# How can I slow down Earth's rotation?

Imagine a world where the Earth's rotational cycle is 20 hours. It has wreaked havoc on ecosystems that need sunlight. Although some plant life can adapt, others are endangered. Is there a way to slow down earth as to save our plant life and "ecosystems"?

For example: Could we point all the rockets we have West and then fire them up, which might make earth slow down slightly?

EDIT The amount of time in which the Earth's rotational cycle was changed from 24 to 20 hours took 1 hour.

• Note that life that cannot withstand a different length of day because it cannot withstand a different duration of daylight would also likely have the same problem with seasons. Of course there might be other problems as well, since the effects aren't exactly the same. – Darth Wedgius May 8 '15 at 4:29
• I would think that doing something involving the moon would be your best bet, but could quickly become disastrous. – Michael May 8 '15 at 16:59
• @MichaelKjörling Aliens came and worked their magical alien powers to eradicate humanity. They came upon the brilliant idea of speeding up Earths rotation to destroy our food supply. The change from 20 to 24 hours happened within a one hour period. – Kit May 8 '15 at 18:58
• Not sure if this is a duplicate, but this other question does a pretty good job of going over how to change the rotational speed of earth: worldbuilding.stackexchange.com/questions/8425/… – ckersch May 8 '15 at 20:42
• @ckersch - not a dupe but certainly related. That asks about speeding up, which may be different. – ArtOfCode May 8 '15 at 23:27

A note to those more mathematically minded: Yes, I am an actual engineer but I haven't used my book learnin' in a while. Please feel free to correct my napkin math.

So aliens have sped up the planet and we want to slow it back down?

To achieve this within the realm of physics as we know it, the answer is that anything powerful enough to slow down the earth will also kill everything on it. Rockets on the surface wouldn't work because the exhaust goes into the atmosphere and counteracts the effects. Hitting it with asteroids would work but the energy transfer would kill everything. The What If? section over at xkcd actually examined this exact problem except they were trying to speed the planet up.

The big problem is the energy transfer required. The rotational energy of the actual Earth is about $2.138*10^{29}J$. The Wikipedia article on Rotational Energy actually works this as an example. It is directly related to the Earth's mass and to the square of the radius as well as inversely related to the rotational period. You want to change that period so you must change either the mass or the radius to achieve this without massive energy transfers out of the system. The aliens reduced the period by 16.7%, presumably without changing the mass or radius, which means they pumped $3.6*10^{28}J$ into the system. That's pretty close to the total kinetic energy of the moon orbiting Earth so... A lot. Removing that given our current technology would not be easy.

If you're willing to incorporate some handwavium, then I can offer a few ideas for changing the rotational period but I can't promise that everything won't still die.

• Increase Radius: Maybe some humans had an idea for infinite free energy by tapping into the Earth's core and accidentally caused expansion. A relatively minor effect would be earthquakes, volcanoes, oceans disappearing into the crust, etc.
• Add Mass: Humans colonize other worlds and start sending back materials. They loaded huge amounts of minerals using their super-efficient space elevator or something to avoid just launching them at the planet. This would take a long time to avoid the "kill everything on the planet" problem. Note that this would have to be a lot of material to have an impact. Earth's mass is something like $6*10^{24}kg$ and you want to significantly change its angular momentum.
• Energy Transfer Out of the System: An advanced civilization - perhaps humans, perhaps not - transfers energy out of the planet and converts it to, I dunno, radiation. Say that they converted the energy to a Buridan field whose satellite generator network was orbiting the Earth at high speed. Of course, this much energy basically turns the planet into a small star for a while if you offload it too quickly. That might cook our planetary neighbors. However, it could be explained that such a field is how the aliens sped us up in the first place. Perhaps we captured one of their engineers or a ship with the schematics and figured it out.

Again, any of these would cause drastic environmental changes although the "Add Mass" option would be the least jarring as it would simply take so long.

• If you look in my question, I don't want to sped up earth: I want to slow it down to a manageable 24 hours. – Kit May 8 '15 at 20:40
• @ThoughtfulCat Ah, OK. I couldn't quite follow but I can see that form the comments now. So aliens sped it up in a very short time frame and now we want to slow it back down. I guess my answer would be the same except in reverse. I'll edit. – Engineer Toast May 8 '15 at 21:06

Slow the planet down... what? again?

You're too young to remember it but way back, about 50 years ago, near the beginning of the 21st century, we accidentally ripped hours out of each day, and nearly wrecked the planet doing it.

Back then we were trying to figure things out. We built this enormous machine in Switzerland, an enormous magnetic tunnel which looped back on itself to form an perfect circle. We used it to smash atoms together at speeds just shy of the speed of light. Then we studied the collisions to learn what those atoms were made of. We were looking for one particular part called a boson. That was the part which hooked each atom into the underlying fabric of reality, giving the atom weight and mass. We weren't sure it really existed or that reality was really like a fabric, but we kept searching; enhancing the machine with each successive experiment.

It was only a matter of time, till we found one. It was on the 9th of May in 2015, and nobody who survived will ever forget that day. For what nobody had expected, was that first liberated boson might lodge itself in the space-time fabric, creating a fixed motionless point in all three dimensions.

Up until then, everything in the universe had been sliding along the frictionless fabric of reality, spinning around themselves and each other in beautifully interrelated orbits. Then, for one fraction of one millisecond, a fraction of one swedish atom got stuck, and the entire mass and momentum of the planet pivoted violently around that point.

The boson disintegrated under the strain, charring a third of the planet as it unbecame. It happened in the nick of time. If that little particle had lasted any longer, we would probably have lost our orbital momentum. The perfect balance of speed and weight, which keeps us from falling into the sun, was barely preserved.

The planet lost alot of its spin in that moment... and a lot of buildings, and cars, and people, and just about everything else that wasn't strapped down. Its oceans washed over the mountains and down into the plains.

It was a cosmically bad day, and the last 24 hour day that our planet will ever know.

Now our light times are nearly a week long, but then again, so are our nights.

• Err... What? But I do have to say it makes a very intriguing story... +1 for that – Kit May 28 '15 at 0:23

? The cycle of the life is not concerned by the earth rotational cycle, even not by the sun or the ecosystem so 20 hours, 10 or 36 does not matter ... (actually 24 here is not at all good for the life & health). But it is not your question : how can you slow down the earth's rotation (as planet)? Does it mean that you want travel from the past to the future , change plants & animals or living with a new human group ? Slow down the earth or best, change its rotation will change all & every thing : you will have less plant, less animals & diseases will come back ; for that goal you need to be a little bit far of the sun and a little bit turned on another angle. Few hundred of nuclear bomb at the equinox will achieve it or cheaper, shoot on the moon, destroy or move it : the moon balance the earth rotation.

• Shooting the moon might cause all sorts of other dangerous problems. what about a large shifting of the magma beneath the earths surface? A large gravitational even like that has the power to move worlds.... (pun intended. – Vince Scalia May 8 '15 at 16:58

We could stop the rotation of the earth relativitly quickly by making the earth part of a binary system with an equal massed planet i;e; increase the moon's mass.

As the source says, we can launch asteroids.

With some handwavium (or a highly advanced civilization), we can add more mass to the earth. Angular momentum is conserved, so the rotation of the earth will slow.

Launch this rod a the Earth's side.

There are several methods, all probably impractical for quick changes.

Flatten the Earth

We cannot make it flat flat, but in the most general sense, we can make it flatter by moving more mass from the poles to the equator. You know that trick where a spinning ice-skater draws her limbs in to spin faster? This is the opposite. By moving mass away from the axis of rotation, we can have the Earth spin slower while keeping the angular momentum constant.

Equal and Opposite Reaction

Just putting a rocket engine on the equator and lighting it off won't work, as Engineer Toast mentioned above. I'm pretty sure this would only help if the exhaust reaches escape velocity. We could build mountains along the equator (helping a little in that "flatten the Earth" direction), mount mass drivers atop the mountains, and fling garbage or rocks or paparazzi into space against the rotation of the earth. But it would be slow. We could use neutrino rockets, too. It's probably best to put those up on our new equatorial mountain range as well. Neutrinos pass through just about anything, but if these beams are to be strong enough the slow the earth, the fraction that do interact with people could be enough to cause ill health for those hit by the beams. This is not only slow, though, but inefficient. Our ways of making uni-directional neutrino beams aren't very energy efficient, IIRC. Of course if we have have fusion power then that would help offset this. Still, it's a slow method.

Move the Moon Closer

Slam asteroids and comets into the moon to move it closer to the Earth. Tidal forces will start to transfer angular momentum from the Earth's rotation to the moon's orbital motion. Tidal forces are doing that now, in fact, but this will speed it up a little. Downsides: it's slow (like everything else), you're getting higher and lower tides than some coasts are ready for, you're probably getting more earthquakes and volcanism, and, worst of all, the calendars have to be re-done because the length of the month just changed. This requires an international government committee, and all the good-will toward a fixed calendar was used up whe the aliens changes the length of the day. Making Russia, China, the U.S., and France agree on a new system would make moving the moon look easy.

Which Leads Me to My Very Best Suggestion

Ask the aliens, very, very nicely, to put the earth back the way it was.

Although, not strictly an answer to your question as asked, it would probably be easier to set up shades (to provide darkness) and orbital mirrors (to provide daylight) for crop-growing areas than to slow the Earth down.

I was just reading on how a probe or an asteroid slingshoting some planets orbit slightly slow down or speed up the planets rotation.

So maybe a sufficiently advanced society could sling asteroids around earth and with enought/big enought asteroids we could actually change our rotation