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> (1) The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible. What a terrible explanation. I've done a p

> (1) The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible. What a terrible explanation. I've done a physics course or two so I understand the physical process of how ocean tides are responsible (I think). But for your average person that's just silly. Q. How do trees grow? A. Nuclear Fusion

exo762 13y ago
Tide goes in, tide goes out. No miscommunication.
- stonemetal 13y ago
  Yes but last I checked when the tide went out it doesn't go out to the moon and push it a little bit, so blaming the tide is nonsensical at best. If you want to make a one line quip about it. It would be more accurate to say it is because the Earth spins faster than the moon revolves around the Earth. After all once the Earth stops spinning faster than the moon revolves there will still be a tide but the moon will stop moving away from the Earth.
- dlhavema 13y ago
  say no more... wait. say more.
adaml_623 13y ago
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 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.
- stephth 13y ago
  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!
- Tuna-Fish 13y ago
  ... 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.
  - mixmastamyk 13y ago
    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.
    - Tuna-Fish 13y ago
      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.
      - ars 13y ago
        > 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.
        rohansingh 13y ago
        Exactly. See: http://en.wikipedia.org/wiki/Roche_limit 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.
        13y ago
- ChuckMcM 13y ago
  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)
  - jacquesm 13y ago
    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!
    - ChuckMcM 13y ago
      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 :-()
      - jacquesm 13y ago
        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 :)
- educating 13y ago
  > 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?
  - adaml_623 13y ago
    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.

exo762 13y ago

Tide goes in, tide goes out. No miscommunication.

stonemetal 13y ago
Yes but last I checked when the tide went out it doesn't go out to the moon and push it a little bit, so blaming the tide is nonsensical at best. If you want to make a one line quip about it. It would be more accurate to say it is because the Earth spins faster than the moon revolves around the Earth. After all once the Earth stops spinning faster than the moon revolves there will still be a tide but the moon will stop moving away from the Earth.
dlhavema 13y ago
say no more... wait. say more.

adaml_623 13y ago

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 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.

stephth 13y ago
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!
Tuna-Fish 13y ago
... 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.
- mixmastamyk 13y ago
  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.
  - Tuna-Fish 13y ago
    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.
    - ars 13y ago
      > 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.
      - rohansingh 13y ago
        Exactly. See: http://en.wikipedia.org/wiki/Roche_limit 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.
        13y ago
ChuckMcM 13y ago
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)
- jacquesm 13y ago
  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!
  - ChuckMcM 13y ago
    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 :-()
    - jacquesm 13y ago
      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 :)
educating 13y ago
> 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?
- adaml_623 13y ago
  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.