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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 sunlight has to reach the dark side of the Moon somehow differently.

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

* 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
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    $\begingroup$ 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
  • $\begingroup$ The dark side is NOT the far side. The far side of the moon gets just as much sunlight as the near side. The dark side of the moon is whatever side is facing away from the sun, and the near side and far side go theough that every 28 days. And the answer to the question is that when the moon is new (dark side facing us), it is illuminated by Earth shine. The Earth is much larger than the moon, and The earth's albedo is also much higher than the Moon's. Therefore, Earthshine on the moon is about 40 times brighter than a full moon on Earth. $\endgroup$ – Dan Hanson Sep 21 at 20:50
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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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    $\begingroup$ Traditional name for earthshine: "Old moon in the new moon's arms". $\endgroup$ – bitchaser Jul 7 at 4:17
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As the older answers stated, Earthshine is what you're seeing.

Compared to the Moon as seen from Earth, Earth as seen from the Moon has almost four times the apparent diameter, and close to 14 times the apparent surface area. If Earth's reflectivity were the same as the Moon's, that would make Earthlight on the Moon 14 times brighter than moonlight on the Earth.

But Earth is shinier than the Moon. The Moon's "albedo" (the proportion of light it reflects instead of absorbing) averages 0.12, about the same as a well-traveled asphalt road; that means it reflects back only 1/8 of the light that hits it. Earth's albedo averages 0.37, three times greater -- and if there's a lot of cloud cover, it can be even higher (since clouds are white).

So, think about how well you can see the Earth's landscape under a nearly-full Moon. If you were standing on the Moon's night side under a nearly-full Earth, there would be nearly 50 times as much light. No wonder we can see it from Earth!

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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
  • $\begingroup$ But it is close to full Earth. You see the dark side of the moon clearly only when the illuminated crescent is thin; that's also when Earth as seen from the Moon is nearly full, if you think about the geometry. $\endgroup$ – jeffB Jul 8 at 18:12
  • $\begingroup$ The Earth has an Albedo about 2.5 times the moon's, and is four times larger in diameter. So even a half Earth will shine much more light on the moon than a full moon will on Earth. $\endgroup$ – Dan Hanson Sep 21 at 20:53

protected by HDE 226868 Sep 18 at 14:52

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