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Some days ago, about 1 ~ 1.5 hours after the sunset I could see a very bright half moon, and the interesting part is, I think I could see the edge of the dark half of the moon, i.e. a whole circle, but with very little contrast to the surrounding black space. It was very weak.

I'll use my impressive graphic design skills to try to explain it better:

enter image description here

The line I'm talking about is that grey one on the left, closing the moon.

My hypotheses are:

  • My brain made it up, as if it was trying to "close the circle"
  • The earth was lighting the moon up, as in: light from the sun was hitting the earth and being re-emitted to the moon (as a very bright moon can actually provide you enough light to see in the dark).

Thanks!

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    $\begingroup$ Have a look at en.wikipedia.org/wiki/Planetshine It is always okay to post an answer to your own question if you like :-) $\endgroup$
    – uhoh
    Jul 30 '20 at 9:00
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    $\begingroup$ @uhoh thank you very much! sometimes it's just a matter of finding the right term. $\endgroup$
    – bgusach
    Jul 30 '20 at 9:43
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    $\begingroup$ Here is an APOD (Astronomy Picture Of the Day) showing the phenomenon: apod.nasa.gov/apod/ap020419.html $\endgroup$
    – Bit Chaser
    Jul 31 '20 at 0:08
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    $\begingroup$ Not really related, yet quite fun (especially the caption): xkcd.com/1738 $\endgroup$
    – DarioP
    Jul 31 '20 at 8:18
  • $\begingroup$ My then-four year old noticed this just a few months ago. He exclaimed "I can see the moon's little baggie!" (אני רואה את השקית של הירח!‏), we've since enjoyed three additional new moons and each time he goes out to make what is now a joke. $\endgroup$
    – dotancohen
    Aug 2 '20 at 13:44
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As uhoh has written in the comments, this phenomenon is known as "planetshine", and in this case more specifically "earthshine". The sunlight hits the earth and is reflected to the moon, illuminating what cannot be directly reached by the sunlight.

enter image description here

https://en.wikipedia.org/wiki/Planetshine

https://en.wikipedia.org/wiki/Earthlight_(astronomy)

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    $\begingroup$ of course there still is always the possibility that your "brain made it up, as if it was trying to 'close the circle'" but I think the chances are low ;-) $\endgroup$
    – uhoh
    Jul 30 '20 at 12:21
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    $\begingroup$ A bit unrelated, but here researchgate.net/figure/… is an image whose optical illusion relies on the brain making up/completing things $\endgroup$
    – Jonas
    Jul 30 '20 at 18:15
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    $\begingroup$ An additional point about your brain "making it up" - the dark side of the moon would have blocked out any stars behind it, making an area that could be distinguished from the rest of the stars. $\endgroup$
    – IronEagle
    Jul 31 '20 at 7:58
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    $\begingroup$ How come then we only see the edge slightly lit and not the whole dark side? The pictures posted in the comments show all, but in my experience I see only the edge of the dark side lit $\endgroup$
    – Hakaishin
    Jul 31 '20 at 8:26
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    $\begingroup$ @Araucaria-Nothereanymore. technically you're right... but I am the OP 😅. $\endgroup$
    – bgusach
    Aug 3 '20 at 7:10
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Here's a very overexposed image of the old Moon in the new Moon's arms.

https://www.flickr.com/photos/doegox/5507240324

The bright side is the overexposed sunlit side. The dimly lit side is a result of the bright Earth shining in the Moon's sky.

The Earth lights up the Moon's surface as does the Moon light up the Earth at night, except the Earth is much brighter in the Moon's sky than the Moon is in the Earth's sky.

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  • $\begingroup$ That's really, really beautiful! $\endgroup$
    – uhoh
    Jul 31 '20 at 2:17
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Yes, it is pretty visible in the right conditions.

And before you wonder if the dark side is visibly smaller - yes, it is. A peculiarity of the human vision makes brighter objects larger.

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