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Is the moon leaving us?

  1. Dec 2, 2003 #1
    I've mentioned this before, the problems that might arise if the moon were to leave our orbit. However, I never really grasped why it is moving away from us, if it is even really the case that it is moving away from us.

    Is it? If so, why? And what effects will it have on Earth, if it does completely leave our orbit (gradually)?

    Answers to some or all of these questions are greatly appreciated.
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  3. Dec 2, 2003 #2


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    The main reason the Moon is getting farther away is tides (you do know that the tides also operate on the solid part of the Earth, stressing and flexing the rocks slightly. It's tiny but it can be measured).

    The tidal up and down converts some of the gravitational energy into heat, which is lost to the system (radiated away). So the Earth-Moon system is losing energy. This makes the Earth turn slower ("Leap seconds"), and by conservation of angular momentum the Moon must respond too; it does so by moving to a higher, slower rotation orbit. All of these effects are truly tiny, but they are all in the same direction so they will build up over time (the astronomers' name for that is "secular". Yeah that's right).
  4. Dec 2, 2003 #3


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    <cut>"the tides also operate on the solid part of the Earth, stressing and flexing the rocks slightly. It's tiny but it can be measured)."

    At the Luner Ranging Station (using lasers) just below McDonald Observatory, they measused that particular part of the mountain rises (like tides) at just about exactly 6 inches "per tide". Somewhere before on PF this subject came up and Janus posted about all you would need to know about tides. But, of course, I don't remember where..

    This is a good spot:
  5. Dec 8, 2003 #4
  6. Dec 8, 2003 #5
    The Moon is actually moving away from us at about 4cm per year
  7. Dec 8, 2003 #6


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    As to why the moon is moving away, as has been mentioned, it is due to the lunar tidal bulges.

    The Moon causes two bulges to form on the Earth, Ideally. they would form on a line joining them with the moon.

    The Earth however rotates faster than the Moon revolves around the Earth. And as has also been mentioned, there is a friction between the rotating Earth and the Tidal bulges. This causes the tidal bulges to tend to rotate along with the Earth. Thus there are two forces acting on the bulges that must balance out. When they do, the result is that the tidal bulges tend to lead the Moon a little.

    And this tug of war has two effects, the friction between the bulges and the Earth tends to slow the Earth, and the moon's tendancy to want to line up with the title bulges tends to pull it forward in its orbit.

    Now the thing is, that when an object is given a forward thrust it moves into a higher orbit. (basically if given a forward thrust at any given point of an orbit, The radius on the opposite side of the orbit increases. Since the moon gets this thrust along its entire orbit, it follows an outward spiral.)

    Left alone, this will continue until The Earth's rotation slows to the point where it matches the period of the Moon's orbit. (Since the Moon's orbital period is also increasing, this will happen when they both are about 2 months long.)

    If we only had to concern ourselves with the Moon and Earth, this situation would be stable and wouldn't change.

    However, the Moon isn't the only body that causes tides, the sun does too. These tides are about 1/2 the magnitude of the Earth's (so even if we did lose the Moon completely, there would still be tides.)

    The Solar tidal bulges create friction with the Earth too, and results in a continuing slowing of the Earth's rotation.

    This, in turn, causes the Earth to start to rotate slower than the Moon revolves, causing the Lunar tidal bulges to lag behind the Moon and tug it backward, which will pull the Moon into a lower, faster orbit. (And the Lunar tides will start to increase in magnitude)

    The Moon will continue to fall inward until it passes the Roche limit and it breaks up.

    The kicker is, "if left alone". Chances are that the Earth and Moon will not be left alone long enough for this scenerio to take place. Long before then the Sun will expand into a Red giant and likely engulf both the Earth and Moon.
  8. Dec 9, 2003 #7
    Job done.
    Last edited: Dec 9, 2003
  9. Dec 9, 2003 #8


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    Re: Re: Is the moon leaving us?

    I saw a program on Discovery or somesuch addressing this issue of the moon-- how it got there and what would happen if it left. Well, according to this program, the moon stabilizes the earth on its axis, and without it the axis would tilt wildly-- north pole would become equator in the flash of a geological eye, and then over again, with no end in sight. At the very least, in such a scenario evolution would be faced with its largest challenge since life on earth began. At the worst, earth would be changing too rapidly to support complex life on its surface for very much longer.
  10. Dec 9, 2003 #9


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    Please refrain from replying to serious questions with your own personal "theories". Continuing to do so will just get your posts deleted in the future. Such posts belong in Theory Development.
  11. Dec 12, 2003 #10
    The moons destiny

    I am sure you are more of an expert than i am but i would like to mention another theory that i discussed on a our amateur astronomer radio program in 1997. Is the Roche limit then the base parameter used on the latest computer analized theory?

    As the Earth rotates, friction between the ocean floors and water, is constantly slowing down the Earth, as if tidal bulges were immense brake shoes. As a resutlt the lenght of the Earth day is increasing by a fraction of a second each century. Also gravity between the moon, the sea, and the tidal bulges is slowing down the moon and causing it to move slowly away from the Earth. The net result will occur in billions of years in the future.
    At this time the length of the lunar month and the Earth day will have increased to the same amount of time, and will then be equal to 47 of our present days. Then just as the same side of the moon always faces us, the Earth will likewise always have the same side facing the moon.
    In this tidally locked position the moon will always occupy the same relative place in the Earths Sky. It would be great if it faced Chicago, it would look great over the Lake Michigan.
    However the tides would continue, but no longer compelled by the moon, these future tides will only be caused by the sun. As such compelled to the tides of today, future tides will be much smaller and less frequent, barely recognizable as the heirs to the tides which once helped to shape the plantet Earth we live on.
  12. Dec 12, 2003 #11


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    Re: The moons destiny

    This looks good, as far as it goes.

    But, as you state yourself, the Sun will still cause tides. These tides will continue to slow the Earth's rotation, as the Sun tries to tidally lock the Earth to itself. The result will be that the Earth will be pulled out of tidal lock with the Moon, causing the moon to spiral inward.
  13. Dec 13, 2003 #12
    Re: Re: The moons destiny

    I have googled the Roche limit below, its clear what you say.
    There must be other examples in our solar sytem to demonstrate the effects of the Roche limit. With Jupiter and Saturn approaching the 100 limit of satellites, is there an example of any of those moons that is getting close to this limit? What about Pluto and Charon that appear to be locked now and near this limit? The two links below give a nice demonstation of what you have said. It also explains why rings form around planets and at what distance they are and also why the ring particles do not further break apart.
  14. Dec 13, 2003 #13


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    Re: Re: Re: The moons destiny

    There are quite a number of moon in the solar system which are being "pulled in " by their respective planets, the majority because they have retrograde orbits (They orbit in the opposite direction of the Planet's rotation.)

    A lot of these are small captured asteroids.

    There are a few examples of moons that actually match the pattern of having direct orbits with periods less than the rotation period of the planet, notably, Phobos of Mars, and Metis and Adrastea of Jupiter.

    The Uranus system is odd in that it is the planet that rotates retrograde while the majority of the planets orbit directly.

    As to whether a moon will break up and form a ring system once it passes within the Roche limit, this also depends on the size of the Moon. The Roche limit generally only applies to bodies that are large and massive enough for their own gravity to force them into a spherical shape. Bodies much smaller than this can pass within the Roche Limit without being torn apart. Examples of this are Metis and Phobos, both small, irregularly shaped moons that are actually already inside their respective Roche limits.

    The majority of moons that are presentally approaching their planet are these small bodies.

    Of the large bodies, we have Miranda, Ariel, Umbrial, Titania, and Oberon of Uranus, and Triton of Neptune.

    These bodies will eventually pass inside the Roche limit, break up and contribute to these planet's already existing rings.
  15. Dec 13, 2003 #14
    Re: Re: Re: Re: The moons destiny

    Thanks for the great insight on the Roche limit. I missed this one on our radio program, do not know why, this is very interesting to me. We will discuss Uranus later but first would like to discuss the Roche limit in regards to Pluto and Charon. One question i am sure many would like to know. Was the Roche limit found first by observation then comparison to GR or GR calculations first and then observation? Even though Galileo could see Saturns rings, it was not clear what they were then. On this one, did observation and GR kind of come at the same time?

    Our Earth and Moon orbit there collective center of mass, their barycenter, this cosmic "balance point" lies beneath the Earths surface. The Earth is much larger than the Moon in comparison to Pluto and Charon. The masses of Pluto and Charon posses "mutual orbits" unique among planets. Their barycenter is not beneath Plutos surface, but a good distance from it. As the two bodies orbit their barycenter, it is as if they were connected by a long bar in a perpetual celestial dance. The two bodies Pluto and Charon orbit "one another" highly inclined to the Ecyliptic, the two bodies tidally bonded , facing each other all the time. Then every 6.38 Earth days, they not only complete one orbit around their barycenter, but also complete one rotation on there axis.

    Charon has a diameter of 1,190 kilometers or 740 miles, it is about half the size of Pluto. Charon has a low density about twice that of water, which suggests that Charon is composed of water ice and rock. This would be an easy candidate to breakup.

    Question: Is Pluto and Charon a candidate of the Roche limmit or is this a case apart? If so does GR predict this and what is its name?
    Last edited: Dec 13, 2003
  16. Dec 13, 2003 #15


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    Re: Re: Re: Re: Re: The moons destiny

    The Roche Limit was first predicted by the math. GR is not involved as it fits perfectly well within classical physics.

    There is a factor working against Pluto pulling Charon in close enough for it to pass inside the Roche limit and break up.

    There is the distance of Pluto/Charon from the Sun. It averages about 40 times that of the distance of Earth to Sun. Since tidal forces fall off by the cube of distance, the tidal effect of the Sun on Pluto would be about 1/64000 that of that on the Earth.

    It would take trillions of years for it to take place. The chances of the system remaining undisturbed for that period of time are pretty slim.
  17. Dec 13, 2003 #16
    Re: Re: Re: Re: Re: Re: The moons destiny

    Regarding the Roche limit to Uranus and its five major moons,Miranda, Ariel, Umbrial, Titania, and Oberon. The Roch limit seems to have the same effect whether or not the planet has a retrograde rotation. The orbital revolution or axial rotation of a planetary or other celestial body that moves clockwise from east to west, in the direction opposite to most celestial bodies is not the norm, it is the exception.

    All Uranus moons are "tidally locked", the same side of each all face the planet. All are thought to be composed of ice and rock. The Uranian moons may contain slightly more rocky material, but not enough to explain geological resurfacing, unexpectedly found on the five major moons. This far from the sun all were expected to be completely frozen. But all five appear to have been reshaped since the formation of the solar system. The resurfacing is believed to be carved by the gravity of Uranus. The surface effects on all the moons vary to more or less geological activity, depending on distance from Uranus. We are talking about 365,000 miles to 80,000 miles at the closest. Moons in size between 300 and 1,000 miles in diameter. The surface features include craters with scarps, rolling terrain, valleys, scarps and ridges,frost on crater ridges,carbonous liquid flows.

    I mention all the above to arrive at the questions:

    Does the Roche limit understanding predict, the geological features found on the moons of Uranus? Is it a prepartion of what is to come? Is then the state of the moons in a transition period of a type of semi-volcanic geological heating, before breakup? Also does the Uranus inclination to axis of 97.9 degrees have an effect on the moons, as there is a doble curiousity of the moons having a 0 percent inclination. If you line up a celestial body with its north and south pole pointing towards a moon does it pull more or faster when within the Roche limit? Does the north pole or south pole pointing first make a difference in attactive force when a moon is with in the Roche limit?
    Last edited: Dec 13, 2003
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