardyTempest
CC BY-SA 4.0
2018-08-09T20:09:14.687Z
Some of the comments on [this post](https://meta.stackexchange.com/questions/262264/uploading-images-that-are-too-large) might help?  How large is the image you're trying to attach?  I don't think there was a real mixup, as you've done properly as far as I can tell.  We'll see if we can get some additional input... I'm considering if I can offer anything useful on the illusion posted in the one answer, it does seem to have some bearing to me.  But hopefully someone really passionate about optics and physics can help give something really understandable to us all.
JeopardyTempest
CC BY-SA 4.0
2018-08-23T13:45:14.593Z
The reason this question isn't getting much attention here might be that there is a separate Astronomy Stack Exchange site. It might suite better there...
Communisty
CC BY-SA 4.0
2018-08-24T15:06:27.743Z
Thank you for voting to reinstate it. However with the greatest respect to members of this forum but I find its not very user friendly. The explanations offered for the anomaly do seem to hold water and that it why I produced a new drawing and restating that it cannot be an illusion.
Jackamus
CC BY-SA 4.0
2018-08-26T18:28:50.530Z
Did you mean to say "do not seem"?  Good to remember that the ultimate goal of this site isn't discussion, but to make the ultimate answers easily digestible, so people can quickly learn (especially useful on computer programming related questions, where I constantly find the site invaluable... but also has its benefits in preventing questions ending up being opinions.)  It still awaits a definitive answer unfortunately... but it's a little rough you're dismissive of most attempts at answering.  Many times in life I've thought people wrong only to later finally better understand what they said
JeopardyTempest
CC BY-SA 4.0
2018-08-27T12:52:04.297Z
Yes I did mean "do not seem to hold water". Why is so difficult to accept that a perpendicular line can be drawn from the ball to the torch but not from the moon to the sun? Nobody has commented on my statement that curved surfaces in space do not actually exist or my drawing?
Jackamus
CC BY-SA 4.0
2018-10-05T16:32:39.213Z
Can I ask if this was taken as a simple photograph, or did it use the "panorama" feature that many modern digital cameras have (where you hold the shutter while you scan the camera across the scene, which is then digitally combined into a single image)? This might be relevant as this can introduce additional distortions. It might be worth doing an experiment: With the same camera with the same setting, do a similar photograph in a city where house walls are all vertical. This would give an idea of the distortions.
uUnwY
CC BY-SA 4.0
2018-10-06T17:10:47.900Z
The panorama photo that was shown was merely to illustrate what I observed. I have two more photos that I took of the moon and the sun separately to avoid using the 'panorama' argument against. My observation showed the sun at a different position from that which was illuminating the moon. This should be simple and no different from using a torch and a football except for scale.
Jackamus
CC BY-SA 4.0
2021-10-15T20:17:41.640Z
If you wanted an answer from an astronomer it would have been better to have posted the question on SE **Astronomy**, not SE **Earth Science**.
Fred
CC BY-SA 4.0
2021-10-16T00:06:45.090Z
The "direction of illumination" arrow you have drawn is wrong... the arrow should be pointing directly at the observer (us), perpendicular to the image... not pointing "left" as if the sun was next to the Earth/Moon system.  The sun is 93 million miles away... and it's really big.
f.thorpe
<sun><astronomy><visualization><earth-moon>
What is this Sun and moon photographic anomaly?
Jackamus
# What is this Sun and moon photographic anomaly?

(Asked by: username_0 on 2018-08-08)

Below is a photo that my son took in Scotland showing the sun and moon at the same time. I immediately noticed this anomaly that the light illuminating the moon could not possibly come from the sun. I sent the photo to 4 University astronomy departments and only one responded and that was Cambridge University which is near where I live. The response came from the department librarian (not an astronomer) who said he had never heard of this before. He gave me two possible solutions, one was was from an engineer (not an astronomer) in which he got confused between perspective and light ray tracing and the other was referring to Einstein's theory of light bending by gravity. I check out Einstein and the effect was so small as to be almost immeasurable.

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

I have looked at the various 'complex' explanations for what to me is a very simple model. What need is there to introduce 'curved planes' and 'starry sky domes' all of which do not exist in reality?
It is only referred to as an 'illusion' because observation doesn't fit the conventional model hence the complex explanations to try and make it work.
The anomaly is acknowledged to exist with or without photos. Since everyone believes that the moon is illuminated by the sun then simple normal physics do not seem to work. Either the physics is wrong or the sun does not illuminate the moon. I realise that is a heavy statement!

Therefore I state once again:

1. The sun and the moon are two objects (like a torch and a football) that are suspended in a 3 dimensional space and size should not matter.
2. The moon/football are illuminated by the sun/torch and a perpendicular line or light ray can be drawn between them.
3. It doesn't matter where in space you choose to view them, a perpendicular line or light ray can still be drawn between them.

This drawing explains my doubts:

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

I'm very surprised that some of you have never noticed it before hence the suggestion asking me to post a video. This is a very common occurrence and I have seen it many many times as I go for my morning walk at about 8.00am every morning. I have never thought of actually tabulating my observations.

**Tags:** sun, astronomy, visualization, earth-moon

> **Comments:**
> **username_1**: One thing that just stood out to me... is that the moon seems more lit than I'd expect from being as close to the sun as the picture hints visually. I did find in further investigation that the moon on 5/16/16 should've been roughly 135 degrees from the sun in the sky ([suncalc path](http://suncalc.net/#/56.4907,-4.2026,6/2016.05.16/15:58), [mooncalc path](https://www.mooncalc.org/#/56.4907,-4.2026,6/2016.05.16/15:58/1/0)).  
> **username_1**: Given the picture has some vertical tilt (up)... I would think the image should have a curved ground if a panorama, or is using some sort of lens to adjust the curvature maybe?  I wonder if that might be a key factor?  
> **username_1**: Some of the comments on [this post](https://meta.stackexchange.com/questions/262264/uploading-images-that-are-too-large) might help?  How large is the image you're trying to attach?  I don't think there was a real mixup, as you've done properly as far as I can tell.  We'll see if we can get some additional input... I'm considering if I can offer anything useful on the illusion posted in the one answer, it does seem to have some bearing to me.  But hopefully someone really passionate about optics and physics can help give something really understandable to us all.  
> **username_2**: The reason this question isn't getting much attention here might be that there is a separate Astronomy Stack Exchange site. It might suite better there...  
> **username_0**: Thank you for voting to reinstate it. However with the greatest respect to members of this forum but I find its not very user friendly. The explanations offered for the anomaly do seem to hold water and that it why I produced a new drawing and restating that it cannot be an illusion.  
> **username_1**: Did you mean to say "do not seem"?  Good to remember that the ultimate goal of this site isn't discussion, but to make the ultimate answers easily digestible, so people can quickly learn (especially useful on computer programming related questions, where I constantly find the site invaluable... but also has its benefits in preventing questions ending up being opinions.)  It still awaits a definitive answer unfortunately... but it's a little rough you're dismissive of most attempts at answering.  Many times in life I've thought people wrong only to later finally better understand what they said  
> **username_0**: Yes I did mean "do not seem to hold water". Why is so difficult to accept that a perpendicular line can be drawn from the ball to the torch but not from the moon to the sun? Nobody has commented on my statement that curved surfaces in space do not actually exist or my drawing?  
> **username_3**: Can I ask if this was taken as a simple photograph, or did it use the "panorama" feature that many modern digital cameras have (where you hold the shutter while you scan the camera across the scene, which is then digitally combined into a single image)? This might be relevant as this can introduce additional distortions. It might be worth doing an experiment: With the same camera with the same setting, do a similar photograph in a city where house walls are all vertical. This would give an idea of the distortions.  
> **username_0**: The panorama photo that was shown was merely to illustrate what I observed. I have two more photos that I took of the moon and the sun separately to avoid using the 'panorama' argument against. My observation showed the sun at a different position from that which was illuminating the moon. This should be simple and no different from using a torch and a football except for scale.  
> **username_4**: If you wanted an answer from an astronomer it would have been better to have posted the question on SE **Astronomy**, not SE **Earth Science**.  
> **username_5**: The "direction of illumination" arrow you have drawn is wrong... the arrow should be pointing directly at the observer (us), perpendicular to the image... not pointing "left" as if the sun was next to the Earth/Moon system.  The sun is 93 million miles away... and it's really big.  

---

## 7 answers

### Answer by username_6 on 2018-08-08. Score: 21

The apparent anomaly, known as the Lunar Terminator or Moon Tilt Illusion, is indeed a matter of perspective. A brief explanation can be found here: <http://chrisjones.id.au/MoonIllusion/>

The essence: 'The illusion occurs when the moon and sun are separated by a wide angle, so that they are perceived relative to the horizon, as if in a panorama. A panoramic photograph is a cylindrical projection. In this projection, most straight lines project as sinusoidal curves. The moon-sun line is curved, unless the moon and sun are on the horizon or directly above one another.'

A long, technical explananation is in the PDF [The moon tilt illusion](https://www.seas.upenn.edu/~amyers/MoonPaper20June.pdf). Quoting from page 21:

> Modern cameras use lenses whose properties are designed to deliver a rectilinear
> or curvilinear image. Rectilinear lenses reduce barrel or pincushion distortion from
> the image but such lenses are difficult to manufacture for the wide angles (90°
> and above) needed to record both the sun and the moon on a single photograph. A
> photograph [3] of the moon and sun at an azimuth difference of 80°
> containing a
> leaning tower and unnaturally leaning trees illustrates the difficulty of eliminating
> distortion in a wide-angle photograph

That reference [3] is to [A Different Moon Illusion](http://www.astropix.com/html/l_story/moonill.html) on Jerry Lodriguss astrophotography site *Catching the light* where he shows and explains the exact same situation (picture taken from there):

[![enter image description here](https://i.sstatic.net/NOym6.jpg)](https://i.sstatic.net/NOym6.jpg)

> **Comments:**
> **username_0**: Since the phenomena can be clearly seen by the naked eye it is therefore a real phenomena and that appeals to photographic optical illusions are not necessary. If you were to draw on a blackboard what was observed then optical illusions are ruled out.  
> **username_7**: A phenomenon that you observe, incorrectly, with the naked eye is the definition of an optical illusion. This answer explains how the illusion occurs, in spite of what you think you are seeing.  
> **username_0**: If a torch were placed on a pole coincident with the sun and a football also on a pole and coincident with the moon, the torch would not illuminate the football. I think the reason it is called an illusion is because it doesn't fit the standard paradigm.  
> **username_0**: The idea that this is being called an illusion is because it doesn't fit the official narrative because everyone and his dog KNOWS the Sun illuminates the Moon. It is impossible for this accepted Moon-Sun system to be viewed in any other way than the way I have been maintaining. A bright light illuminating a distant smaller spherical object can only behave in the way that a smaller light (torch) and smaller closer spherical object (football).  
> **username_0**: Out there in space there are no curved planes or celestial spheres on which to build an explanation to try and compensate for the obvious anomaly. In space there is no up or down or left or right. Whatever you observe is what is actually there. Viewing it from a position here on Earth will make no difference to what is observed - a light shining on a sphere.  
> **username_0**: If only the illuminated sphere is observed then it is a simple extrapolation to determine from where it is being illuminated. You have to make a choice between using common sense and basic physics or denying them in order to protect the status quo by appealing to optical illusion. There was one animation on this thread that showed a Moon and Sun projected onto a vertical curved plane and a curved line connecting the Moon and Sun.  
> **username_0**: It then animated the curved plane into a flat vertical plane showing a straight line between the Moon and Sun. This part of the animation was showing reality whilst the first part (curved plane) was showing a false way to explain the anomaly. Is it possible to add jpg graphic to a Comment?  

---

### Answer by username_8 on 2021-10-14. Score: 8

[![enter image description here](https://i.sstatic.net/WvOM5.jpg)](https://i.sstatic.net/WvOM5.jpg)Here my wife holds a globe with the moon in view. I carefully processed in a bit-mode to find the curve of light on the globe and drew perpendicular lines. Plainly we can see that the moon-lite side is facing the sun in the same direction of my globe.

The illusion is due to the sun being so far that the difference in angles shown in the drawing would not be noticeable. Imagine two planes where both of them contain the sun and moon, the level plane would be how an observer would see the moon when both the moon and the sun were at the horizon. The tilted plane is how an observer would normally see the moon high in the sky even though the sun was at the horizon during sunset. The observers on both planes would see the same half-moon shape even though the relative heights in the sky vary greatly. This construct might only work for quarter moon illuminated.[![enter image description here](https://i.sstatic.net/S2QrE.jpg)](https://i.sstatic.net/S2QrE.jpg)

> **Comments:**
> **username_0**: You seem to agree that one would expect the moon to be tilted more towards the sun but it isn't. The moon's illumination seems to be coming from another direction. I realise that what I am saying goes against what everyone believes but that may be the problem - it is a 'belief' that the sun illuminates the moon and not a fact.  
> **username_9**: @username_0 It is very much fact that the moon is illuminated by the sun. https://www.livescience.com/45979-why-does-the-moon-shine.html  
> **username_10**: Humans can't judge linear metrics beyond 8km. So, when a person visualizes the location of a light source illuminating the moon, the problem is 2D only, involving ORTHOGONAL ANGULAR DISPLACEMENTS (e.g, right ascension/declination, altitude/azimuth, etc.).   One traces a "line" from the moon's day/night terminator, orthogonal to its midpoint. This "line" is actually a 2-D "HALF-INFINITE" TRIANGULAR PLANAR SECTION which continues from moon to the light source, with the observer at the vertex.  In this case, the moon is on the left edge, and the section continues rightward UNTIL a light source!  
> **username_10**: @username_0, per my previous comment, one's eyes cannot perceive distance. Thus, it is only an optical illusion that leads to one to think one perceives that an imaginary "line" from the moon is receding into a point in space. One can only see in angles at astronomical distances. So, where would one stop when sweeping from the terminator in the direction of the light source? Well, one stops at the light source! One must not stop before then, because literally there is no justification to stop. Any perception of a "line" in 3-D space that has terminated is a self-imposed illusion.  
> **username_0**: Looking at the link offered by username_9 username_9 above I noticed that the intensity of the light on the moon's surface is even right across its face. If the Moon is a sphere shouldn't the intensity of the reflected light fall-off towards the edges?  
> **username_0**: Comment to username_10 - Creating a triangle plane between me, the Moon and the Sun will not actually change anything. All you are doing is simply joining-up the three points with a line which means that at my position I will be observing a straight line between the Moon and Sun as I have been saying all along. Claiming it to be an illusion is a false way to try and explain why what is observed is different from what is believed - I use the word 'believed' deliberately.  
> **username_0**: A further comment to username_10 - You are trying to complicate a very simple property of physics with complicated geometry which finishes up, not demonstrating anything but stating it is an illusion. You are trying to use scale as a 'Get out of jail card'. Supposing I showed you a photo of a half illuminated sphere without any reference to its size (football or planet). How would you go about determining the line that would lead you to the light source?  
> **username_10**: I would determine the line as I said by tracing an arc that subtends that line. Start by pointing your arm and finger at the lit object (e.g. moon). Now imagine a vector extending perpendicular to that lit hemisphere. Start sweeping your finger and arm in an arc to trace that vector. **Note: Because the line you are tracing is in a plane involving you, the object, and the light source, tracing that line with your finger can only be done by continuing to sweep in that plane.** If you keep sweeping in that plane, you follow the line and eventually point at the light source. It works every time.  

---

### Answer by username_11 on 2021-10-20. Score: 4

You can see this effect on the powerline above the railroad tracks in the picture you posted too, it doesn't look straight, even though it likely was in reality! The straight line between the sun and the moon is just even *more* curved. Just like what you see at <http://chrisjones.id.au/MoonIllusion/> , like the top answer already posted. If you accept that the curvature of the powerline on this picture is just an optical illusion/distortion introduced by the camera, then so is the sun-moon illumation!

[![the picture curves straight lines](https://i.sstatic.net/TqHHU.png)](https://i.sstatic.net/TqHHU.png)

The error in your diagram is that you assume a rectangular and linear (0, 1 or 2 point foreshortening) perspective. As you can see [in this VSauce video](https://youtu.be/Y2gTSjoEExc?t=190), straight lines in our surroundings actually become curved lines from a single eye (or photocamera) perspective. After you've seen the VSauce video, you can check that this holds true for you too: if you rotate your head, you'll notice that the place where your wall and ceiling meet will curve when you're moving from side to side. Straight lines, when viewed from below from left to right cease to be straight! A 5-point curved perspective better captures the way straight lines appear to us.

[![5 point perspective](https://i.sstatic.net/X9lKd.jpg)](https://i.sstatic.net/X9lKd.jpg)

Note, by the way, that even from a "torch" perspective, as you insist, and without any optical illusion, we can get the moon shining "upward". Consider the following overview picture: the moon (left object) is illuminated by the sun (right object) in exactly the way you expect. Now, however, consider that you are an observer (where the two red lines converge), in particular, that you are standing *below* the sun and moon. As you know, if you stand below things, you cannot see what is on top -- and similarly, you can see more of what is on the bottom of that object. That is what's drawn in the dotted lines from the observer view: they tangentially hit the moon on the dark side on the bottom, and cannot observe the top of the moon. Just because we're below that object.

[![Overview picture of moon and sun](https://i.sstatic.net/VaU9B.png)](https://i.sstatic.net/VaU9B.png)

Thus, if you place yourself in the position of the observer, the objects will appear as if they have both rotated away from the horizon, because we can see **both** bottoms of the objects!

If you put the two effects together (one, the strange effects of panning from left to right to lines above us; and the effect of being located below the moon), we end up with the following effect, where it appears that the "moon" is facing upwards to the right in the first few frames when we're looking to the left, but once we zoom out you can see that the sun is actually level with the moon!

[![sun-moon system](https://i.sstatic.net/EhVnt.gif)](https://i.sstatic.net/EhVnt.gif)

The effect nearly disappears entirely once I add a line connecting the two bodies:

[![sun-moon system with added lines](https://i.sstatic.net/2DM0E.gif)](https://i.sstatic.net/2DM0E.gif)

Now the effect looks like it isn't even there anymore, because somehow our brains are able to make sense of the perspective. Note how in every single picture, the lines are all straight (so, no trickery with wide-angle or fish-eye lenses), but when we rotate from left to right, they seem to "change direction" from pointing northeast to northwest and back.

> **Comments:**
> **username_0**: This answer is in response to username_8 as a 'Comment' was rejected for being too long.  username_8 I think your animation is flawed. What you do not take into account is the observing eye level (horizon) is always coincident with a straight (perpendicular line) between the centre of the Moon and the Sun. You cannot be looking straight ahead with the Moon and Sun above your eye level (possibly out of your area of vision) and observe the same thing as when you tilt your eyes upwards to see the underside of the Moon and Sun (now in the centre of your area of vision). To be continued in another comment.  
> **username_0**: Your animation is not showing what you will actually observe when you move your eyes to keep the Moon and Sun always in the centre of your vision. This is why wide angle lens cameras show curved lines at their peripherals  Imagine that the centre of the Moon (football) has a rigid bar connected to the centre of the Sun (torch) no matter where your position of observation is that rigid bar will always be straight and not curved. It will only appear curved when you move your head to observe them with your peripheral vision just as you would with a wide angle lens at its edges. to be continued.  
> **username_0**: Therefore your animation is showing what would be observed through a wide angle lens that moves up and down. If you kept them in the centre of your vision as you move up and down then the rigid bar would always be straight and not curved.  This can be confirmed by looking at some NASA footage that shows the Earth's curve changing from concave to horizontal to convex as the camera moves up and down just like your animation. End of comment.  
> **username_11**: @username_0: Okay, so I have replaced the two animations with two new ones, which hopefully show the full effect. You can see a moon (on the left) that appears to "point upwards", but when zooming out the effect disappears. Even if the sun/moon are level with the camera (so, we're still looking up, but they are positioned in the center line of vision) as in https://imgur.com/a/W1aVBEy, you can see that the effect persists.  
> **username_12**: I think those new animations are the most intuitive answer I've seen on here (and they match the answer I never got around to posting 3 years ago).  Note you can easily reproduce this effect IRL by looking up at a wall/ceiling join through a directors viewfinder formed by your hands and then swivelling.  
> **username_0**: Your argument still doesn't stand up. If you zoom out so that you can see the sun and the moon at the same time they will be joined by the purple line between their respective centres thus confirming what I have been saying. It doesn't matter where in space you view the system there will always be a direct line between their contres if one illuminates the other.  
> **username_11**: @username_0 It'd be helpful to make sure you agree (or not) on two points first. (1) do you agree that the powerline in your picture appears more curved than it would be in real life? (2) do you agree that in the zoomed-in perspective (without lines present) it kind of looks like the "moon" is illuminated from above-right rather than from just from the right? These are relevant points for me!  
> **username_0**: Yes I agree that the power lines look curved because of the wide angle lens effect. The zoomed-in image of the Moon shows that it is lit from slightly above. It is that angle that is the problem. It is not possible to observe a straight light ray between the centre of the Sun and the centre of the Moon because of the Moon's upward facing lit area. Can please confirm that must be a straight line light ray that is perpendicular from the centre of the two bodies and which causes normal illumination as would a torch and a football. As yet nobody has acknowledged this to be a given truth.  
> **username_11**: @username_0 I agree that the moon should be (and is) illuminated by straight rays following the shortest path from the sun to the moon. Just like a torch and a football. But doesn't the animation show that the moon (or football) might *seem* to be lit from above (from a zoomed in perspective below the object, looking to the side), even when it really is lit from the side only (as visible from a zoomed out perspective)? To me, the animation [and curved power line] confirms that your picture is an optical illusion: a stitched-together picture of the zoomed-in perspective on the moon.  
> **username_11**: @username_0 Next time when you see the sun and the moon together, don't take a picture but take a piece of string between your left and right hands. Hold your left hand at any point between your eyes and the moon, and your right hand at a point between your eyes and the sun. Following that piece of string, it should be a straight line from the moon's illumination towards the sun. Just like the first frame in the animation shows a straight line [https://imgur.com/FsPoa2g], perpendicular to the moon's terminator, which goes straight to the sun. That should prove the whole issue.  
> **username_11**: @username_0 from https://www.seas.upenn.edu/~amyers/MoonPaperOnline.pdf, “...the line connecting the horns of the moon, between its first quarter and full moon, for instance, does not appear to be at all perpendicular to the direction from sun to moon; we apparently think of this direction as being a curved line. Fix this direction by stretching a piece of string taut in front of your eye; however unlikely it may have seemed to you at first you will now perceive that the condition of perpendicularity is satisfied”  
> **username_0**: I haven't tried the string idea because trying to sight one end of the string on Moon whilst trying to sight the other end on the Sun would be very difficult to do with any accuracy. This is because I cannot view the Moon and the Sun at the same time and the chance of my hands moving are very great. You accept the torch-football idea so why not Moon - Sun?  
> **username_0**: I know why appeals are being made to it being an illusion is because observation does not fit the official narrative. If some suspended a huge light in the sky say over a football stadium, that was coincident with the Sun and the same with a huge ball coincident with the moon and viewed from a seat in the stands, what you would observe is the ball being directly illuminated by the light but it would not look the same as the Moon-Sun observation. Can someone explain why the football - torch arrangement is different from the Moon - Sun arrangement?  
> **username_11**: @username_0 Try harder, you only have to move your head while keeping your hands (nearly) at the same spot. Or get a friend to do it. There's no way you'll find an effect as large as what your picture suggests. The other questions are amusing me, because they are exactly the point: the effect exists *because* you cannot see the sun and moon at the same time. If you could, you would realize that the moon isn't illuminated from seemingly "above" at all. It would be the same with a torch/football setup, if you would set them up s.t. you couldn't see both at the same time, you'd see the same effect!  

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### Answer by username_13 on 2021-12-02. Score: 3

When looking at the sky, spherical coordinates are used as a convenience. We turn our head and body in azimuth and altitude to compare the position of objects. In this regards, the sky looks like a sphere. (At least I visualize it as a sphere.) A Cartesian XY (or XYZ) grid cannot be easily used.

The question then becomes: what is the path of a light beam in spherical coordinates? Realizing that light takes the shortest distance from point A to point B, another way to phrase the question is **what is the shortest distance between two points in the sky? The answer to the re-phrase question is a great circle**.

As username_10's Oct 25 2021 answer was indicating, the problem is it is difficult to visualize a great circle on the sky, and the path created by a great circle is not intuitive to most people (including me). Fortunately, you can easily observe where one particular great circle exists and solve the “Moon illumination anomaly” at the same time: go out and observe the Moon! The Moon orbits the Earth in a plane, and the intersection of the orbital plane and the sky is a great circle.

For example, observe the position of the Moon for several days starting a few days after New Moon. Dec 5 through the 13 2022 is a good opportunity. Make the first observation just before or at sunset and observe 4 minutes earlier on each succeeding day. (The 4 minute difference compensates for the Earth's motion around the Sun. Unless your plot of the Moon's position in the sky is precise to a fraction of a degree, you do not need to be so precise with your timing.) If you plot the position of the Moon, it will look something like the following plot: (created from [Sky and Telescope's Interactive Sky Chart](https://skyandtelescope.org/interactive-sky-chart/))

[![Moon's changing position Dec 5 to Dec 13 2021](https://i.sstatic.net/T9XdJ.png)](https://i.sstatic.net/T9XdJ.png)

Note that:

1. The “straight line” of the great circle is a curve going across the sky. It does not “feel” straight. (The black line in the figure is the ecliptic, which is the great circle created by the Sun’s apparent path for 1 year. The Moon’s orbit is inclined approximately 6 degrees to the ecliptic -- close enough for reference.)
2. The illuminated face of the Moon points toward the position of the Moon on the previous day.
3. When connected together, the illuminated directions make a “line” (technically a great circle) that connects to the Sun. That is, the illuminated direction is in the plane of the Moon’s orbit and always faces the Sun. (The Moon's great circle crosses the Sun when there is an eclipse. At other times, the great circle may pass above or below the Sun by 6 degrees.)

On May 16 2016, the all-sky plot looks like the following:

[![May 16 2016 sky](https://i.sstatic.net/li4OZ.png)](https://i.sstatic.net/li4OZ.png)

When shown in a horizon projection, the image looks like the following. The illumination on the Moon appears to be coming from the upper right because that is the shortest direction to the Sun!

[![May 16 2016 horizon](https://i.sstatic.net/iskmJ.png)](https://i.sstatic.net/iskmJ.png)

All images are approximations, just like the original photograph, due to projections from 3D onto 2D. The last image is stitched from several images which introduces more distortions.

> **Comments:**
> **username_0**: With the greatest respect you start from a position that the Moon IS illuminated by the Sun whereas I start from the simple position of what I observe. Because of your presupposition you have to construct a very complicated method to support your belief. What I observe does not confirm that the Moon is illuminated by the Sun. I then compared the Sun- Moon relationship with a football and torch which is an accurate model its only the scale that is different.  
> **username_0**: Continued from above:  
> **username_0**: In order to try and accommodate your great-circle idea, which doesn't actually exist i.e. when looking at the sky a straight line can be drawn between any two points in the sky but why is then necessary to call it a 'great-circle'. I suggest that this idea only applies to the Sun and Moon. This being the case why doesn't this apply to any object i.e.mountains or tall buildings, attached to the Earth. Even though the earth be curved it is still a straight line.  
> **username_0**: Concluded from above. A good example of this is the way lasers are used. We are told that a laser was used to reflect off a mirror placed on the Moon. Would this mean that an observer outside the Moon-Earth connection would see it as a curve? There are two points I keep making that nobody has addressed directly and that is comparing the Moon-Sun with a football and torch and that it is only a question of scale.  
> **username_13**: Have you tried your torch example? I predict that when you do, you will find the illumination is different, and your theory is not supported by observation. Secondly you need a theory for how the moon is lit if not by the Sun.  
> **username_0**: Common sense tells me that if I set-up a torch and a football it would be possible to see/detect a straight line between them. Its just plain simple optics/physics. It applies to everything - all illuminated objects except the Moon.  
> **username_0**: Continued - Just because I have stated that the Moon does not appear to be illuminated by the Sun does not mean that I have to, by default, offer an alternative explanation. How the Moon is actually illuminated, if not by the Sun, is for others to hypothesize. Also you have not commented on my two unanswered point in my previous comment.  
> **username_13**: Sorry, but I have no idea what your "two unanswered points" are.  
> **username_0**: The last sentence of the 5th paragraph above -'There are two points...'  
> **username_10**: username_0, per your comment on 12/5/2021 @16:48, you should not have said common sense. You should have "misperception". Perceiving a straight line involves perceiving the 3-D angle to the line and the *distance* (radius) to the line. It is commonly known and thus common sense that you can NOT properly judge the distance to a line at celestial distances. Thus, you may misperceive how far away each segment of the line is. Then you end up thinking the line of illumination is curved - WHEN IT IS NOT. That is you perceiving it incorrectly, just like everyone else. Welcome to being human.  

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### Answer by username_10 on 2021-10-25. Score: 1

Here is what you do. Look at the moon. See which way the sunlit face of the moon is pointing, not in a picture but in a real-life view of the moon. Using your own perception, trace a line INCREMENTALLY through space that goes out in the direction "pointed to" by the lit side is pointing. Keep tracing that line until your line hits an object bright enough to light the moon OR until the line extends past the horizon. This process ill eventually lead you across the sky to the sun or in the direction of the sun at the horizon. Every. Single. Time.

If you find otherwise, then you have an insufficient sense of three-dimensional direction and that requires either some kind of skills therapy or else just acceptance that you can't accurately imagine 3-D line tracing in the real world.

> **Comments:**
> **username_13**: The issue is a "straight" line in the sky is not straight; for example, a line from the Moon to the Sun. A straight line in the sky is a great circle, and a great circle "arcs across the sky". This is the phenomenon that needs to be explained. The light from the Sun to the Moon is in the plane of the ecliptic (within 6 degrees of the ecliptic), and the ecliptic projected onto the sky is curve. After removing the distortions due to photographic lenses, the curving line of the ecliptic is exactly like the original photo. The illuminated part of the Moon points in the ecliptic plane to the Sun.  
> **username_10**: @JphnHoltz - You are right. I realized that after my comment. Still, username_0 should follow a great circle across the sky. Unfortunately, she doesn't seem to intuit why she should do that. Instead, she uses fallacies to discredit explanations that she will not understand. She clearly demonstrates frustration with explanations that are not convenient for her.  The reality is she simply doesn't "get" how she is confusing 2-D and 3-D geometry and line tracing. Further, she seems to want only validation of her emotions, not a logical explanation of her misperception.  
> **username_0**: With a name like 'username_0' I would have thought that my gender was obvious - Male! Regarding the 'Great circle' it doesn't exist - there is no celestial sphere. I said this in earlier comments when someone mentioned it. The celestial sphere is simply a convenient way to plot stars with respect to each other. BTW I was a technical illustrator for over 30 years and part training was understanding perspective geometry and light ray tracing. So I do know the difference.  
> **username_10**: username_0,- You have a good handle on perspective? Fine. Then see my explanatory comment under username_8 username_8's answer. Hopefully it puts to rest your confusion.  
> **username_0**: Would someone care to explain to me why appeals to the 'celestial sphere' and the 'great circle' are constantly made when we all know it doesn't exist. It only enables you to plot on star relative to another star using spherical coordinates for the sake of convenience. It cannot be used to answer my Moon-Sun anomaly. Would someone please confirm this?  
> **username_10**: username_0, they are not appeals but explanations limited by how the brain perceives. The brain can't see 3-dimensionally at large distances. PERIOD. That means you CAN'T judge 3D lines accurately since the points will be at varying, unperceived distances. Your binocular acuity is only good for a short distance, other visual cues like atmospheric attentuation help to extend that a bit. Your brain ONLY collects angular information about celestial objects. Thus, following straight lines between celestial objects means your eyes sweep across a "celestial sphere" of undefined ("infinite") radius.  

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### Answer by username_14 on 2021-10-15. Score: 0

EDIT: I am making two simplifications which still display the "anomaly".

1. I consider the 2 dimensional case.
2. Sunbeams (blue lines) are nearly parallel, since the Sun is so far away.

At sunset the observer can see the unlit part of the Moon at the bottom, so it appears illuminated from above.
[![enter image description here](https://i.sstatic.net/mz7U1.jpg)](https://i.sstatic.net/mz7U1.jpg)

> **Comments:**
> **username_0**: Sorry but no. The observer will see exactly what you have drawn. The light coming from the right is perpendicular to  the illumination of the moon. The place of observation will make no difference to what is observed. In your drawing a second observer is observing the first observer and what that first observer sees.  
> **username_14**: The observer will see the shaded portion of the moon at the bottom. The new moon always appears illuminated at the bottom at sunrise or sunset, for the same reason.  
> **username_0**: You still seem to be missing the point. It doesn't make any difference where I view the moon from. If it is being illuminated by the sun then there will be a perpendicular between the moon and the sun. I cannot understand why this is not obvious to you as indeed you would have to agree that a football and a torch would look just the same. The problem is, with the greatest respect, 'cognitive dissonance'. You are being shown an observable fact that conflicts with what you believe.  
> **username_14**: "there will be a perpendicular between the moon and the sun" Usually we speak of a line perpendicular to another line (or perhaps a plane). What do you mean by this?  
> **username_0**: By 'perpendicular I mean a straight line drawn between the centre of the Moon and the centre of the Sun.  

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### Answer by username_15 on 2019-01-23. Score: -1

I agree that this problem is very counterintuitive! However, the answer is very simple: **the horizon is curved** (it is actually a circle that passes behind us as well as in front of us).

Although it is curved, we generally perceive it as straight unless we specifically look for the curvature. This "correction" in our brains is what makes the angles seem wrong. You can see the curvature of the Earth anywhere there is a wide angle of open water (greater than 90 degrees is best) reaching the horizon. If you were standing on the moon and the moon's surface was smooth instead of mountainous, the curvature would be more apparent.

The apparent direction the lit side of the moon is facing will always be higher than the apparent position of the sun due to the convex nature of the horizon. The greater the angular distance between the sun and the moon, the greater the effect.

Another optical illusion explained the same way is that the apparent angles make it seem that the sun is as close as, or closer than the moon in your photo since the moon is at least half illuminated by the sun.

> **Comments:**
> **username_0**: I think you are trying to make the facts fit the theory. Why can't you compare the anomaly with a torch shining on a football? Surely it is only a matter of scale.  
> **username_5**: @username_0 It's more like a nuclear blast shining on a grain of sand 1000 miles away.  
> **username_0**: Light travels in a straight line and the grain of sand will still be illuminated on a perpendicular line from the nuclear blast. The moon does not appear to be illuminated in this way.  

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