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Thread: Mystery of the moon...

  1. #1 Mystery of the moon... 
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    The moon has maintained a darkside that is permanently away from the observer on earth since the dawn of mankind. But to do this the moon's rotation about it's axis and its rotation about the earth must be perfectly matched (time period about 29.5.... days).
    Even if this was by some extraordinary coincidence, large meteorites hit the moon once in a while and shouldn't that change the fine balance between the two rotational motions, thus eventually leading leading to the far side of the moon being seen. But we all know know that didn't happen.
    Please explain this to me.....Also can someone calculate the probability for the moon to have this fine balance for thousands of years.......


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    Quote Originally Posted by ashwinvinoo View Post
    The moon has maintained a darkside that is permanently away from the observer on earth since the dawn of mankind. But to do this the moon's rotation about it's axis and its rotation about the earth must be perfectly matched (time period about 29.5.... days).
    Even if this was by some extraordinary coincidence, large meteorites hit the moon once in a while and shouldn't that change the fine balance between the two rotational motions, thus eventually leading leading to the far side of the moon being seen. But we all know know that didn't happen.
    Please explain this to me.....Also can someone calculate the probability for the moon to have this fine balance for thousands of years.......
    With the given parameters, 100%.

    Tidal locking - Wikipedia, the free encyclopedia

    Also, moving to Astronomy. And welcome.


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    It is also worth noting that because of small variations in orbit, rotation and nutation we can see more than 50% of the lunar surface from the Earth.
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    Thanks ,I understand this perfectly now
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    I distinctly remember in the 6th. grade, we were told in Science class that the Moon DOES NOT ROTATE about an AXIS, as does the Earth. I immediately questioned how that was possible, when the moon, in circling about the Earth, ALWAYS showed the same face towards us.

    The teacher, and others, being typical "locked-in" "Professionals", unable to think for themselves, answered that the textbooks were written by experts, and should not be questioned. I wonder today whether "No Child Left Behind" has altered the comedy to any great extent? joc
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    Quote Originally Posted by jocular View Post
    I distinctly remember in the 6th. grade, we were told in Science class that the Moon DOES NOT ROTATE about an AXIS, as does the Earth. I immediately questioned how that was possible, when the moon, in circling about the Earth, ALWAYS showed the same face towards us.
    Everything that rotates does so about an axis - otherwise it wouldn't rotate.
    Maybe they meant doesn't rotate about its "own" axis.
    That one's simple:


    It could achieve the same effect by rotating about the Earth's.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by jocular View Post
    I distinctly remember in the 6th. grade, we were told in Science class that the Moon DOES NOT ROTATE about an AXIS, as does the Earth. I immediately questioned how that was possible, when the moon, in circling about the Earth, ALWAYS showed the same face towards us.
    Everything that rotates does so about an axis - otherwise it wouldn't rotate.
    Maybe they meant doesn't rotate about its "own" axis.
    That one's simple:


    It could achieve the same effect by rotating about the Earth's.
    The Moon rotates on its own axis once per orbit of the Earth. So it might not look like it does but when you play with objects (coins) and make one orbit the other and try and keep one face always facing the orbited body you will find you have to rotate the orbiting body
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    Quote Originally Posted by Robittybob1 View Post
    The Moon rotates on its own axis once per orbit of the Earth.
    I'm aware of that, perhaps you missed the words "achieve the same effect ",

    So it might not look like it does but when you play with objects (coins) and make one orbit the other and try and keep one face always facing the orbited body you will find you have to rotate the orbiting body
    Perhaps you missed the diagram.
    If the line between "Earth" and "Moon" is taken as a solid bar 1 then it can be seen and UNDERSTOOD that exactly the same effect is achieved - that the Moon has one face constantly toward the Earth - WITHOUT rotation (on an independent axis) of the orbiting body.

    1 The "solid bar" is not, of course, necessary - but it may help those somewhat slow on the uptake to realise that the Moon doesn't rotate on a separate axis in this particular case.
    Last edited by Dywyddyr; October 26th, 2013 at 09:24 AM.
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    Actually, it's because aliens have a secret base on the far side of the moon...my dog told me this with telepathy. Fortunately my tin-foil hat keeps their alien brain scans blocked.
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    Quote Originally Posted by MacGyver1968 View Post
    Actually, it's because aliens have a secret base on the far side of the moon...my dog told me this with telepathy. Fortunately my tin-foil hat keeps their alien brain scans blocked.

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    what about earth, is it also 50% visible to somebody living n moon?
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    Quote Originally Posted by sir ir r aj View Post
    what about earth, is it also 50% visible to somebody living n moon?
    No. The moon can be seen from anywhere on earth, so a person on the moon could see all parts of the earth, some time of the day. Unless they were on the far side of the moon, in which case they would never see any of it.
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by sir ir r aj View Post
    what about earth, is it also 50% visible to somebody living n moon?
    No. The moon can be seen from anywhere on earth, so a person on the moon could see all parts of the earth, some time of the day. Unless they were on the far side of the moon, in which case they would never see any of it.
    does earth goes into phases like moon?
    can we not invent a telescope which has the power to bend/ reflect light without touching any solid and help in seeing black side f moon. a sort of peeping (peep means to look through, over, or around something)telescope
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    How would you "bend light without touching anything solid"? We require lenses to "bend light", even in our own eyes. Even atmospheric disturbance is due to light interaction with gases. Being that there is no atmosphere in space, how would you go about altering the path of light without interaction with a solid object?
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    Quote Originally Posted by MacGyver1968 View Post
    Actually, it's because aliens have a secret base on the far side of the moon...my dog told me this with telepathy. Fortunately my tin-foil hat keeps their alien brain scans blocked.
    now your life is at risk...
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    Quote Originally Posted by Flick Montana View Post
    How would you "bend light without touching anything solid"? We require lenses to "bend light", even in our own eyes. Even atmospheric disturbance is due to light interaction with gases. Being that there is no atmosphere in space, how would you go about altering the path of light without interaction with a solid object?
    nicely put, now that is my question. thanks flick
    a sort of "boomerang effect"
    Last edited by sir ir r aj; October 26th, 2013 at 11:08 AM.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Robittybob1 View Post
    The Moon rotates on its own axis once per orbit of the Earth.
    I'm aware of that, perhaps you missed the words "achieve the same effect ",

    So it might not look like it does but when you play with objects (coins) and make one orbit the other and try and keep one face always facing the orbited body you will find you have to rotate the orbiting body
    Perhaps you missed the diagram.
    If the line between "Earth" and "Moon" is taken as a solid bar 1 then it can be seen and UNDERSTOOD that exactly the same effect is achieved - that the Moon has one face constantly toward the Earth - WITHOUT rotation (on an independent axis) of the orbiting body.

    1 The "solid bar" is not, of course, necessary - but it may help those somewhat slow on the uptake to realise that the Moon doesn't rotate on a separate axis in this particular case.
    The science around here is shocking! Just 2 more days and I can go back to Physforums.
    "achieve the same effect " and solid bars between the Earth and Moon!!!!
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by jocular View Post
    I distinctly remember in the 6th. grade, we were told in Science class that the Moon DOES NOT ROTATE about an AXIS, as does the Earth. I immediately questioned how that was possible, when the moon, in circling about the Earth, ALWAYS showed the same face towards us.
    Everything that rotates does so about an axis - otherwise it wouldn't rotate.
    Maybe they meant doesn't rotate about its "own" axis.
    That one's simple:


    It could achieve the same effect by rotating about the Earth's.
    Not sure what you are trying to say here. Rotation is all about an axis (could be more than 1) within the body in question, while orbits are of the whole body around some common centre of gravity with something else. Given that, the moon in your graphic completes a single rotation around it's own axis while completing one orbit of the planet. In other words, that moon would still experience a slight equatorial bulge due to centrifugal forces.

    No?
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    Quote Originally Posted by KALSTER View Post
    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by jocular View Post
    I distinctly remember in the 6th. grade, we were told in Science class that the Moon DOES NOT ROTATE about an AXIS, as does the Earth. I immediately questioned how that was possible, when the moon, in circling about the Earth, ALWAYS showed the same face towards us.
    Everything that rotates does so about an axis - otherwise it wouldn't rotate.
    Maybe they meant doesn't rotate about its "own" axis.
    That one's simple:


    It could achieve the same effect by rotating about the Earth's.
    Not sure what you are trying to say here. Rotation is all about an axis (could be more than 1) within the body in question, while orbits are of the whole body around some common centre of gravity with something else. Given that, the moon in your graphic completes a single rotation around it's own axis while completing one orbit of the planet. In other words, that moon would still experience a slight equatorial bulge due to centrifugal forces.

    No?
    You might be the first to ever consider the equatorial bulge of the Moon! It definitely would be "slight", since there is no longer a molten mantle and the rotation rate is so slow, and the radius value so small. Any distortion in the Moon's shape would be more likely attributed to the one-sided pull of the Earth -Moon gravity rather than the residual rotation about the Moon's own axis.
    http://www.space.com/2701-moon-stran...explained.html
    Moon's Strange Bulge Finally Explained

    by Sara Goudarzi, Staff Writer | August 03, 2006 02:10pm ET

    An eccentric orbit in the Moon's distant past might be responsible for the mysterious bulge around its middle, scientists say.
    The excess material around the lunar equator has been known since 1799 when French mathematician Pierre-Simon Laplace first noticed it. The reason, however, has been a mystery until now.
    The Moon's peculiar shape can be explained if the satellite moved in an eccentric oval-shaped orbit 100 million years after its violent formation, when the satellite hadn't yet solidified, the researchers say.


    It was like a big ball of molasses and all around the equator it got deformed, study team member Ian Garrick-Bethell of the Massachusetts Institute of Technology told SPACE.com.
    Around that time, conditions, such as orbit shape and position, were optimal for this "ball of molasses" to cool down and become the solid moon that we now know.


    Today, the Moon's orbit around the Earth is nearly circular.
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by sir ir r aj View Post
    what about earth, is it also 50% visible to somebody living n moon?
    No. The moon can be seen from anywhere on earth, so a person on the moon could see all parts of the earth, some time of the day. Unless they were on the far side of the moon, in which case they would never see any of it.
    They might have a giant periscope topeep around the edge of it. joc
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    Quote Originally Posted by jocular View Post
    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by sir ir r aj View Post
    what about earth, is it also 50% visible to somebody living n moon?
    No. The moon can be seen from anywhere on earth, so a person on the moon could see all parts of the earth, some time of the day. Unless they were on the far side of the moon, in which case they would never see any of it.
    They might have a giant periscope topeep around the edge of it. joc
    That was particularly good science! NOT!
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    Quote Originally Posted by Robittybob1 View Post
    The science around here is shocking!
    Perhaps if you bothered to read the post, and take some some time to place it in the context of the overall discussion then you'd realise what being said.

    "achieve the same effect " and solid bars between the Earth and Moon!!!!
    And then you wouldn't come up such ridiculous "objections".

    Quote Originally Posted by KALSTER View Post
    Not sure what you are trying to say here. Rotation is all about an axis (could be more than 1) within the body in question, while orbits are of the whole body around some common centre of gravity with something else. Given that, the moon in your graphic completes a single rotation around it's own axis while completing one orbit of the planet.
    No.
    I was pointing out that there IS a explanation for one face of the Moon to face the Earth all the time WITHOUT rotation about its own axis (and this is, in fact, a common phenomenon here in everday life 1). I mentioned the solid bar to show that the Moon doesn't (can't) rotate about its own axis (in this case).
    And it is this that leads people into thinking that the Moon doesn't rotate on its own axis, hence lengthy explanations are required to show that it in fact does.

    1 Anything spun around one's head on a piece of string for example will only show one face, hand wheels with rim bulges will always show only one "face" of the bulge to the central pivot, etc. etc.
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    Quote Originally Posted by sir ir r aj View Post
    Quote Originally Posted by Flick Montana View Post
    How would you "bend light without touching anything solid"? We require lenses to "bend light", even in our own eyes. Even atmospheric disturbance is due to light interaction with gases. Being that there is no atmosphere in space, how would you go about altering the path of light without interaction with a solid object?
    nicely put, now that is my question. thanks flick
    a sort of "boomerang effect"
    That effect would be caused by the shape of the object moving through the atmosphere. Throw it in space and it will just travel straight on. It doesn't apply to photons.

    In my experience, analogies are not particularly helpful in understanding scientific phenomena unless they are highly related, in which case it is often just as simple to understand the phenomenon scientifically.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Robittybob1 View Post
    The science around here is shocking!
    Perhaps if you bothered to read the post, and take some some time to place it in the context of the overall discussion then you'd realise what being said.

    "achieve the same effect " and solid bars between the Earth and Moon!!!!
    And then you wouldn't come up such ridiculous "objections".

    Quote Originally Posted by KALSTER View Post
    Not sure what you are trying to say here. Rotation is all about an axis (could be more than 1) within the body in question, while orbits are of the whole body around some common centre of gravity with something else. Given that, the moon in your graphic completes a single rotation around it's own axis while completing one orbit of the planet.
    No.
    I was pointing out that there IS a explanation for one face of the Moon to face the Earth all the time WITHOUT rotation about its own axis (and this is, in fact, a common phenomenon here in everday life 1). I mentioned the solid bar to show that the Moon doesn't (can't) rotate about its own axis (in this case).
    And it is this that leads people into thinking that the Moon doesn't rotate on its own axis, hence lengthy explanations are required to show that it in fact does.

    1 Anything spun around one's head on a piece of string for example will only show one face, hand wheels with rim bulges will always show only one "face" of the bulge to the central pivot, etc. etc.
    Sorry, but that doesn't work, All that would show is that the Moon doesn't rotate on it own axis relative to the rotating frame of its orbit around the Earth, but relative to the non-rotating frame, it is rotating on its own axis while traveling in a circle.

    The solid rod doesn't prevent it from rotating on its axis, but forces it to do so. Imagine that instead of the Moon being held by the solid bar it is attached by a frictionless spindle through its axis.

    You now start to spin the solid arm around. What happens to the Moon? does it behave like it does in your first example? No. it maintains the same orientation with respect to its original non-rotating frame. If it was not rotating about its own axis before you starting turning the arm, and it maintains the same orientation after the bar starts rotating, it is not rotating about its own axis then either. Now place your first device right above the second so that the two "Moons" are aligned and traveling at the same speed. Relative to the Moon on the spindle, the Moon solidly attached to the bar is rotating. Since the spindle moon is not rotating on it's axis, the solidly attached Moon is.

    Now besides the above, the argument that the Moon does not rotate on its own axis fails in two other ways:

    1. Its axis of rotation is not parallel to the axis of its orbit, but is tilted to it by some 6 degrees.
    2. the angular speed of its rotation differs from the angular speed of its orbit over the course of its orbit. This is because while its rotational velocity is constant through out the orbit, due to its eccentric orbit, the orbital angular speed varies. Sometimes it is faster than the axial rotation and sometimes it's slower.
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    Quote Originally Posted by Janus View Post
    Sorry, but that doesn't work, All that would show is that the Moon doesn't rotate on it own axis relative to the rotating frame of its orbit around the Earth, but relative to the non-rotating frame, it is rotating on its own axis while traveling in a circle.
    The point being: we are looking at the Moon from the (rotating frame of) Earth.
    I know what you're saying, but the example was given to illustrate why some people consider that it doesn't.

    Meh, I shouldn't have bothered...
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    Sorry, but that doesn't work, All that would show is that the Moon doesn't rotate on it own axis relative to the rotating frame of its orbit around the Earth, but relative to the non-rotating frame, it is rotating on its own axis while traveling in a circle.
    The point being: we are looking at the Moon from the (rotating frame of) Earth.
    I know what you're saying, but the example was given to illustrate why some people consider that it doesn't.

    Meh, I shouldn't have bothered...
    Well what would it take to get the Moon to begin rotating wrt the Earth again and would there be an advantage to this?
    When the Moon was rotating WRT the Earth the internal friction of the Moon's mass created heat and that has since cooled and probably the core has settled into an off balance position, and now is virtually locked into this position (like a yacht with it lead filled keel it resists rolling over).

    So does that mean the Moon slowed itself to the point where it spins on it axis at the same rate as it orbits the Earth? Or did it transfer its momentum to the Earth in a similar way the Earth is transferring its momentum to the Moon currently?
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    Sorry, but that doesn't work, All that would show is that the Moon doesn't rotate on it own axis relative to the rotating frame of its orbit around the Earth, but relative to the non-rotating frame, it is rotating on its own axis while traveling in a circle.
    The point being: we are looking at the Moon from the (rotating frame of) Earth.
    The Earth rotates at neither the same rate or on the same axis as the Moon.

    I know what you're saying, but the example was given to illustrate why some people consider that it doesn't.
    And I was pointing out why such a conception is totally false. If you are viewing the moon from a frame that rotates at the same rate as the Moon orbits, then relative to you, the Moon does not rotate, but neither does the moon orbit the Earth relative to this frame. But the people who say that the Moon doesn't rotate on its axis don't also claim that the Moon doesn't orbit the Earth. Instead they claim the the Moon orbits the Earth while at the same time doesn't rotate on its own axis. This is wrong, because it involves mixing two different frames with different rotations.

    Meh, I shouldn't have bothered...
    Last edited by Janus; October 27th, 2013 at 11:31 AM.
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    [QUOTE=Janus;477736]And I was pointing out why such a conception is totally false. If you are viewing the moon from a frame that rotates at the same rate as the Moon orbits, then relative to you, the Moon does not rotate, but neither does the moon orbit the Earth relative to this frame. But the people who say that the Moon doesn't rotate on its axis don't also claim that the Moon doesn't orbit the Earth. Instead they claim the the Moon orbits the Earth while at the same time doesn't rotate on its own axis. This is wrong, because it involves mixing two different frames with different rotations.[quote]
    Okay.
    So which axis IS the Moon rotating around in the diagram?
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    Quote Originally Posted by Dywyddyr View Post
    Okay.
    So which axis IS the Moon rotating around in the diagram?
    It is rotating around its own axis while that axis travels around in a circle around another axis.
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by Dywyddyr View Post
    Okay.
    So which axis IS the Moon rotating around in the diagram?
    It is rotating around its own axis while that axis travels around in a circle around another axis.
    You're seriously losing me, I can't see it and it's bugging the sh*t out of me.
    By "its own axis" do you mean an axis that's geometrically inside the "circle" of the Moon - the centre of the Moon's circle?
    If so how come the drawn in diameter remains at 90 degrees to the Earth-Moon line - i.e. if actually constructed as a model there'd be no twisting of that diameter or the "solid bar"?
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by Dywyddyr View Post
    Okay.
    So which axis IS the Moon rotating around in the diagram?
    It is rotating around its own axis while that axis travels around in a circle around another axis.
    You're seriously losing me, I can't see it and it's bugging the sh*t out of me.
    By "its own axis" do you mean an axis that's geometrically inside the "circle" of the Moon - the centre of the Moon's circle?
    If so how come the drawn in diameter remains at 90 degrees to the Earth-Moon line - i.e. if actually constructed as a model there'd be no twisting of that diameter or the "solid bar"?
    That's why I made my post, as each part of your drawing is closely analogous to how the moon behaves now, tidally locked as it is.

    Like swinging a ball on a string. The string provides the centripetal force that makes the ball orbit around your hand, just like gravity does for the moon around the common centre of gravity in the earth/moon system. Then because the string is tied to the ball, the ball is forced to present the same face to your hand the whole time, just like the tidal locking has done to the moon. Only difference is that the ball does not have an intrinsic spin, while the moon still does have angular momentum left over from its formation.

    In both cases, the diameter line connected to the earth/hand remains in tact (more or less). My confusion then was where you saw a difference between the ball analogy and the earth moon system in terms of whether the moon rotates on its own axis or not.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by KALSTER View Post
    That's why I made my post, as each part of your drawing is closely analogous to how the moon behaves now, tidally locked as it is.

    Like swinging a ball on a string. The string provides the centripetal force that makes the ball orbit around your hand, just like gravity does for the moon around the common centre of gravity in the earth/moon system. Then because the string is tied to the ball, the ball is forced to present the same face to your hand the whole time, just like the tidal locking has done to the moon. Only difference is that the ball does not have an intrinsic spin, while the moon still does have angular momentum left over from its formation.

    In both cases, the diameter line connected to the earth/hand remains in tact (more or less). My confusion then was where you saw a difference between the ball analogy and the earth moon system in terms of whether the moon rotates on its own axis or not.
    Exactly!
    I'm not denying at all that the Moon does rotate, just highlighting the fact that a similar effect can occur without that rotation (pending Janus - whose posts I do appreciate - managing to drill it into my thick skull where my misunderstanding is) and is "more common" (e.g. ball on a string) which would/ could explain why there's "resistance" to the idea of Lunar rotation among some people.
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    Quote Originally Posted by Dywyddyr View Post
    If so how come the drawn in diameter remains at 90 degrees to the Earth-Moon line
    Because if it weren't rotating, then the diameter line would always point in the same direction.
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    Quote Originally Posted by Strange View Post
    Because if it weren't rotating, then the diameter line would always point in the same direction.
    Which axis? That's what I can't fathom.
    If it's rotating on its own axis why doesn't the angle between the Earth-Moon line and the diametral (real word?) line alter?

    Oh, @ Janus: no I'm not claiming, nor intending, to indicate that the Moon rotates at the same rate the Earth: but I didn't think to indicate rotation rates on the diagram.

    Edit; when the penny drops I can see myself giving myself a good slap on the head, but as yet...
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    Quote Originally Posted by Dywyddyr View Post
    If it's rotating on its own axis why doesn't the angle between the Earth-Moon line and the diametral (real word?) line alter?
    Because it orbits the earth at the same rate. Why is that confusing? (I know there are endless silly arguments about the semantics of "rotate" but that doesn't really change the facts.)
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    Quote Originally Posted by Strange View Post
    Because it orbits the earth at the same rate. Why is that confusing?
    You're talking about the real Moon or the "fake" one with a solid bar? (E.g. the example that Kalster noted out doesn't have any intrinsic rotation).


    Again, I regret posting the diagram: I should have left well alone.
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    Quote Originally Posted by Dywyddyr View Post
    You're talking about the real Moon or the "fake" one with a solid bar?
    I assumed your diagram was supposed to represent the real moon.

    (E.g. the example that Kalster noted out doesn't have any intrinsic rotation).
    He said:
    Quote Originally Posted by KALSTER
    the moon in your graphic completes a single rotation around it's own axis while completing one orbit of the planet.
    which is what I said.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Strange View Post
    Because it orbits the earth at the same rate. Why is that confusing?
    You're talking about the real Moon or the "fake" one with a solid bar? (E.g. the example that Kalster noted out doesn't have any intrinsic rotation).
    While I think it does. If your diagram and the real situation has direct key analogous aspects to each other, then the moon in both should be considered to be rotating. It does from the top down perspective of your diagram.

    The job the steel bar is doing is the same thing as gravity is doing in reality. To get it to halfway, imagine a positively charged disc rotating in place in the centre of a board and another, smaller disc with half of it oppositely charged fixed to, but free to rotate on, a moving ring. Here again the smaller disc will present one side to the bigger one the whole time, while still rotating in place on the ring. This would work whether the big disc's rotation was in sync with the smaller one's orbit or not.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by Dywyddyr View Post
    Okay.
    So which axis IS the Moon rotating around in the diagram?
    It is rotating around its own axis while that axis travels around in a circle around another axis.
    You're seriously losing me, I can't see it and it's bugging the sh*t out of me.
    By "its own axis" do you mean an axis that's geometrically inside the "circle" of the Moon - the centre of the Moon's circle?
    If so how come the drawn in diameter remains at 90 degrees to the Earth-Moon line - i.e. if actually constructed as a model there'd be no twisting of that diameter or the "solid bar"?

    Yes, I mean around the center of the Moon.
    Here's the thing, you are trying to define "spinning on its axis" as rotating relative to the solid bar, which is itself rotating relative to another frame. You are using two different frames to define rotation. One for the solid bar at which the moon on the end of, and another for the moon on its axis.

    Go back to the "Moon on a spindle". Add a second moon on a spindle, but not on the end of the solid bar. Start with neither bar or Moon rotating. Spin up just the Moon's so that they rotate at the same rate. . Now start slowly rotating the bar. As you do so, the Moon's don't change their rotation with respect to Each other (Since the spindle is frictionless no torque is applied to the axis of the Moon that in on the end of the bar.). At first you have a moon spinning on its axis at one speed while the bar carries it around in a circle at another.

    Eventually, you will reach a point where the bar is rotating at the same speed as the Moon that isn't attached to the bar.

    So you have two moon's spinning at the same speed and the arm spinning at the same speed swinging one of the Moon's in a circle, resulting in this moon keeping one face constantly towards the axis of rotation of the Bar. But remember, the rotation of the Moon on its own axis is independent of the rotation of the bar. (spinning the bar did not impart any rotation to the Moon, it had that rotation before we started spinning the bar). They just happen to have the same rate of spin.

    Now apply a brake to the spindle so that the Moon can no longer rotate freely with respect to it. This mimics the moon being attached rigidly to the bar. What changes?. A Moon being swung around rigidly attached to the bar, is no different from the moon spinning freely at the same rate as the bar rotates. Both signify moons that rotate on their axi as they orbit another.
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    Quote Originally Posted by Strange View Post
    I assumed your diagram was supposed to represent the real moon.
    I stated it wasn't.

    He said:
    Quote Originally Posted by KALSTER
    the moon in your graphic completes a single rotation around it's own axis while completing one orbit of the planet.
    Not with regard to a ball on string.

    Quote Originally Posted by Kalster
    While I think it does. If your diagram and the real situation has direct key analogous aspects to each other, then the moon in both should be considered to be rotating. It does from the top down perspective of your diagram.
    The job the steel bar is doing is the same thing as gravity is doing in reality. To get it to halfway, imagine a positively charged disc rotating in place in the centre of a board and another, smaller disc with half of it oppositely charged fixed to, but free to rotate on, a moving ring. Here again the smaller disc will present one side to the bigger one the whole time, while still rotating in place on the ring. This would work whether the big disc's rotation was in sync with the smaller one's orbit or not.
    Okay, what if the diameter is steel bar - welded to the Earth-Moon one?
    Rotation around the Moon's own axis?


    @ Janus:
    Here's the thing, you are trying to define "spinning on its axis" as rotating relative to the solid bar
    That's exactly what I was doing.
    And yes, I think I've got it now...

    Sorry for the long drawn-out (and mostly irrelevant) continuation, and thanks to everyone.
    Last edited by Dywyddyr; October 27th, 2013 at 02:45 PM.
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    mystery solved?
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    Quote Originally Posted by Dywyddyr View Post
    Okay, what if the diameter is steel bar - welded to the Earth-Moon one?
    Rotation around the Moon's own axis?
    Yep.

    To borrow from Janus; Once the small disc presents the same face to the big one, you'll get exactly the same rates of rotation for the small and big discs whether you then fasten the small disc to the big one or not. You are just substituting one method of syncing the "moon's" orbit and rotation with another. With the steel bar though, you'll be syncing it with the rotation of the "earth" as well.

    Sorry for the long drawn-out (and mostly irrelevant) continuation, and thanks to everyone.
    Hey man, I have made a much worse ass of myself here before and more than once.
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    The moon and the earth are massive objects and tidally locked so it would take a lot to dislodge them. However the moon is slowly drifting away from the earth and will escape earths orbit in about 6 billion years. There is a risk that the moon may pollute the earths orbit and crash into the earth. However the earth may be eaten by the sun before this time.
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    If the Moon is moving away how is it going to "crash into the Earth"?
    How do you "pollute an orbit"?
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    The moon is moving away. Eventually it will reach a distance where the moon is no longer captured by the earth. The moon will then drift in an orbit around the sun instead. That orbit may cause it to cross the orbit of the earth and the two will collide.
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    Quote Originally Posted by uptonryan View Post
    The moon is moving away. Eventually it will reach a distance where the moon is no longer captured by the earth. The moon will then drift in an orbit around the sun instead. That orbit may cause it to cross the orbit of the earth and the two will collide.
    Even if enough time were allowed for it to take place, the Moon would not escape the Earth. The Moon is presently receding from the Earth due to tidal interaction and the fact that the Earth rotates faster than the moon orbits. The Earth is transferring angular momentum to the Moon, pushing it to a higher orbit while slowing its own rotation. (the transfer is not perfect as a great deal of the rotational energy the Earth gives up is lost to tidal friction.). The upshot is that long before the Moon recedes far enough to escape the Earth, the Earth will become tidally locked to the Moon. With no difference between the Earth's rotation rate and the Moon's orbit ,the Moon will stop receding.

    Now if it was only the Earth and Moon involved, this would be the end of the story. However, the Sun also has a tidal effect on the Earth, ~1/2 that of the Moon's at present.

    Tidal friction between the Sun and Earth will act as a brake on the Earth's rotation and it will start to rotate slower than the Moon orbits. When this happens, the reverse of the Moon-Earth tidal interaction of today will occur. The Moon will start transferring angular momentum to the Earth and start slowly spiraling back in towards the Earth.

    When it gets close enough to the Earth, it will be torn apart by tidal forces and the Earth would get a ring.
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    Janus do you have some mathematical modelling to support that theory? My model says that it will escape earth orbit but only way after the sun has become a super red giant.
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    Quote Originally Posted by uptonryan View Post
    Janus do you have some mathematical modelling to support that theory? My model says that it will escape earth orbit but only way after the sun has become a super red giant.
    "The drag from the solar atmosphere may cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross the Earth's Roche limit. Tidal interaction with the Earth would then break apart the Moon, turning it into a ring system. Most of the orbiting ring will then begin to decay, and the debris will impact the Earth. Hence, even if the Earth is not swallowed up by the Sun, the planet may be left moonless."
    (cf. http://en.wikipedia.org/wiki/Future_of_the_Earth)

    What is your model?
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

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    Quote Originally Posted by Cogito Ergo Sum View Post
    Quote Originally Posted by uptonryan View Post
    Janus do you have some mathematical modelling to support that theory? My model says that it will escape earth orbit but only way after the sun has become a super red giant.
    "The drag from the solar atmosphere may cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross the Earth's Roche limit. Tidal interaction with the Earth would then break apart the Moon, turning it into a ring system. Most of the orbiting ring will then begin to decay, and the debris will impact the Earth. Hence, even if the Earth is not swallowed up by the Sun, the planet may be left moonless."
    (cf. http://en.wikipedia.org/wiki/Future_of_the_Earth)

    What is your model?
    "Like this is very much into the future. Man will have become an extinct species long before that happens for sure." Agree or disagree?
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    Man will have become an extinct species long before that happens for sure
    The best way to establish that is to see if there are any tourists from the future. If we assume that there will be some sort of technological singularity then we may even assume time travel. If that is the case then were are all the visitors from the future? If we all go on forever, or even if someone else does, then why don't they visit. We may assume the maybe we are too boring to visit or too dangerous but still. Still 6 Billion years is a very very long time surely long enough to develop time travel. So maybe we do all die very shortly. There certainly are a lot of things out there to kill us. However there are some things that are kinda suss. The way the moon and mars are placed kinda look like stepping stones. Venus as a warning. Sort of looks like messages being sent. To me at least. Plus the way the dinosaurs were killed off when they were showing no promise, but Our Killer KY event is long past its due date. Yeah maybe we are being babied and looked after. Then again maybe the reason why our children never visit is because they are inconsiderate.
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    Quote Originally Posted by uptonryan View Post
    Man will have become an extinct species long before that happens for sure
    The best way to establish that is to see if there are any tourists from the future. If we assume that there will be some sort of technological singularity then we may even assume time travel. If that is the case then were are all the visitors from the future? If we all go on forever, or even if someone else does, then why don't they visit. We may assume the maybe we are too boring to visit or too dangerous but still. Still 6 Billion years is a very very long time surely long enough to develop time travel. So maybe we do all die very shortly. There certainly are a lot of things out there to kill us. However there are some things that are kinda suss. The way the moon and mars are placed kinda look like stepping stones. Venus as a warning. Sort of looks like messages being sent. To me at least. Plus the way the dinosaurs were killed off when they were showing no promise, but Our Killer KY event is long past its due date. Yeah maybe we are being babied and looked after. Then again maybe the reason why our children never visit is because they are inconsiderate.
    What is the difference you say here and the basic concept of the Rapture? There seemed to be a hint of Hope and Faith in some higher power there.
    I am Christian but I have no idea what the Rapture would mean. I take life as it comes is my motto, but I think Global Warming is going to bite humans in the butt.
    I can do my little recycling project but I'm still part of the problem not the solution.
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    What is the difference you say here and the basic concept of the Rapture? There seemed to be a hint of Hope and Faith in some higher power there.
    I am Christian but I have no idea what the Rapture would mean. I take life as it comes is my motto, but I think Global Warming is going to bite humans in the butt.
    I can do my little recycling project but I'm still part of the problem not the solution.
    Well there is always faith. Human Beings have come a long way and done amazing things. We just need to remain strong, look towards the future and resist the sins of apathy, ignorance and intolerance. Remember God helps those that help themselves and do unto others as you would have them do unto you.
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    so, we cant live on moon in future. i had booked my plot through Authorized Lunar Embassy Agent
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    so, we cant live on moon in future.
    Well you can for the next 6 billion years at least.
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    Quote Originally Posted by uptonryan View Post
    so, we cant live on moon in future.
    Well you can for the next 6 billion years at least.
    we have 6 billion years to think. cant we save our moon?
    edit: even if we save moon, how can we save both from expanding sun. can anybody tell how much it will take for sun to engulf earth and moon?
    edit: if we move to europa or mars , before being scorched by Sun, what guarantee we have that sun will not follow us there?
    Last edited by sir ir r aj; October 30th, 2013 at 04:24 AM. Reason: new
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    Quote Originally Posted by uptonryan View Post
    What is the difference you say here and the basic concept of the Rapture? There seemed to be a hint of Hope and Faith in some higher power there.
    I am Christian but I have no idea what the Rapture would mean. I take life as it comes is my motto, but I think Global Warming is going to bite humans in the butt.
    I can do my little recycling project but I'm still part of the problem not the solution.
    Well there is always faith. Human Beings have come a long way and done amazing things. We just need to remain strong, look towards the future and resist the sins of apathy, ignorance and intolerance. Remember God helps those that help themselves and do unto others as you would have them do unto you.
    And another bit of advice is not to tell an administrator he doesn't need to read the posts! Sorry Harold I'm working on an old computer so your name didn't show up! Well All is good. Thanks.
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by uptonryan View Post
    The moon is moving away. Eventually it will reach a distance where the moon is no longer captured by the earth. The moon will then drift in an orbit around the sun instead. That orbit may cause it to cross the orbit of the earth and the two will collide.
    Even if enough time were allowed for it to take place, the Moon would not escape the Earth. The Moon is presently receding from the Earth due to tidal interaction and the fact that the Earth rotates faster than the moon orbits. The Earth is transferring angular momentum to the Moon, pushing it to a higher orbit while slowing its own rotation. (the transfer is not perfect as a great deal of the rotational energy the Earth gives up is lost to tidal friction.). The upshot is that long before the Moon recedes far enough to escape the Earth, the Earth will become tidally locked to the Moon. With no difference between the Earth's rotation rate and the Moon's orbit ,the Moon will stop receding.

    Now if it was only the Earth and Moon involved, this would be the end of the story. However, the Sun also has a tidal effect on the Earth, ~1/2 that of the Moon's at present.

    Tidal friction between the Sun and Earth will act as a brake on the Earth's rotation and it will start to rotate slower than the Moon orbits. When this happens, the reverse of the Moon-Earth tidal interaction of today will occur. The Moon will start transferring angular momentum to the Earth and start slowly spiraling back in towards the Earth.

    When it gets close enough to the Earth, it will be torn apart by tidal forces and the Earth would get a ring.
    ~ and that before that can happen the central body of this solar system will have started to expand. The star Sol. will engulf the Earth Moon system before they get to play out this little model..
    We would do better to concern ourselves with detection of Earth crosser s.. and how to make our path clear.. and get off this planet..
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    We would do better
    Terraform mars. Collide asteroids into it to increase its mass and atmosphere. Give it a moon so that the mars core becomes molten and it develops plate tectonics as a magnetic field. Make it habitable in a 100 years time. Create a Mars hotel that constantly orbits between earth and mars picking up and putting down travellers. Mars is easy. Finding The next step after that is a lot harder. I don't think Europa is going to be all that nice.
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    Quote Originally Posted by Robittybob1 View Post
    "Like this is very much into the future. Man will have become an extinct species long before that happens for sure." Agree or disagree?

    Personally, I find it very likely that mankind is extinct before any of this has happened.
    One species surviving for billions of years does not seem plausible.

    Quote Originally Posted by uptonryan
    Janus do you have some mathematical modelling to support that theory? My model says that it will escape earth orbit but only way after the sun has become a super red giant.

    Have you already presented your model?
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    Personally, I find it very likely that mankind is extinct before any of this has happened
    We have survived quite a long time so far. You have to ask what would replace us? Machines? Aliens? We are fairly well suited to earth. Maybe the planets start to drift. Jupiter decides it likes an inner orbit. A wandering planet enters the solar system or a nearby star goes super nova. However in the 4.5 billion years so far none of those has happened. Our biggest problem is the Easter Island effect. On Easter Island a group of natives set up camp and cut down all the trees to build large stone statues to their gods. They quickly consumed all the resources and starved themselves to death. If we consume all Earths resources before we make it to Mars we may eat ourselves to extinction.
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    Quote Originally Posted by uptonryan View Post
    Personally, I find it very likely that mankind is extinct before any of this has happened
    We have survived quite a long time so far. You have to ask what would replace us? Machines? Aliens? We are fairly well suited to earth. Maybe the planets start to drift. Jupiter decides it likes an inner orbit. A wandering planet enters the solar system or a nearby star goes super nova. However in the 4.5 billion years so far none of those has happened. Our biggest problem is the Easter Island effect. On Easter Island a group of natives set up camp and cut down all the trees to build large stone statues to their gods. They quickly consumed all the resources and starved themselves to death. If we consume all Earths resources before we make it to Mars we may eat ourselves to extinction.

    It is, according to Wikipedia, agreed upon that modern humans appeared on Earth about 200.000 years ago. This is insignificant when compared with the events that might happen 4.5 - 6 billion years into the future.

    Is there any evidence for your statements concerning the fate of the Solar System (such as Jupiter decreasing its distance to the Sun)? If there is none, then that would make a quite worthless model.

    Furthermore, I support the notion of space exploration and I think that Mars, Europa and Titan might be suited for extraterrestrial camps.
    But even then, surviving for a few billions years (even in another environment) seems unlikely, in my opinion.
    Last edited by Cogito Ergo Sum; November 9th, 2013 at 02:15 PM.
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    such as Jupiter decreasing its distance to the Sun
    There is some evidence that the outer planets have changed orbits over their lifetimes. Jupiter may have been closer to the sun earlier on and drifted outwards. Uranus is tilted on its side which suggest a strange encounter. The solar system orbits are roughly circular. Of course this would easily change if we drifted closer to another gravitational body. Plenty of other stars seem to have there Jupiter's closer to their suns. It is unusual to have large planets on the outer like we do. Currently mercury is evaporation and reducing in size. This means its orbit is changing and there is an increased chance that may destabilise the other orbits. Sometimes that can result in a planet ejection. Our system is delicately balanced and planetary drift can occur. It is not that rare for Jupiter to enter the inner solar system. Of course that would be a disaster for us if it were to happen. Europa and Titan have issues in that Jupiter bathes them in extreme radiation. That radiation makes life difficult. Christopher Columbus went through all sorts of hell to discover America. I am sure the first space explorers will too.
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    Quote Originally Posted by uptonryan View Post
    such as Jupiter decreasing its distance to the Sun
    There is some evidence that the outer planets have changed orbits over their lifetimes. Jupiter may have been closer to the sun earlier on and drifted outwards. Uranus is tilted on its side which suggest a strange encounter. The solar system orbits are roughly circular. Of course this would easily change if we drifted closer to another gravitational body. Plenty of other stars seem to have there Jupiter's closer to their suns. It is unusual to have large planets on the outer like we do. Currently mercury is evaporation and reducing in size. This means its orbit is changing and there is an increased chance that may destabilise the other orbits. Sometimes that can result in a planet ejection. Our system is delicately balanced and planetary drift can occur. It is not that rare for Jupiter to enter the inner solar system. Of course that would be a disaster for us if it were to happen. Europa and Titan have issues in that Jupiter bathes them in extreme radiation. That radiation makes life difficult.

    May I see these evidences?
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

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    Quote Originally Posted by uptonryan View Post
    Terraform mars. Collide asteroids into it to increase its mass and atmosphere.
    The total mass of asteroids is only a small fraction of the mass of Mars, so even if all were impacted with the planet there would be no significant mass increase.

    Asteroids are, for the most part, low on volatiles. There would be very little atmosphere gain through asteroid impact. On the other hand a handful of comets would work wonders.

    Quote Originally Posted by uptonryan View Post
    Give it a moon so that the mars core becomes molten and it develops plate tectonics as a magnetic field. Make it habitable in a 100 years time.
    How do you intend to acquire a moon large enough to produce the required flexure in the parent to generate enough heat to yield a molten core? And 100 years is ludicrous as a timescale using the methodologies you have proposed.

    Quote Originally Posted by uptonryan View Post
    Create a Mars hotel that constantly orbits between earth and mars picking up and putting down travellers. Mars is easy.
    This is a sensible and well-established proposal.

    The best way to establish that is to see if there are any tourists from the future. If we assume that there will be some sort of technological singularity then we may even assume time travel.
    You cannot assume time travel if it is prohibited by physics. While closed time like curves are permitted in relativity theory, it is not clear that time travel would actually be possible, or - if possible - practical. The assumption is groundless.

    However there are some things that are kinda suss. The way the moon and mars are placed kinda look like stepping stones. Venus as a warning. Sort of looks like messages being sent. To me at least.
    Badly placed stepping stones! In fact the planetary distributions and sizes look as if they will turn out to be typical for planetary systems. Nothing to see here - move along.

    Plus the way the dinosaurs were killed off when they were showing no promise,
    The dinosaurs were showing a great deal of promise and were in the midst of developing significant intelligence.

    Our Killer KY event is long past its due date.
    Really? Please explain what regularity you see in the previous five major extinction events and justify your assertion that we are overdue for the next one. (I'll give you a hint - you can't.)

    There is some evidence that the outer planets have changed orbits over their lifetimes. Jupiter may have been closer to the sun earlier on and drifted outwards. Uranus is tilted on its side which suggest a strange encounter. The solar system orbits are roughly circular. Of course this would easily change if we drifted closer to another gravitational body. Plenty of other stars seem to have there Jupiter's closer to their suns. It is unusual to have large planets on the outer like we do. Currently mercury is evaporation and reducing in size. This means its orbit is changing and there is an increased chance that may destabilise the other orbits. Sometimes that can result in a planet ejection. Our system is delicately balanced and planetary drift can occur. It is not that rare for Jupiter to enter the inner solar system. Of course that would be a disaster for us if it were to happen. Europa and Titan have issues in that Jupiter bathes them in extreme radiation.
    There are several strange ideas here. Let's amend them.

    1. The migration of giant planets is common during the formative years of a solar system. At present the solar system is quasi-stable and gross changes are extremely unlikely.
    2. Uranus doubtless suffered a major impact during those formative years.
    3. The numerous hot Jupiters are almost certainly an artifact of the detection methods that favoured massive planets in close orbits.
    4. Mercury is not evaporating. If you think it is please provide a relevant citation.
    5. Titan orbits Saturn and so is not bathed int Jupiter's radiation.
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    Quote Originally Posted by John Galt View Post
    There are several strange ideas here. Let's amend them.

    1. The migration of giant planets is common during the formative years of a solar system. At present the solar system is quasi-stable and gross changes are extremely unlikely.
    2. Uranus doubtless suffered a major impact during those formative years.
    3. The numerous hot Jupiters are almost certainly an artifact of the detection methods that favoured massive planets in close orbits.
    4. Mercury is not evaporating. If you think it is please provide a relevant citation.
    5. Titan orbits Saturn and so is not bathed int Jupiter's radiation.
    This information is new for me.
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    Quote Originally Posted by uptonryan View Post
    Janus do you have some mathematical modelling to support that theory? My model says that it will escape earth orbit but only way after the sun has become a super red giant.
    First, let's consider your 6 billion year prediction for Moon escape.

    For the Moon to escape, it would have to get beyond the Earth's Hill sphere. This has a radius of 1.5 million km. If you take the difference between this and the Moon's present distance and then divide that by 4cm/yr you find that even if the moon constantly receded at the same rate it does now, it would take ~28 billion years to reach the edge of the Hill sphere.

    Now let's consider as to whether the Earth even can transfer enough energy to the Moon in order to lift it to the edge of the Hill sphere.

    The total rotational energy of the Earth works out to ~2.6 x 10^29 joules. This is the maximum energy it could possibly transfer to the Moon in lifting its orbit. (and it would have to come to a dead stop to do so. )

    The orbital energy of the Moon can be found by



    Where M is the mass of the Earth, m the mass of the Moon and a the Earth-Moon distance.

    If you take the difference between the Moon's orbital energy now, and its energy at the edge of the Hill sphere, you get an answer of

    2.8 x 10^28

    Now this is ~1/10 what the Earth has to give up, so, at first glance, it looks possible. However, we must go back to something that I mentioned earlier, not all of the energy the Earth loses is given to the Moon. To find out how much, we compare the energy given up by the Earth compared to the energy gained by the Moon.

    The Earth presently slows at a rate of 17 milliseconds/century.

    This works out to an energy loss of:

    ~10^23 joules.

    In that same century, the Moon recedes by 4 meters, and gains:

    4 x10^20 joules.

    or 1/250 as much energy as the Earth loses. This means the transfer of energy from Earth to Moon is not very efficient. The rest is given up as waste heat.

    It also means that even though the Earth has almost 10 times the energy stored in its rotation needed to cause the Moon's escape, it actually can only transfer 1/250 of that energy to the Moon.

    The Earth can't transfer enough energy to the Moon to get it outside of the Hill sphere.

    That only leaves the possibility of the Earth tidal locking to the Moon.

    Now, as far as the Moon spiraling back in, that is due to tidal deceleration:

    Tidal acceleration - Wikipedia, the free encyclopedia

    Of course, all of this is a mute point considering the life-cycle of the Sun, and the time scales needed for the scenario to play out.
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    I think you are right about that Janus, but how do you feel about the Moon origin
    -was it"
    1. Moon capture or
    2 Giant impact with Theia
    as the cause?
    I go against the grain and favour moon capture myself.
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    1. The migration of giant planets is common during the formative years of a solar system. At present the solar system is quasi-stable and gross changes are extremely unlikely.

    Planetary migration - Wikipedia, the free encyclopedia read under in the solar system.

    2. Uranus doubtless suffered a major impact during those formative years.

    several impacts as above

    3. The numerous hot Jupiters are almost certainly an artifact of the detection methods that favoured massive planets in close orbits.

    Well we just don't know.

    4. Mercury is not evaporating. If you think it is please provide a relevant citation.

    It was one of the outcomes of the Messenger mission. Planet Mercury is evaporating and this will eventually potentially destabilise the orbits of the planets in the solar system.

    5. Titan orbits Saturn and so is not bathed int Jupiter's radiation.

    Jupiter has a lot of radiation. Hiding from Jupiter's Radiation
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by uptonryan View Post
    Janus do you have some mathematical modelling to support that theory? My model says that it will escape earth orbit but only way after the sun has become a super red giant.
    First, let's consider your 6 billion year prediction for Moon escape.

    For the Moon to escape, it would have to get beyond the Earth's Hill sphere. This has a radius of 1.5 million km. If you take the difference between this and the Moon's present distance and then divide that by 4cm/yr you find that even if the moon constantly receded at the same rate it does now, it would take ~28 billion years to reach the edge of the Hill sphere.

    Now let's consider as to whether the Earth even can transfer enough energy to the Moon in order to lift it to the edge of the Hill sphere.

    The total rotational energy of the Earth works out to ~2.6 x 10^29 joules. This is the maximum energy it could possibly transfer to the Moon in lifting its orbit. (and it would have to come to a dead stop to do so. )

    The orbital energy of the Moon can be found by



    Where M is the mass of the Earth, m the mass of the Moon and a the Earth-Moon distance.

    If you take the difference between the Moon's orbital energy now, and its energy at the edge of the Hill sphere, you get an answer of

    2.8 x 10^28

    Now this is ~1/10 what the Earth has to give up, so, at first glance, it looks possible. However, we must go back to something that I mentioned earlier, not all of the energy the Earth loses is given to the Moon. To find out how much, we compare the energy given up by the Earth compared to the energy gained by the Moon.

    The Earth presently slows at a rate of 17 milliseconds/century.

    This works out to an energy loss of:

    ~10^23 joules.

    In that same century, the Moon recedes by 4 meters, and gains:

    4 x10^20 joules.

    or 1/250 as much energy as the Earth loses. This means the transfer of energy from Earth to Moon is not very efficient. The rest is given up as waste heat.

    It also means that even though the Earth has almost 10 times the energy stored in its rotation needed to cause the Moon's escape, it actually can only transfer 1/250 of that energy to the Moon.

    The Earth can't transfer enough energy to the Moon to get it outside of the Hill sphere.

    That only leaves the possibility of the Earth tidal locking to the Moon.

    Now, as far as the Moon spiraling back in, that is due to tidal deceleration:

    Tidal acceleration - Wikipedia, the free encyclopedia

    Of course, all of this is a mute point considering the life-cycle of the Sun, and the time scales needed for the scenario to play out.
    You missed something from your simulation. 1) the moons orbit will not be circular it will be oval. 2) the moon orbits the sun as well as the moon. 3) the other planets tug on the moon also.
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    Quote Originally Posted by uptonryan View Post

    You missed something from your simulation. 1) the moons orbit will not be circular it will be oval. 2) the moon orbits the sun as well as the moon. 3) the other planets tug on the moon also.
    1)If you mean the that Moon's orbit is elliptical, it only has an eccentricity of 0.05. The tidal acceleration which increases the Moon's orbital distance will not significantly alter this value. Even accounting for the Moon's eccentricity only decreases the Hill sphere to a little over 1.4 million km.

    2) This is what the whole concept of the Hill sphere is based on, it is the maximum distance at which a body can hold on to a satellite while orbiting a larger body without the satellite breaking free into an independent orbit around the larger body. The Earth's Hill sphere marks the point w#here the Earth will no longer dominate the moon's orbit and the Sun will take over.

    3) The perturbing effects of the other planets are negligible compared to the main effect of tidal acceleration and would not significantly change the results.

    IOW, the
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by uptonryan View Post

    You missed something from your simulation. 1) the moons orbit will not be circular it will be oval. 2) the moon orbits the sun as well as the moon. 3) the other planets tug on the moon also.
    1)If you mean the that Moon's orbit is elliptical, it only has an eccentricity of 0.05. The tidal acceleration which increases the Moon's orbital distance will not significantly alter this value. Even accounting for the Moon's eccentricity only decreases the Hill sphere to a little over 1.4 million km.

    2) This is what the whole concept of the Hill sphere is based on, it is the maximum distance at which a body can hold on to a satellite while orbiting a larger body without the satellite breaking free into an independent orbit around the larger body. The Earth's Hill sphere marks the point w#here the Earth will no longer dominate the moon's orbit and the Sun will take over.

    3) The perturbing effects of the other planets are negligible compared to the main effect of tidal acceleration and would not significantly change the results.

    IOW, the
    1) it will become more elliptical the further out it goes. This is because of Mars and the uneven nature the forces are distributed.
    2) You assumed the Earth's Hill sphere is constant. However it changes depending on how close Mars is and the alignment of the other planets.
    3) Saying they are negligible does not allow you to not calculate them. The fact is as the moon gets further out the orbit becomes more precarious and the negligible forces start to be more significant.
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    Of course, your model, you know the one you haven't had the decency to present (despite asking for others to present theirs and despite being asked a number of times for yours), clearly shows this, doesn't it?
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    My model is amazing. Its like actually being there.
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    Yet you still haven't had the integrity to present it.
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    Quote Originally Posted by uptonryan View Post
    My model is amazing. Its like actually being there.
    This sort of smartass post is not appreciated, especially on a hard science forum like astronomy and cosmology. Straighten up and fly right.
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    I don't think it is reasonable to be asked to present my model, especially if it is commercial. In my country science and technology is frowned upon. Scientist are often assaulted in the street. We don't even have a science Minister. It is alright if you come from a country were science is appreciated and respected. Out were I live we have to hide our science away living on poverty. You guys don't know how lucky you have it.
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    That's strike two, Ryan. I think you're trolling. Three strikes and you're out.
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    Quote Originally Posted by uptonryan View Post
    I don't think it is reasonable to be asked to present my model
    Since you yourself brought up the subject of your model, and made claims for it then it's entirely reasonable for us to ask you to present it.
    It's also dishonest of you to ask others to present theirs while refusing to support your own claims with your own model.
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    My model uses the Bullet Physics game engine. Real-Time Physics Simulation That makes it rather hard to present in this forum. Have you heard of it?
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    Quote Originally Posted by uptonryan View Post
    My model uses the Bullet Physics game engine. Real-Time Physics Simulation That makes it rather hard to present in this forum. Have you heard of it?
    So how do rigid body interactions (or even soft body ones) apply in celestial mechanics?

    How about this:
    Celestial mechanics particle system
    Something like
    the Open Dynamics Engine (ODE) or the Bullet Physics Library can be used, but a proper particle or celestial mechanics system such as libnova or changa (which is unfortunately license incompatible) will probably be more appropriate given the physics involved.

    I.e. Bullet Physics is ill-suited to the job.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by uptonryan View Post
    I don't think it is reasonable to be asked to present my model
    Since you yourself brought up the subject of your model, and made claims for it then it's entirely reasonable for us to ask you to present it.
    It's also dishonest of you to ask others to present theirs while refusing to support your own claims with your own model.
    Could you tell me please what is a model? is it like Mathematica Restricted Three-Body Problem - Wolfram Demonstrations Project?

    Last edited by Robittybob1; October 30th, 2013 at 09:30 PM.
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    Quote Originally Posted by Robittybob1 View Post
    Could you tell me please what is a model?
    Asks the "scientist".
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Robittybob1 View Post
    Could you tell me please what is a model?
    Asks the "scientist".
    I am going to buy Mathematica for I would like to run the Three Body Problem with the Moon Sun and Earth and see what variations of masses will achieve a stable orbit arrangement. I imagine the program is possibly described as a model since it models the Earth Moon Sun system.
    Have you worked with "the Restricted Three-Body Problem - Wolfram Demonstrations Project"?
    It is quite an investment though.
    Last edited by Robittybob1; October 31st, 2013 at 01:38 PM. Reason: clarity
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    uptonryan, you are implicitly denying the corrections I made to some of your earlier posts. This is potentially misleading to casual readers of this thread. It also casts you in an unfavourable light. I trust you will welcome this opportunity to accept those corrections gracefully, or provide substantive evidence that your assertions are correct. (In the quotes below I have distinguished my original clarifications by using italics and blue font.)
    Quote Originally Posted by uptonryan View Post
    1. The migration of giant planets is common during the formative years of a solar system. At present the solar system is quasi-stable and gross changes are extremely unlikely.

    Planetary migration - Wikipedia, the free encyclopedia read under in the solar system.
    This confirms precisely what I said - that the migration of planets is thought to be commonplace in the formative years of the solar system. You implied that it could occur now. Do you now accept that what I said and what is confirmed by your link is correct?

    2. Uranus doubtless suffered a major impact during those formative years.
    several impacts as above
    The linked Wikipedia article does not refer to any major impacts relating to Uranus. If you wish to assert that its present orientation is a consequence of several major impacts please provide appropriate citations. In particular please identify why this is favoured over the original suggestion of a single impactor by Safronov. (Safronov, V.S. 1972 Evolution of the Protoplanetary Cloud and Formation of the Earth and Planets Nauka translation from Russian Israel Program for Scientific Translation). Alternatively you could concede that you are mistaken.
    3. The numerous hot Jupiters are almost certainly an artifact of the detection methods that favoured massive planets in close orbits.
    Well we just don't know.
    Well, we pretty well do know. I have read scores of paper on detection of exoplanets and I do not recall a single dissenting voice from this view. Once again, if you have contrary information please provide the appropriate citations.

    4. Mercury is not evaporating. If you think it is please provide a relevant citation.
    It was one of the outcomes of the Messenger mission. Planet Mercury is evaporating and this will eventually potentially destabilise the orbits of the planets in the solar system.
    When I posted the absolute statement, "Mercury is not evaporating", I hesitated. I wondered if someone would raise the point that, well, actually, it is. Then I thought, "No. No one is going to be so pedantic that they take the miniscule mass loss that is occurring and make the implicit claim that the planet is slowly losing a substantial portion of its mass." Clearly I was mistaken. Would you like to provide citations that indicate the evaporation rates are sufficient to produce significant mass changes that could destabilise the Mercurian orbit?

    5. Titan orbits Saturn and so is not bathed in Jupiter's radiation.
    Jupiter has a lot of radiation. Hiding from Jupiter's Radiation
    Are you seriously suggesting that Jupiter's radiation can extends, in any significant way, as far as Titan? If you are claiming this, would you explain why this has not had a marked effect on the Earth, since Jupiter is closer to the Earth than it is to Saturn?

    uptonryan, I am not trying to put you down here, but I do believe it is important - on a science forum - that when assertions are made with confidence that those assertions be correct, or be corrected. I am ready to concede I am incorrect on each of these five points, if you produce sound evidence to support your position. As far as I can see that is unlikely, but the door is open for you.
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    Quote Originally Posted by John Galt View Post
    ....

    4. Mercury is not evaporating. If you think it is please provide a relevant citation.
    It was one of the outcomes of the Messenger mission. Planet Mercury is evaporating and this will eventually potentially destabilise the orbits of the planets in the solar system.
    When I posted the absolute statement, "Mercury is not evaporating", I hesitated. I wondered if someone would raise the point that, well, actually, it is. Then I thought, "No. No one is going to be so pedantic that they take the miniscule mass loss that is occurring and make the implicit claim that the planet is slowly losing a substantial portion of its mass." Clearly I was mistaken. Would you like to provide citations that indicate the evaporation rates are sufficient to produce significant mass changes that could destabilise the Mercurian orbit?
    John I have done a study on Mercury and I'm sure there has been claims of quite a substantial loss of mass from Mercury since its formation. I always wondered how they knew this, what evidence did they have, but there must have been the science behind it.
    Over the weekend I will see if I can find references to support this. (I'm working off memory at the moment).
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    [QUOTE=uptonryan;479498

    1) it will become more elliptical the further out it goes.

    [/quote] How so?
    There are three ways to increase the eccentricity of an orbit:
    1) decrease the orbital velocity at apapis
    2) Increase the orbital velocity at periapis
    3) tidal stretching[quote]

    The first doesn't doesn't help your argument because it actually lowers the average orbital radius
    The second would require that the perturbing effect be always be properly timed to occur when the Moon is at perigee and then always in the right direction. Not only that, but any gravitational pull from a planet on the Moon also acts on the Earth, which means that you wold get no net change in orbital speed for the Moon relative to the Earth, which is what is needed to increase orbital speed at perigee.

    That leaves number three. But even this requires the the tidal stretching only is applied when it is aligned with the existing semi-major axis of the orbit.(If the stretching is at a right angle it decreases the eccentricity) Since the alignment of the moon's orbit and the pull from the various planets are constantly changing there would be no regular effect increasing the Moon's orbit.
    2) You assumed the Earth's Hill sphere is constant. However it changes depending on how close Mars is and the alignment of the other planets.
    The hill sphere depends on three factors:

    The mass of the Sun
    The mass of the Planet
    The distance from Sun to planet.

    It is indirectly related to the Sun's mass (an increase in Sun mass causes a decrease in Hill sphere radius)
    It is directly related to the Earth's mass (an increase in Earth mass causes an increase in the radius)
    and direstly proportional to the Sun-planet distance ( an increase in Sun-planet distance causes an increase in Hill sphere.

    Basically it comes down to the balance between the tidal forces from the Sun pulling the Moon orbit apart and the Earth's gravity holding on the Moon. The only effect the other planets would have is through their tidal influence across the Moon's orbit. This is related to their mass and the cube of their distance. To use Mars as an example: at its closest, it is some 78 million km away. It mass is 1/3,300,000 that of the Sun. This works out to it having ~7/3,300,000 or 1/47,1428 the tidal effect of the Sun. In other words, an insignificant effect even when it is at it's closest.
    3) Saying they are negligible does not allow you to not calculate them.
    I gave one example of such a calculation above. But here's the thing, you keep going on about the effect of Mars, when it is not even the most significant player. Venus is both closer, and more massive and would have a greater effect. ( but still only averaging 1/41,0677 that of the Sun.) Jupiter while further away has enough mass, that its tidal influence varies between 1/16,000 and 1/36,000 that of the Sun. So even adding up the effect of all the planets, you don't get more than a tiny fraction of an effect of the Sun's

    The fact is as the moon gets further out the orbit becomes more precarious and the negligible forces start to be more significant.
    But so does the Sun's. As the Moon gets further from the Earth, the tidal influence of the Sun increases due to the increased radius. IOW, the relative tidal influence of Sun relative to the that of the planets stays the same. The radius of the Hill sphere is calculated taking this into account. Adding the influence of the other planets will not change the result of this calculation significantly, and no where near enough to decrease it by the factor need for the Moon to escape in 6 billion years.

    A couple of more points:

    Earlier you said that the I assumed that the Hill sphere would remain constant. This is true. there are two factors that could effect the Hill sphere and could chnage over time; The mass of the Sun and the Earth-Sun distance. The Sun is constantly converting mass to energy and thus is losing mass, as a result of this loss of mass the Earth will increase its orbital distance. Both of these effects will work to increase the radius of the Hill sphere with time.

    Also, when I estimated the amount of time it would take to reach the Hill sphere limit, I assumed a constant 4cm/year recession speed. As the Moon recedes, its tidal force on the Earth will decrease by the cube of its distance. Since this tidal force is what causes the recession in the first place, the effect will weaken and you cannot expect the Moon to continue receding at the same rate as it does now. An additional factor is that tidal acceleration is also tied to the k/Q ratio, or basically drag effect the rotating Earth has on the tidal bulges. Presently, because of the arrangement of the continents, this is particularly high. As the continents continue to drift, this will change and lower, causing a further decrease in the recession rate.
    Last edited by Janus; November 1st, 2013 at 11:34 AM.
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    Quote Originally Posted by Robittybob1 View Post
    John I have done a study on Mercury and I'm sure there has been claims of quite a substantial loss of mass from Mercury since its formation. I always wondered how they knew this, what evidence did they have, but there must have been the science behind it.
    Over the weekend I will see if I can find references to support this. (I'm working off memory at the moment).
    Bob, you are absolutely correct, but this mass loss occurred during the formative years of the system. Mercury appears to have lost a substantial portion of its silicate mantle. This is thought most likely to have occurred when the proto-Mercury was hit by one or more substantial planetesimals, or other proto-planets. (This would be akin to the process that produced the moon through a giant impact of a a proto-planet with the Earth.)

    However, this is quite different from what uptonryan is proposing. He is arguing that current processes are leading to a significant mass loss. I am asking him to substantiate this, or to acknowledge that he is incorrect.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by Robittybob1 View Post
    John I have done a study on Mercury and I'm sure there has been claims of quite a substantial loss of mass from Mercury since its formation. I always wondered how they knew this, what evidence did they have, but there must have been the science behind it.
    Over the weekend I will see if I can find references to support this. (I'm working off memory at the moment).
    Bob, you are absolutely correct, but this mass loss occurred during the formative years of the system. Mercury appears to have lost a substantial portion of its silicate mantle. This is thought most likely to have occurred when the proto-Mercury was hit by one or more substantial planetesimals, or other proto-planets. (This would be akin to the process that produced the moon through a giant impact of a a proto-planet with the Earth.)

    However, this is quite different from what uptonryan is proposing. He is arguing that current processes are leading to a significant mass loss. I am asking him to substantiate this, or to acknowledge that he is incorrect.
    The most recent findings didn't confirm that impact theory. Things keep on changing as more information is found. I have the link somewhere, the direct evaporation losses would not be that great as you say.
    The thing that can catch you out is the effect of small changes over long periods of time. Like if even a thin layer of rock is evaporated by the extreme heat on Mercury say even 0.1 mm per year over a billion years the planet might disappear.

    Rough calculation if it lost 0.5 mm/year for the next 5 billion years it would disappear altogether.
    Last edited by Robittybob1; November 1st, 2013 at 05:03 AM.
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    Quote Originally Posted by Robittybob1 View Post
    The most recent findings didn't confirm that impact theory. Things keep on changing as more information is found.
    Nor did the findings disprove the giant impact theory. It does require modification, but still remains a viable explanation for the mantle stripping. For example, see Revisiting Collisional Stripping of Mercury's Mantle.

    Do you have specific, confirmed information on the evaporation rates?
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by Robittybob1 View Post
    The most recent findings didn't confirm that impact theory. Things keep on changing as more information is found.
    Nor did the findings disprove the giant impact theory. It does require modification, but still remains a viable explanation for the mantle stripping. For example, see Revisiting Collisional Stripping of Mercury's Mantle.

    Do you have specific, confirmed information on the evaporation rates?
    No but the surface temperatures would be doing damage to the rocks alright.
    Once the molecules are released from the rock the solar wind just whipped them into space. I didn't look into that sorry.
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    I have no issue with the reality of mass loss from evaporation. But I don't even know if it exceeds the mass gain from bolide and metoroid impact. And I strongly suspect, until I am shown contrary data, that it is wholly inadequate to produce the effects uptonryan appears to be claiming exist. I hope he will respond shortly to my previous post and address this and the other issues.
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    Quote Originally Posted by John Galt View Post
    I have no issue with the reality of mass loss from evaporation. But I don't even know if it exceeds the mass gain from bolide and metoroid impact. And I strongly suspect, until I am shown contrary data, that it is wholly inadequate to produce the effects uptonryan appears to be claiming exist. I hope he will respond shortly to my previous post and address this and the other issues.
    I think you will find a meteorite impact will cause a nett loss of mass rather than a gain.
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    Quote Originally Posted by PhDemon View Post
    Is this just more of your made up nonsense or do you have any evidence to back this up?
    That is on Mercury not on Earth. Without the atmosphere and with the strong solar radiation pressure and solar wind, I think the dust vaporised would exceed the mass of the meteorite.
    I have had a longstanding interest in Mercury. So it is not hard cold facts but a possible explanation of why the planet is losing mass rather than gaining it.

    But if it is important I'll see if i can find a reference.
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    Quote Originally Posted by Robittybob1 View Post
    Quote Originally Posted by John Galt View Post
    I have no issue with the reality of mass loss from evaporation. But I don't even know if it exceeds the mass gain from bolide and metoroid impact. And I strongly suspect, until I am shown contrary data, that it is wholly inadequate to produce the effects uptonryan appears to be claiming exist. I hope he will respond shortly to my previous post and address this and the other issues.
    I think you will find a meteorite impact will cause a nett loss of mass rather than a gain.
    This appear s to be true for Mercury and the Moon, for large impactors >1,000km across. It is doubtful if this applies to small impactors. http://arxiv.org/pdf/1306.4325.pdf. Of course, this supports my earlier assertion that the loss of silicate mantle was the consequence of one or more large impacts.
    Please provide a citation that supports the notion that meteoroid impacts will have a net erosive effect.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by Robittybob1 View Post
    Quote Originally Posted by John Galt View Post
    I have no issue with the reality of mass loss from evaporation. But I don't even know if it exceeds the mass gain from bolide and metoroid impact. And I strongly suspect, until I am shown contrary data, that it is wholly inadequate to produce the effects uptonryan appears to be claiming exist. I hope he will respond shortly to my previous post and address this and the other issues.
    I think you will find a meteorite impact will cause a nett loss of mass rather than a gain.
    This appear s to be true for Mercury and the Moon, for large impactors >1,000km across. It is doubtful if this applies to small impactors. http://arxiv.org/pdf/1306.4325.pdf. Of course, this supports my earlier assertion that the loss of silicate mantle was the consequence of one or more large impacts.
    Please provide a citation that supports the notion that meteoroid impacts will have a net erosive effect.
    John once we start sizing the impactors I sometimes get a bit lost in the descriptions, was it an asteriod or was it a meteor or a meteorite. But after finishing here last night for me, one thing to realize is that the impact speed of an object on Mercury is going to be possibly 2 -3 times faster than on Earth. So the kinetic energy is vastly different yet the mass stayed the same, so the amount of damage done to a planetary surface will be so much more. (KE = V^2 so if impact speed is 3 times faster then the kinetic energy will be 9 times - 9 times as much damage from the same sized impactor.)

    Now this "fact" has not been mentioned much in the articles I read on the subject. Is that a valid conclusion? After it has passed through the orbit of the Earth it continues to fall in toward the Sun speeding up. So how much faster is it going when it impacts Mercury?
    If I'm right the same sized impactor on Earth causes less damage than on Mercury. Dust thrown up on Mercury can get blown into space much easier than on Earth. No atmosphere to slow it down, less gravity, and strong Solar Winds.
    So those two points the extra kinetic energy and the ease of vaporized materials to be lost, points to a continual mass loss situation on Mercury. Whether it has been determined at what sized object causes no nett loss of mass I'm not sure, I have not seen it.
    Last edited by Robittybob1; November 1st, 2013 at 03:57 PM. Reason: KE estimation added
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    I see that you have a good understanding of some of the issues for impacting meteors.. and let me clear this for you.. they are asteroids that can become meteors or comets. When the explode in the atmosphere they are a bolide.. a fire ball, and that explanations of what a bolide is is still a argument... It's a meteorite only after it has landed.
    Some place under the cobwebs here is a program that I have often played with.. and understand that many variables exist;
    'Earth impactors' you give it size and construction, and velocity and angle.. and it tells you how far away from it was safe.. or if the size of the new asteroid field that was Earth... as for your questions.. A greater deal of mathematics than I can cope with is essential to calculations of impact damage.. and calculating impact velocities of/at mercury.. is above my simple point and shoot astronomy. Mark.
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    Quote Originally Posted by astromark View Post
    I see that you have a good understanding of some of the issues for impacting meteors.. and let me clear this for you.. they are asteroids that can become meteors or comets. When the explode in the atmosphere they are a bolide.. a fire ball, and that explanations of what a bolide is is still a argument... It's a meteorite only after it has landed.
    Some place under the cobwebs here is a program that I have often played with.. and understand that many variables exist;
    'Earth impactors' you give it size and construction, and velocity and angle.. and it tells you how far away from it was safe.. or if the size of the new asteroid field that was Earth... as for your questions.. A greater deal of mathematics than I can cope with is essential to calculations of impact damage.. and calculating impact velocities of/at mercury.. is above my simple point and shoot astronomy. Mark.
    That implies there should be no bolides on the Moon or on Mercury as there is no atmosphere for the meteors to heat up in as they approaches the planet.
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    Quote Originally Posted by Robittybob1 View Post
    John once we start sizing the impactors I sometimes get a bit lost in the descriptions, was it an asteriod or was it a meteor or a meteorite.
    Well it's not going to be a meteor is it? By definition.

    So those two points the extra kinetic energy and the ease of vaporized materials to be lost, points to a continual mass loss situation on Mercury.
    You don't think that simple energy transfer/ losses indicates there'd be a net gain from impact? (I.e. whatever energy is there is going in cannot be the same as that ejecting material simply due to losses of energy in vaporising material on impact. And even if it were the ejecta still has to overcome gravity, weak though it is).
    Plus which: Mercury recaptures a large fraction of its ejecta.
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Quote Originally Posted by Dywyddyr View Post
    ....
    So those two points the extra kinetic energy and the ease of vaporized materials to be lost, points to a continual mass loss situation on Mercury.
    You don't think that simple energy transfer/ losses indicates there'd be a net gain from impact? (I.e. whatever energy is there is going in cannot be the same as that ejecting material simply due to losses of energy in vaporising material on impact. And even if it were the ejecta still has to overcome gravity, weak though it is).
    Plus which: Mercury recaptures a large fraction of its ejecta.
    We are only talking about gain in mass. Energy gains probably cancel as impacts will be from random directions. So even if the material falls back down the surface area exposed to the elements has increased and hence will aid in increasing evaporative losses.
    The article really supports my view. What they don't compare as far as I can see is the mass difference between the impactor and the amount of material ejected. It could be 1:1000 (It will be a high number for sure). Thousands of times the mass of material is lost compared to the mass of the impactor. (well that is my guess) and even if half of that falls back down again there is still substantial loss of mass from the planet.
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