# Thread: How does the moon stabilize Earth's axis?

1. ## How does the moon stabilize Earth's axis?

I know the moon is supposed to have a stabilizing effect on Earth's axis, preventing the planet from having big swings in polar inclination like Mars is thought to have and this is thought to have played a role in making the planet more habitable for life.

The problem is I haven't been able to find a really thorough explanation on the internet of how the mechanics of how this effect works. I've read that it has something to do with the Earth's equatorial bulge but I'd like to understand how it works. Is it a factor of how the moon's gravity pulls on different parts of the Earth at different strengths depending on how far away they are, like the tides? Or is it something else, maybe dependent on the general strength of the gravitational force of the moon on Earth (though I find that hard to believe, as IIRC the sun's gravity at Earth is stronger) or the ratio of the masses of Earth and the moon?

Part of the reason I ask this is because I'm working on a hypothetical Earthlike planet, and it orbits a dimmer star than our sun, making an Earthlike moon problematic because of the reduced Hill sphere. I figured a good solution might be to have a moon 1/8 the mass of ours at 1/2 the orbital distance - the planet should experience the same tides as Earth. But I read somewhere (it was on Google Books, I forget the book) that a moon 1/2 the mass of ours or less would not stabilize the Earth's axis - it didn't give any context for it though, like whether they were assuming this smaller moon was at the same distance as ours, or if it was some more absolute factor. So I want to understand exactly how the stabilization mechanism works, so I can figure this out.

Also, would a planet, say, twice the mass of Earth need a moon twice as big to stabilize its axis, or 1/2 as massive as Earth only need a moon 1/2 as massive?

Could someone explain exactly how the stabilization effect of the moon works to me?

Thanks, that would be really helpful.

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Originally Posted by Somes J
I know the moon is supposed to have a stabilizing effect on Earth's axis, preventing the planet from having big swings in polar inclination like Mars is thought to have and this is thought to have played a role in making the planet more habitable for life.

The problem is I haven't been able to find a really thorough explanation on the internet of how the mechanics of how this effect works. I've read that it has something to do with the Earth's equatorial bulge but I'd like to understand how it works. Is it a factor of how the moon's gravity pulls on different parts of the Earth at different strengths depending on how far away they are, like the tides? Or is it something else, maybe dependent on the general strength of the gravitational force of the moon on Earth (though I find that hard to believe, as IIRC the sun's gravity at Earth is stronger) or the ratio of the masses of Earth and the moon?

Part of the reason I ask this is because I'm working on a hypothetical Earthlike planet, and it orbits a dimmer star than our sun, making an Earthlike moon problematic because of the reduced Hill sphere. I figured a good solution might be to have a moon 1/8 the mass of ours at 1/2 the orbital distance - the planet should experience the same tides as Earth. But I read somewhere (it was on Google Books, I forget the book) that a moon 1/2 the mass of ours or less would not stabilize the Earth's axis - it didn't give any context for it though, like whether they were assuming this smaller moon was at the same distance as ours, or if it was some more absolute factor. So I want to understand exactly how the stabilization mechanism works, so I can figure this out.

Also, would a planet, say, twice the mass of Earth need a moon twice as big to stabilize its axis, or 1/2 as massive as Earth only need a moon 1/2 as massive?

Could someone explain exactly how the stabilization effect of the moon works to me?

Thanks, that would be really helpful.
Our moon is a very big moon compared to the size of the Earth. Other such large moons encircle vastly bigger planets like Jupiter, Saturn, and Neptune. Some have suggested that the Earth Moon system could be considered a double planetary system. If you are thinking of writing sci-fi you might consider such an idea concerning equally sized planets. It's supposedly a very unlikely circumstance that presently is considered to have been created by a great impact. The moon is in a locked revolution, the same side always faces the Earth. In the same way the moon is also slowing down the revolutionary velocity of the Earth, now ~24 hours per day but supposedly much faster in the past. In the same way the moon keeps the axis of the Earth more constant than it otherwise might be. This is thought to be do to a type of resonance of the Earth's rotational axis with the moons orbital axis relative to the sun, a type of steadying influence. More variation of the Earth's axis would not necessarily be catastrophic but certainly more destabilizing concerning regional climate changes on Earth as it might relate to ice ages, hotter climates, and the evolution of the Earth's climate and life in general.
Last edited by forrest noble; 2010-Dec-23 at 02:47 AM.

3. I'm actually working on Earthlike double planets as well, but for this particular planet I want something more Earthlike.

4. One way to understand the connection between the Moon's orbit around the Earth and the stability of the Earth's rotational axis is to consider angular momentum. The Earth, by itself, spinning on its axis, has some amount of angular momentum; the exact amount depends on a combination of the Earth's mass, its radius (squared), and its angular velocity. The faster it spins, the more angular momentum it has; the bigger its mass, or its radius (to a greater degree), the larger its angular momentum.

The more angular momentum a body has, the harder it is to change its rotation -- either the rate at which it rotates, or the orientation of its rotational axis. So, if the Earth were alone, and spinning, it would take some definite amount of perturbing influence to change the direction of its rotational axis. Fine.

But the Earth isn't alone -- it has the Moon. The Moon's orbit around the Earth has some angular momentum of its own, again a combination of mass (the Moon's isn't as large as the Earth, so that's bad) and radius squared (the Moon's orbit is much larger than the Earth's radius, and this is squared, so this is good) and angular velocity (the Moon's angular velocity is about 29 times smaller than the Earth's rotational angular velocity, so that's bad, too). Overall, the various factors combine in a way which cause the orbital angular momentum of the Moon to be of the same order of magnitude as the Earth's rotational angular momentum. Having a big Moon means that the Earth-Moon system has (very very) roughly twice the angular momentum as the Earth would have alone.

Because the Earth is not a perfect sphere, but bulges out slightly at the equator, the spinning angular momentum of the Earth is linked to the orbital angular momentum of the Moon. If you try to change one, you'll end up changing the other as well, by some indirect mechanisms I'd prefer to skip at the moment.

What this all boils down to is pretty simple: the total angular momentum of the Earth-Moon system is significantly increased by the Moon's orbital motion. And what THAT means is that it is significantly harder to alter the Earth's rotational properties than it would be if the Earth were all alone.

Perhaps an analogy would help: consider these two situations:

a) Katerina, a female figure skater, is spinning on her toe

b) Elizabeth, a female figure skater, is spinning on her toe, while she holds a sledgehammer so that it sticks out horizontally

Which skater would have a more easily influenced rate (or direction) of spin?

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