Astronomers at the Mauna Loa Observatory in Hawaii, used a telescope and sensitive camera to take measurements and multiple images of the moon for the last two years.
The dark side is not the same as the far side, which receives the same amount of light as the bright side. The dark side is not lit by sunlight, but by light reflected off Earth, which can be seen at the new moon.
"Astronauts standing on the moon and looking up at the Earth described it as a blue marble," said first author of the study Peter Thejll. "Having not been into space myself, I don't know what they meant exactly, but once that blue light strikes the moon's surface, it shifts to a blue-green colour. We can call it turquoise."
To measure the color of the dark side they had to first eliminate the light coming from the bright side, which tends to scatter in Earth's atmosphere and create a halo that makes it difficult to measure the dark side, an effect similar to the fuzzy glow seen around a street light when viewed from a distance.
Using two filters astronomers took hundreds of pictures of the moon, and found two of the waning crescent moon on January 18, 2012 with the same halo. When they subtracted one image from the other they were able to find the actual color of the dark side.
"We know how unlikely it is that the haloes should cancel out, and yet we found a pair where they did. That says something about the conditions on the night when we took those pictures. Something was unique so the two haloes were identical and they cancelled and frankly we don't know why," said Thejll.
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