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Thread: what is causing the Moon to move away from Earth?

  1. #1
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    what is causing the Moon to move away from Earth?

    I thought it might be the tidal bulge of sea and the Earth tide bulge being dragged by the Earth's rotation that was 'pulling' on the Moon in the direction of its orbit...is that right?
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    It is. As the Earth rotates beneath the moon, the tidal bulge is carried eastwards (in the direction of the Earth's rotation and moon's orbital revolution). This puts it ahead of the moon in its orbit, which places the gravitational vector slightly off-centre, and accelerates the moon along its orbit, thereby "flying" it into a higher (and ultimately slower) orbit. Meanwhile the tidal friction that causes the tidal bulge to be offset is slowing down the Earth's rotation - so angular momentum is transferred from the Earth's rotation to the Moon's revolution.

    There's a very long thread somewhere around here, in which it's pointed out that the continents obstruct the movement of the tidal bulge, and disintegrate it into swirls and gyres to the extent that no unitary "tidal bulge" is evident on maps of the tidal movement of the oceans. But all the gyres and periodic movements average out to something that acts as if it were an offset tidal bulge.

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    ok.

    I suppose the Moon is somehow falling towards the bulge, but rising in altitude.
    Is the barycentre of the Earth-Moon system a bit off from the line from the centre of the Earth to the centre of the Moon, due to the bulge, so that the moon is orbiting a rotating location?
    Formerly Frog march.

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    Quote Originally Posted by Mudskipper View Post
    ok.

    I suppose the Moon is somehow falling towards the bulge, but rising in altitude.
    Is the barycentre of the Earth-Moon system a bit off from the line from the centre of the Earth to the centre of the Moon, due to the bulge, so that the moon is orbiting a rotating location?
    Yes, the moon isn't being attracted directly towards the centre of the Earth, but towards a location that is always a small distance from the centre of the Earth, displaced ahead of the moon's position in orbit. You can parse that into a force directed towards the Earth's centre, and a force accelerating the moon along its orbit. So the moon's velocity is always slightly greater than that required for a circular orbit, which means that it is always moving to a greater radial distance.

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    It should also be mentioned that there is conservation of angular momentum, so the angular momentum that is going into the Moon's orbit must come from somewhere. It comes from the spin of the Earth, so the Earth day is getting longer as the Moon gets smaller.

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    Over eons, does the tiny jittery rate (due to dynamic activity such as tectonic plate movements) of angular momentum exchange have any significant effect on the lunar orbit?
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    Over eons, does the tiny jittery rate (due to dynamic activity such as tectonic plate movements) of angular momentum exchange have any significant effect on the lunar orbit?
    Changes in the form of the continents can change the amount of tidal drag, which would cause the rate of exchange of angular momentum. Much of the drag is from tidal bulges piled up in shallow coastal areas.

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    wouldn't the Earth tide(the rising and falling of the crust) cause a large proportion of the drag?
    Formerly Frog march.

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    Quote Originally Posted by Mudskipper View Post
    wouldn't the Earth tide(the rising and falling of the crust) cause a large proportion of the drag?
    The earth tide leads the moon by only a fraction of a degree, whereas the ocean tides lead by something on the order of ten degrees, so the torque from the oceans is relatively large. This is reflected in a tidal dissipation factor Q for the Earth that is a few times higher than for a terrestrial planet without oceans.

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    Quote Originally Posted by grant hutchison View Post
    It is. As the Earth rotates beneath the moon, the tidal bulge is carried eastwards (in the direction of the Earth's rotation and moon's orbital revolution). This puts it ahead of the moon in its orbit, which places the gravitational vector slightly off-centre, and accelerates the moon along its orbit, thereby "flying" it into a higher (and ultimately slower) orbit. Meanwhile the tidal friction that causes the tidal bulge to be offset is slowing down the Earth's rotation - so angular momentum is transferred from the Earth's rotation to the Moon's revolution.
    The Earth is also moving away from the Sun.
    If I understand it correctly, all the planets and moons in the solar system are drifting outwards.

    So, can we use Tidal as the main explanation for this Phenomenon?

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    Quote Originally Posted by Hornblower View Post
    Changes in the form of the continents can change the amount of tidal drag, which would cause the rate of exchange of angular momentum. Much of the drag is from tidal bulges piled up in shallow coastal areas.
    Yes, that was the given for the question I asked. But without any, highly unexpected, cyclical variations that would be in resonance with the orbital period, then my question is likely without merit.
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    No. For the Earth, tidal anguiar-momentum transfer from the Sun (micrometers per year from couple of online articles, although I don't see a more exact source) is orders of magnitude less important than solar mass loss (combining fusion and solar wind - Phil Plain quoted some numbers[/url], and Peter Noerdlinger gives much more detail). Even so, the total effect is calculated to be ~1.5 cm per year for the Earth, and would be proportional at other distances.

    For hot-Jupiter systems, the situation can be different, and there have been attempts to work out theoretical ranges for the tidal responses (known as Q factors) both of the planets and their stars.

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    Quote Originally Posted by ngc3314 View Post
    No. For the Earth, tidal anguiar-momentum transfer from the Sun (micrometers per year from couple of online articles, although I don't see a more exact source) is orders of magnitude less important than solar mass loss (combining fusion and solar wind - Phil Plain quoted some numbers[/url], and Peter Noerdlinger gives much more detail). Even so, the total effect is calculated to be ~1.5 cm per year for the Earth, and would be proportional at other distances.

    For hot-Jupiter systems, the situation can be different, and there have been attempts to work out theoretical ranges for the tidal responses (known as Q factors) both of the planets and their stars.
    Thanks

    However, it is clear to us that all planets and moons are drifting outwards?
    Why the situation in Jupiter is different?

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    Quote Originally Posted by Dave Lee View Post
    Thanks

    However, it is clear to us that all planets and moons are drifting outwards?
    Why the situation in Jupiter is different?
    The situation for hot Jupiters (gas giants with very close orbits to their parent star) may be different, because some of them orbit faster than their primary rotates. The tidal bulge therefore trails the planet, and the angular momentum transfer goes in the opposite direction, with the planet evolving inwards. The same applies to moons that orbit their parent planets faster than the planet rotates - Phobos is moving towards Mars, for instance. And retrograde moons, like Triton, will do the same thing.

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    Quote Originally Posted by grant hutchison View Post
    The situation for hot Jupiters (gas giants with very close orbits to their parent star) may be different, because some of them orbit faster than their primary rotates. The tidal bulge therefore trails the planet, and the angular momentum transfer goes in the opposite direction, with the planet evolving inwards. The same applies to moons that orbit their parent planets faster than the planet rotates - Phobos is moving towards Mars, for instance. And retrograde moons, like Triton, will do the same thing.

    Grant Hutchison
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    Do we know about just one planet or moon (real one - not a broken moon, comet or Asteroid) in the Solar system or even in the whole galaxy which is drifting inwards for sure?

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    Quote Originally Posted by Dave Lee View Post
    Thanks

    Do we know about just one planet or moon (real one - not a broken moon, comet or Asteroid) in the Solar system or even in the whole galaxy which is drifting inwards for sure?
    I don't know what your definition of "real" moon is but I can't see how the the aforementioned Triton could be considered to be not real.

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    Quote Originally Posted by Dave Lee View Post
    Thanks

    Do we know about just one planet or moon (real one - not a broken moon, comet or Asteroid) in the Solar system or even in the whole galaxy which is drifting inwards for sure?
    Phobos, as mentioned.

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    Don't forget Triton.

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    Some orbital changes are caused by resonance effects with other orbiting bodies. Some gaps in the asteroid belt are due to secular resonance.
    We know time flies, we just can't see its wings.

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    Sorry

    Triton is a broken moon

    https://en.wikipedia.org/wiki/Triton..._2_(large).jpg

    Same issue with phobos, which looks like a big rock.

    https://en.wikipedia.org/wiki/Phobos...olour_2008.jpg

    we can see it clearly!

    Any other idea?

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    What???

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    Quote Originally Posted by Dave Lee View Post
    Sorry

    Triton is a broken moon

    https://en.wikipedia.org/wiki/Triton..._2_(large).jpg

    Same issue with phobos, which looks like a big rock.

    https://en.wikipedia.org/wiki/Phobos...olour_2008.jpg

    we can see it clearly!

    Any other idea?
    Please tell us what you imagine a "broken moon" is, and maybe we can help.

    Grant Hutchison
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  23. #23
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    Quote Originally Posted by Dave Lee View Post
    Sorry

    Triton is a broken moon

    https://en.wikipedia.org/wiki/Triton..._2_(large).jpg

    Same issue with phobos, which looks like a big rock.

    https://en.wikipedia.org/wiki/Phobos...olour_2008.jpg

    we can see it clearly!

    Any other idea?
    My bold. What is "it"? What is it about Triton and Phobos that makes them any less "real" (whatever that is supposed to mean) than the other satellite bodies that are orbiting planets?

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    A complete (at time of writing) list of known prograde moons with orbital periods less than the primary's rotation period looks like this:

    MARS
    Phobos

    JUPITER
    Metis
    Adrastea

    URANUS
    Juliet
    Portia
    Rosalind
    Cupid
    Belinda
    Perdita

    NEPTUNE
    Naiad
    Thalassa
    Despina
    Galatea
    Larissa

    Grant Hutchison
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    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

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    Quote Originally Posted by Dave Lee View Post
    Sorry

    Triton is a broken moon

    https://en.wikipedia.org/wiki/Triton..._2_(large).jpg

    we can see it clearly!

    Any other idea?
    You do know that the part of Triton not lit by the sun is still there, right?

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    Quote Originally Posted by glappkaeft View Post
    You do know that the part of Triton not lit by the sun is still there, right?
    Thanks
    Do you mean the triton has full symmetrical ball shape, however, due to the Sun light it looks as a broken moon?

    How do we know that this moon is drifting inwards?

    Actually, I have just found that triton is a Retrograde satellite.
    If I understand it correctly, based on the current hypothesis, Retrograde satellites must have a Tidal deceleration.
    Which means that they must drift inwards.
    Can we prove it?
    Last edited by Dave Lee; 2018-Jan-03 at 08:38 PM.

  27. #27
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    Quote Originally Posted by grant hutchison View Post
    A complete (at time of writing) list of known prograde moons with orbital periods less than the primary's rotation period looks like this:

    MARS
    Phobos

    JUPITER
    Metis
    Adrastea

    URANUS
    Juliet
    Portia
    Rosalind
    Cupid
    Belinda
    Perdita

    NEPTUNE
    Naiad
    Thalassa
    Despina
    Galatea
    Larissa

    Grant Hutchison

    Do you mean that all of those moons are drifting inwards, as their orbital periods is less than the primary's rotation period.
    Can you please explain why that issue has an effect on the drifting direction?
    Is it just a hypothetical Idea? Can we prove it mathematically.
    Last edited by Dave Lee; 2018-Jan-03 at 08:39 PM.

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    Quote Originally Posted by Dave Lee View Post
    Thanks
    Do you mean the triton has full symmetrical ball shape, however, due to the Sun light it looks as a broken moon?
    Yes, although in that particular example it's probably also partially due to cleaning up the photo mosaic using techniques available in 1998 when the image was created (the edge look to sharp). Triton is much too large to be anything but a sphere. In fact it's the 16th largest object in the solar system and larger than Pluto. Here is a single shot from further away with the sun at a better angle.

    https://commons.wikimedia.org/wiki/F..._Voyager_2.jpg

    How do we know that this moon is drifting inwards?
    It's the same orbital dynamics for why the moon is receding already described in this thread, just the other way around.

  29. #29
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    Quote Originally Posted by Dave Lee View Post
    Thanks
    Do you mean the triton has full symmetrical ball shape, however, due to the Sun light it looks as a broken moon?
    By "broken" do you mean "irregularly shaped" (like a potato, not a sphere)? If so, why is that relevant.

    Quote Originally Posted by Dave Lee View Post
    How do we know that this moon is drifting inwards?

    Actually, I have just found that triton is a Retrograde satellite.
    If I understand it correctly, based on the current hypothesis, Retrograde satellites must have a Tidal deceleration.
    Which means that they must drift inwards.
    Can we prove it?
    Can we prove it by observation? Sure, if we watch for long enough and measure finely enough. I know the latest theory of Triton's orbit (Zhang et al., 2014) includes tidal interactions and is a good match for observation, but I so far haven't been able to find a quoted rate of change for Triton's radial distance. Whereas I know we've observationally measured the tidal change in Phobos's orbital radius.
    Can we prove it mathematically? Sure. There's an entire chapter in Murray and Dermott's Solar System Dynamics devoted to the topic, for instance.

    Grant Hutchison
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    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

  30. #30
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    Quote Originally Posted by Dave Lee View Post
    Do you mean that all of those moons are drifting inwards, as their orbital periods is less than the primary's rotation period.
    Can you please explain why that issue has an effect on the drifting direction?
    Is it just a hypothetical Idea? Can we prove it mathematically.
    It's an observed effect, and it can be shown mathematically, with calculations matching observation.
    If the tidal bulge raised by a moon is always ahead of it in its orbit (as it is when the parent planet rotates faster than than the moon orbits), then the tidal evolution is outwards.
    If the tidal bulge raised by a moon is always behind it in its orbit (as it is when the moon orbits faster than the planet rotates, or in the opposite direction to planet rotation), then the tidal evolution is inwards.

    All the bodies in my list are moving around their parent planets faster than the planet rotates, so all wil evolve inwards tidally.

    Grant Hutchison
    Blog

    Note:
    During life, we all develop attitudes and strategies to make our interactions with others more pleasant and useful. If I mention mine here, those comments can apply only to myself, my experiences and my situation. Such remarks cannot and should not be construed as dismissing, denigrating, devaluing or criticizing any different attitudes and strategies that other people have evolved as a result of their different situation and different experiences.

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