What if the Moon Disappeared?


Today I was asked a question that was motivated by the new movie Oblivion: What would happen to the Earth if the moon was destroyed? “I dunno,” I replied, “What does happen when the moon is destroyed?” When the expected why-the-chicken-crossed-the-road response didn’t come, I decided I’d better try and answer the question.

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The first thing that came to mind is that it depends on the manner of the moon’s destruction. If it was, say, zapped to bits by a Death Star and those bits still floated in a cluster in the same orbit, I expect they would exert the same gravitational pull on Earth as does the intact moon, and not much would change on Earth.

We’d no longer watch the phases of the moon at night, but see a glittering cloud of debris which would probably be a lot brighter than the full moon, what with all those zillions of little surfaces to reflect sunlight. I know some astronomers who would really hate this new interference with their dark skies.

But if the moon were dragged off and completely removed, there would be none of its mass left to tug gravitationally on the Earth. One of the effects would be that we could throw out tide tables for good.

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The ocean tides would still happen, but the bulge of water would follow the sun, so you could expect high tides around noon everywhere, everyday. I know some fishermen who would appreciate this.

Since the solid Earth flexes tidally, it makes sense that there might be some internal grumbling when Earth loses the moon. Earthquakes. Maybe a few volcanoes getting rowdy. That kind of stuff. But there’s no reason to worry (or hope) that California will fall into the Pacific. Sorry New York. Sorry Las Vegas.

The greater concern would be in the long-term, regarding the Earth’s wobbling spin axis. Right now the spin axis of the Earth very slowly wobbles over 26,000 years, like a slowing wobbling top, because of the tug of the sun. The wobble causes true north to not always point at Polaris, a.k.a., the North Star. Experts agree that the moon acts sort of like a shock absorber to this wobble — keeping it from getting out of hand (see the nitty gritty details in this SETI talk).

It’s possible that Earth without a moon would wobble wildly, sort of like Mars does. The Red Planet’s wobble is so extreme that it may be the cause of some cycles of climate change there. If the same thing happened here, Earth might wobble so much that seasons would become inhospitably extreme and Earth would be a much less stable and habitable planet.

Without the moon the tilt of the Earth’s axis could go from its current wobble of 22 to 25 degrees to a wide ride of zero to 85 degrees — zero would eliminate seasons, and 85 is basically the Earth leaning over on its side. If this happened, the current crisis we call global warming would be a very pleasant tea party by comparison.

Luckily, the wobbling would not affect things right away but over many millions of years.

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Until then, we would be busy observing how the loss of the moon messes up the lives of other animals, ruins whole genres of music, poetry, and photography, and we’d probably go extinct just out of sheer boredom.

That’s assuming we survived the alien invaders who destroyed the moon in the first place. Which, to me, raises an even more pressing question: Why would they do that?

Image credit: NASA

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