Why can I see the dark side of the moon?

  • 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

    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.

    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.

    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.

    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.

  • BillDOe

    BillDOe Correct answer

    5 years ago

    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.

    Traditional name for earthshine: "Old moon in the new moon's arms".

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Content dated before 7/24/2021 11:53 AM