Side view of the Moon!

Bad Astronomy
By Phil Plait
Sep 9, 2010 7:00 PMNov 20, 2019 3:33 AM


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Sometimes being stuck on Earth is a bummer. We miss a lot of stuff! Like, f'rinstance, this view of the Moon which is literally impossible to get from the surface of the Earth:

It looks like the Moon, doesn't it? But it also looks different. That's because this mosaic of 3700+ images shows the Moon as if you were seeing it from above its east side -- like you were hovering above it and following it as it orbits the Earth! The images were taken with the Lunar Reconnaissance Orbiter's Wide Angle Camera, which can snap between roughly 50-100 km (30 - 60 miles) of lunar landscape in one image. As LRO circles the Moon, the camera builds up a map of the entire surface, but only one narrow strip at a time. Astronomers can then use those images to create a mosaic of the Moon as seen from any angle... but it's not easy. To get the mosaic to look right, you can't just take an image from one spot of the Moon and stitch it onto an image from another. The lighting angle will be different, for one thing, making shadows go all wonky. Also, the LRO camera points straight down, so whatever is directly beneath it will be close by, while something off to the side will be farther away. Stuff farther away will look smaller, and you have to correct for that as well. It's taken a while, but LRO scientists have figured out how to correct for all that, and are now able to make these cool maps. The big remaining issue are those missing strips, spots the camera missed because another camera was being used at the time. But still, it's pretty amazing they can make this map at all. And why is it different from what we see on Earth? The Moon spins almost exactly once for every time it orbits the Earth -- that's a natural consequence of the effect of the Earth's gravity on the Moon over time. That means we only see one half of the Moon, and the other half is always pointing away from us. This LRO image shows the east side of the Moon which is the hemisphere of the Moon facing away from its direction of orbital motion (it may help to read a description I wrote of this for a moon of Saturn). As seen from this perspective, the Earth is off the image to the left. So everything on the left half of this picture we can see from Earth, and everything on the right we can't*. Over the course of time the black strips will get filled in, and we'll have something very cool indeed: a complete map of the lunar surface at high resolution, roughly 150 meters/pixel (which is better than we can do here from Earth and is somewhat better than we can get observing the Moon with Hubble). This will be an invaluable tool to future lunar explorers... and it'll also make for some pretty awesome pictures. And it'll also open up even more possibilities for people to help astronomers get more accurate ages for the lunar surface... and in the meantime you can amuse yourself zooming in on an interactive LRO map with a resolution up to 400 meters/pixel. NASA/GSFC/Arizona State University

^* Actually, (as usual) the situation is a little more complicated than that; the Moon appears to rock gently left and right over the course of its orbit. This is called libration and it allows us to see a few extra percent of the lunar surface on the farside. It's a small effect though.

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