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

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

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.

  • $\begingroup$ 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, mooncalc path). $\endgroup$ – JeopardyTempest Aug 8 '18 at 13:36
  • $\begingroup$ 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? $\endgroup$ – JeopardyTempest Aug 8 '18 at 13:37
  • $\begingroup$ Some of the comments on this post 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. $\endgroup$ – JeopardyTempest Aug 9 '18 at 20:09
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    $\begingroup$ 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... $\endgroup$ – Communisty Aug 23 '18 at 13:45
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    $\begingroup$ 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 $\endgroup$ – JeopardyTempest Aug 26 '18 at 18:28

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. 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 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

  • $\begingroup$ 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. $\endgroup$ – Jackamus Aug 8 '18 at 16:47
  • $\begingroup$ 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. $\endgroup$ – haresfur Aug 28 '18 at 21:53
  • $\begingroup$ 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. $\endgroup$ – Jackamus Oct 6 '18 at 17:15

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.

  • $\begingroup$ 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. $\endgroup$ – Jackamus Jan 24 '19 at 12:34

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