Why is the Moon Receding?

When the Apollo astronauts set down on the moon, they didn’t just plant the flag and take a selfie–they had science to do.

One of the experiments left by Apollo was a laser range finding experiment. By means of high quality retro-reflectors left by Apollo and two Soviet Lunakod missions, we now know that the moon is receding by nearly 4 cm per year. We also know our day is slowing down. So what gives?

The moon’s gravity constantly creates a bulge in the Earth, mostly in our oceans. But our daily rotation constantly carries that bulge eastward. The moon must then constantly pull it back toward itself–and the bulge constantly pulls the moon forward in its orbit. This has the effect of slowing our rotation and accelerating the moon’s orbit.

tide-frictionDon’t worry, though. Although the moon is now almost a foot farther away that it was when we first set foot on it, it’s not going anywhere. The recession is slow enough, the sun will die before the moon can get away.

 

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Does worrying about the moon keep you up at night? Leave a comment and let me know. And while you’re up, drop by my website for a free scifi sampler.