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Comment by adaml_623 | Hacker News Reader

exo762's reply is making my head hurt so here's a link in case you're curious as to how the tides are making the moon spiral outwards.

http://www.windows2universe.org/kids_space/moon_orbit.html

TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit.

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... I've known about the widening orbit of the moon, but I only just realized that if the moon was rotating in a different direction, it would eventually spiral down to the earth.

Imagine living on such a planet, with a large gravity capture moon, that provides a definite sell by date to living on the surface.

Was thinking about that too. It is sad that the moon is leaving us, but the opposite would be a lot worse. Could we survive such an event? Perhaps by chilling on Mars for a few thousand? years.

Few thousand would be lowballing it rather lot. An impact of that magnitude would melt the entire surface, boil the oceans, and strip off the atmosphere. It would take hundreds of millions of years for earth to recover, if it ever would.

Also, Mars is not that hospitable to life. The best option I see would be putting as much people and industry in orbit as possible, and try to live from asteroids.

> An impact of that magnitude

It wouldn't be an impact. For the moon to reduce its orbit to the point that it impacted the earth it would be moving rather slow as orbital objects go.

It would be more of a gradual merging as chunks of rock and ocean moved between the two bodies.

Exactly. See: http://en.wikipedia.org/wiki/Roche_limit

Basically, in this scenario the moon would either come down in chunks or would form a set of rings.

Yes, I was thinking a -3.8cm per year rate, and did contemplate the "squishing" that the moon Io has to deal with. Just pulled the timeframe from my rear end, probably would take that long just to get the process started. :/

As inhospitable as Mars is, asteroids and space stations are far worse!

Yep, and faced with an end-of-life event we'd probably make a go of it, no?

> Mars is not that hospitable to life

I've always found it rather interesting how the combined mass of Mars and Venus is just about the same as Earth's. Can't we move Venus out to a higher orbit where it would impact with Mars and produce an Earth 2?

I've thought the same thing... assumed we'd need alien help. Someone very powerful, like species 8472 but friendlier.

We already have a sell-by date. Long before the sun goes red giant, in fact in about a billion years from now, solar output will increase to the point that liquid water (and therefore life) will be impossible on the surface of the earth.

In such a world all the extra energy would have to be captured by tides.

Right now a portion of the energy is used to "fling" the moon farther away, and a portion is used by tides.

In the other scenario not only is all the energy available to tides, but you also have the extra added energy of the moon "falling" toward the earth.

I'm not sure tides could capture such energy without smoothing the borders of the oceans, which would cause less friction, and then consequently less momentum transfer to the moon.

Wouldn't the moon leaving the Earth's orbit give us a sell-by date too? I thought the removal of a large object like the moon would change our orbit as well, causing things to be dramatically different on Earth.

Is that wrong? What would happen to the Earth when the moon breaks away?

That's a pretty big if, actually. It may be possible for the earth to give up all its angular momentum without giving the moon enough energy to escape. But then we have eternal day and night on opposite sides of earth, so that's probably not a big improvement.

Actually the angular momentum transfer will stop when the moons orbit is the same length as the earth's rotation. So earth would still have day and night but would only have 10-12 per year.

And of course for the curious, no the moon was not 'skimming the surface of the earth' a million years ago as would be implied by a 'continual' increase of 3.8 cm/yr in the orbit.

The earth is not homogenous, and so the gravitational force experienced by the moon is not constant. This results in a transfer of angular momentum between the two, speeding up the moon slightly (and slowing the earth even more slightly as the transfer is proportional to mass)

Even over a billion years that would only change the distance by 38,000 km which means it wasn't 'skimming the surface of the earth' by any imagination. Unless there is a 260,000 Km inclusive definition of skimming. Even at 4.6 billion years (assuming a linearity for that 3.8 cm / year that likely isn't there) that would still only account for just over half the earth-moon distance.

So I don't think the skimming was implied in any way!

You are correct of course. Now I feel sad that I didn't do the math correctly in my head (in my world apparently kilometers are like 100M or something :-()

I wouldn't worry about it, the mental image of the moon skimming the surface of the earth was worth it :) But at the required orbital speeds the tidal effects would be another matter. Talk about a rip-tide :)

> TL; DR - The earth's rotation drags the tidal bulge faster than the moon's orbit slowing the earth and giving more energy to the moon which moves it into a higher orbit.

Is there no way that this is a misinterpretation of the facts? Couldn't the answer be that the Earth is losing mass through its atmosphere (into space), and due to having less mass, the moon has less gravitational pull with the Earth, and the orbit becomes larger?

A good question. However if the Earth was becoming lighter then that would affect the gravitational interaction with lots of things like satellites, passing asteroids and maybe our orbit around the Sun. I'm quite sure the change with all the satellites would be easily noticed.

Thanks for the link, I appreciated the short but informative explanation.

Today I learned the moon is spinning away from the Earth while the Earth's rotation is slowing down, and by the time this dance movement ends, a day on the Earth will last a month!

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