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Last weekend (29./30. April) the sky in my area (central europe) was pretty clear and the moon was nicely visible. Funnily, one could not only see the sunlit area of the moon, but also the dark side (*) could be easily recognised throughout the whole evening from blue hour until dark night. I attached two photos of the moon that should illustrate the effect. The pictures (sorry for the poor quality) needed to get a little overexposed to capture the illuminated dark side, but with plain eye the effect could be well seen. We were speculating about the reasons why the non-sunlit area of the moon could be seen.

  • diffraction of sunlight through the atmosphere can be neglected as the moon doesn't have an atmosphere
  • reflection of light that in turn was reflected from earth (sun -> earth -> moon -> observer) sounded a little unlikely to me
  • a psychological effect that kind of mentally completes the shape of the moon, because we know it should be circular, sounded ok to me. But could be debunked as the dark side stays visible if one covers the bright side on the photo.

So i guess the sun light has to reach the dark side of the moon somehow differently.

What could be provide enough light to make the dark side visible?

(*) dark side to me is not the far side of the moon, but the non sunlit area of the moon.

visible dark side of the moon in the evening (sorry for the blurred picture) visible dark side of the moon later at night

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    $\begingroup$ Earthshine: en.wikipedia.org/wiki/Planetshine#Earthshine $\endgroup$ – Iván Pérez May 4 '17 at 6:27
  • $\begingroup$ Cool thanks. I was googling for dark side of moon and around that topic to no avail, but i really should have looked for the reflected light of earth, although i didn't believe this could be an option. $\endgroup$ – sargas May 4 '17 at 6:52
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    $\begingroup$ The dark side is the far side, not the half that isn't lit up by the sun. It's the half that faces away from Earth. We can't see the dark side of the moon for the same reason that we can't see the back of a person's head when they're facing us. $\endgroup$ – userLTK May 4 '17 at 13:57
  • $\begingroup$ That's why i defined dark side explicitly in my post in the lack of other words. As Bill pointed out in his answer i could have chosen night/day. It could be another question why the far side is called dark side as this would be only true for full moon. $\endgroup$ – sargas May 5 '17 at 4:23
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You can generally see the unlit side of the moon when a considerable amount of sunlight is reflected off the earth. This reflected sunlight illuminates the unlit side of the moon. This is referred to as earthshine, and a decent explanation can be found at timeanddate.com. I seem to recall reading somewhere (but now can't find a reference) that it is more prominent when it is very cloudy on earth near where the moon is overhead.

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A full Earth on the Moon is much brighter than a full Moon is on Earth, because the Earth is significantly bigger.

Btw, "dark side of the Moon" is not really a technical term (it's rather a song of Pink Floyd). On the Moon, there is (as on Earth) day and night (though the day is as long as month on Earth) and no spot remains dark for longer (except for the pole caps as on Earth). There is, of course, the far side of the Moon, most of which is invisible from Earth (because the Moon is tidally locked to spin with the same rate as it rotates around Earth).

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    $\begingroup$ Except that it isn't "full Earth" on the picture shown - probably only half Earth. If you think about it, you are viewing the Moon from a place on Earth that isn't in daylight... Full Earth occurs roughly when we see New Moon. $\endgroup$ – Rob Jeffries May 4 '17 at 9:35
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    $\begingroup$ I feel like you didn't even actually answer the question. You hinted at an answer, but never explicitly stated it. $\endgroup$ – zephyr May 4 '17 at 13:03

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