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Some quick googling has given me the answer to whether seeing part of the moon that receives no sunlight is an optical illusion or not: it isn't. Earthshine lights the moon enough for the human eye to notice.

However, it seems clear to me that the outline of the unlit part of the crescent moon is brighter than the surface. I was riding my bike home yesterday on a clear winter night, and became confused on why that could be. What mechanism is responsible for making the edge brighter than the dark part of the moon? Or is it my brain trying to make a circle?

Here's an example of what I mean.

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  • $\begingroup$ If I'm understanding you correctly, you're asking me to cover up the crescent completely. Then you are telling me that the edge of the unlit portion of the moon is brighter than the center of the unlit portion of the moon. If this is all correct, first of all, I don't see it. Second of all, this can very easily be tested. You have the image. Import it into your favorite image manipulation program (here's one for free: gimp.org), and see for yourself. $\endgroup$
    – astromax
    Commented Feb 5, 2014 at 2:57
  • $\begingroup$ If you find anything report back here. $\endgroup$
    – astromax
    Commented Feb 5, 2014 at 2:58
  • $\begingroup$ fair enough, I just fiddled with photoshop for 15 minutes, putting a black circle over the moon and using the color picker.. but I am running into pixel issues. Can't really confirm or deny... somehow it still looks brighter to me. $\endgroup$
    – Spork
    Commented Feb 5, 2014 at 9:52
  • $\begingroup$ There are also color variations intrinsic to the surface of the moon - craters, highlands, and lava planes which look lighter and darker depending on their material composition. Perhaps this is why you seeing lighter and darker around the edges. $\endgroup$
    – astromax
    Commented Feb 5, 2014 at 15:12
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    $\begingroup$ I found this ancient question when trying to figure out what I saw one morning. A (terrible) cell phone picture is all I could capture. I leave it here as an additional example for future visitors: full picture, zoomed in. $\endgroup$
    – Engineer Toast
    Commented Oct 16, 2019 at 15:53

3 Answers 3

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In the image you are now refering to the only significant brightness difference is between the mare and the "highlands".

The retina enhances edges. So you'll see edges enhanced, even if they aren't.

By image processing edges can also be enhanced to make contours clearer. This is not the case for the referred image.

To better assess, which part of the perception is real, and which is illusion, here the illusion part: Moon illusion part

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It appears that way because there is more land reflecting sun light to your retina at the edges. This is because the Moon is a sphere. As you move your eye from the center of the moon towards the edges, you are scanning over increasingly more amounts of reflective rock. Hence, the edges appear very bright relative to the center.

It's all about surface area.

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    $\begingroup$ A sphere lit by ambient light appears to be the same brightness all over. A sphere lit by a light near the observer (like in this case) appears brighter in the center and dimmer on the edges. I do not think the surface area analysis holds up. $\endgroup$
    – Dan Bliss
    Commented Feb 14, 2014 at 18:19
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Taking the two answers, the comments, and the lack of a reason why it would actually happen I'll attribute my misperception to the cognitive illusion illustrated by Ernst Mach's Mach Bands. See also the Wikipedia article on lateral inhibition.

Such a shame... I was hoping for some physics effect for bending light around the earth into the moon.

Mach bands illusion

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    $\begingroup$ It's not a misperception. There is simply more light being reflected at the edges because there is more land area. $\endgroup$
    – Stu
    Commented Feb 6, 2014 at 17:35

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