Phew, that was close! A colossal asteroid just came by. It narrowly missed the Earth -- and smashed up the moon instead. Well, no reason to be cheerful. For without the moon, we could be just as doomed.

Just look at that moon. Isnít it lovely? But beware: the moon isnít just there to entertain us. Without the moon, we could be doomed.

Thereís always the small but alarming possibility that some dreadful cataclysm takes out the moon. Some billions of years ago, this almost happened. On one terrible day, a vast comet struck the moon, carving out whatís now known as the biggest impact crater of the entire solar system: the Aitken Basin, a huge, 2,500 kilometers wide scar on the moonís South Pole.

Ok, but whatís the big deal?, you ask.

Well, not too many people know this, but the moon is what keeps our planet stable. Without it, we would find ourselves on a hostile rollercoaster world. Our planet would go berserk.

For one thing, the moon tugs at the oceans. This gives us the tides. No moon, and the floods would immediately be about 2,5 times lower -- some minor tidal motion would remain, because the Sun pulls at the oceans, too.

The consequences would be dramatic. Many fertile deltas would dry up. Other areas would become permanently inundated. There would be all kinds of changes to nature. All over the world, people would face droughts, famines, diseases and wars.

And to cheer you up a little more: thatís not the worst part.


Swing It Out: The moon and the Earth are locked into a twin system, much like an athlete about to hurl a hammer.

Crucially, the moon also stabilizes the axis around which the Earth rotates. You could compare the Earth-moon system to an athlete swinging a hammer around. Take a good look at the picture on the right. Now, ask yourself: what would happen if the rope suddenly snapped? You get the idea: the athlete would fall over.

Exactly the same thing would happen to the Earth if you took away the moon. Right now, our planet rotates at an axis that is tilted about 23 degrees. It has always roughly been that way. But without the moon, the Earthís rotational axis would slowly drift off, because of the pull of the other planets -- especially Venus and Jupiter. So one moment youíre in Africa; the next, youíre on the North Pole!

In the long run, this would cause massive, unpredictable and abrupt climate shifts. The Earth would heat up, freeze up, and heat up again. One moment, your nose freezes off in a massive Ice Age. The next moment, you find yourself sweating your eyeballs out, in a period of soaring heat.

Even worse, our planet could tip over and Ďlie on its backí for some millions of years (or longer). One half of the globe would be in constant sunlight, while the other half would be plunged into everlasting darkness -- and in cold.

Weíd have a two-sided planet. And to be honest, you probably donít want to live on either side. The southern half would become a barren, waterless, roasted desert world. The northern half would be an equally barren, dry, ultra cold ice world.

Ok, so you decide to live in between the sunny and the icy side. There, you might find a small zone with good old days and nights, and mild temperatures. But thereís a downside: the region will be harassed by HUGE, everlasting storms. With temperature differences like that, there would be massive flows of air between north and south. 

And actually, thatís not even the biggest concern. For chances are the massive, abrupt climate shifts will at some point kill our world altogether. With the moon gone, our world could become a lifeless, dead planet in the end. The sweeping climate shifts could at some point disrupt the Earthís atmosphere for good. This is probably what happened to Mars (once watery and friendly, but now as dead as a doornail). Mars indeed tilts like a drunken athlete: it tips over to about 60 degrees! 

And thatís not everything. When the moon gets smashed up or knocked off course by a super big asteroid, itíll probably rain debris on Earth for many years. For years, weíd have to wear hard hats  - only to find that hard hats donít help against big, incoming chunks of moon rock falling down on our world.

Howling at the moon

Still, thereís some good news, too.

According to some doom prophets, without the moon, our planet would start spinning wildly, like a whipped top. A day and night on Earth would last only an estimated four to eight hours. And we would have HUGE, apocalyptic storms, tens to even hundreds of times stronger than anything ever seen on Earth, and each one of them lasting many years. Cities and forests would be flattened, agriculture would become impossible.

But fortunately, the doom prophets saying this have got it all wrong.

The moon has slowed down the rotation of our planet for good. The moon puts the brakes on our planet, because the tides serve like a sort of Ďcounter weightí. Over the billions of years, this has slowed down the rotation speed of the Earth. But if some rogue black hole suddenly snatched away the moon, our planet wouldnít suddenly start to whirl. It would in fact even slow down some more, because of the Sunís pull.

Another often heard, but wrong, horror scenario is that the moon is about to leave us  for good.

Indeed, the moon IS leaving us. Very, very slowly, it creeps away from us, inch by inch, year by year. The moon flies off at the speed at which fingernails grow: 3,8 centimeters per year. 

But that doesnít mean the moon will be lost in space eventually. Calculations show that in about 15 billion years time, the moonís orbit will stabilize at a distance of about 640.000 kilometers. If you looked up from our planet, you would notice that the disk of the moon was much smaller than it is now -- only about three quarter its present size.

Not that you would be here looking at the moon. Within 5 billion years or so, the Earth will become a dead, inhabitable world because the Sun has swallowed us by then -- but thatís another story.

So all in all, next time you see that lovely moon up there, better be grateful. For now, things are just right the way they are. No sweeping storms, no ice ages, no falling over of the Earth. Letís hope itíll stay like that for some time.

(A big celestial thanks to dr. Frank Israel (Leiden University) for technical advise)


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