Back Side of the Moon

Note: I’m sorry this video doesn’t fit. I thought I knew how to resize it, but my method didn’t work. I’m going to leave it, anyway. I think you can see it well enough.

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This is NOT a simulation. This is an actual photographic video of the back side of the moon, passing between the earth and the camera.

The moon rotates on its axis at the same rate it orbits the earth, so it always shows us the same face. It wobbles a little bit, but this is not noticeable to the naked eye. That’s why you always see the same “man in the moon” or “rabbit in the moon” or whatever you see when you look at the full moon. I personally see the rabbit, but it’s not very convincing.

The reason for this strange behavior is that one side of the moon is flatter than the other, so (when the moon rotated faster in the distant past) the bulgy side was slightly closer to earth when it turned around, and was affected more by its gravity. Over many millions of years, this slowed the moon’s rotation until the bulgy side always faces earth. Several other objects in the solar system also behave this way.

NASA’s Deep Space Climate Observatory (DSCOVR) orbits the sun at the Sun-Earth L1 Lagrangian point, 930,000 miles toward the sun from Earth. At this point it is affected about equally by the gravities of earth and the sun, so it maintain its position and always lines up almost perfectly between earth and the sun. The video above was made by its Earth Polychromatic Imaging Camera and telescope on July 16, and shows the moon moving across the Pacific Ocean and America.

The moon is only about a quarter of a million miles away, so the camera was almost four times farther. With the sun at its back, the camera turned its eye toward earth just as the moon was moving between them in its orbit. So we have a video of a full moon crossing (or transiting) the earth, and we’re seeing the far side of the moon. The side never visible from earth.

As expected, it looks different. Because it is different. It’s the flat side. Really, it’s not flat enough that you could notice, but the whole surface is unfamiliar. And it looks dark compared to earth.

“The lunar transit is the back side of the moon — very different looking than the familiar face — no man in the moon, no mares, no cheese,” said Adam Szabo, a heliophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

The surprising darkness of the moon compared to earth actually makes sense. Both objects are receiving the same amount of light from the sun, but the powdery regolith that covers the surface of the moon is not as reflective as the surface of the Earth, which is largely water.

“Water is rather reflective almost like a mirror, so it shouldn’t be a surprise,” he said. “The earth is a shinier, better reflecting object.”

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