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Thread: space and gravity in relation to planets

  1. #1 space and gravity in relation to planets 
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    Hi, I'm currently watching a documentary about the great Albert Einstein and there is a discussion about the planets and space pushing the eart and the sun around. I would like to know if it is gravity alone that keeps the planets aline and rotating or does magnetism play a part. There is all this talk about space curvature too.

    Can someone explain it in laymans yerms.

    Thanks!


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    Space curvature is gravity.
    Gravity is pulling the earth toward the sun.
    The momentum of the Earth is at a right angle to the pull of the sun.
    The forces balance with the earth traveling around the sun.

    Of course this leaves out all of the extra little forces involved, but magnetism plays an insignificant part.

    This is loosely similar to swinging a weight around on a string.
    If you have seen a spiral funnel wishing well, they can show how the momentum of a coin produces the force to resist going down a gravity well. The coin slows quickly due to friction loss and goes down the funnel, the Earth travels through the relative vacuum of space and therefor will not fall into the suns gravity well.


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    Thanks Gort.
    What I found fascinating was the planet Mercury which had an unusual rotation. It didnt follow the basic one track circling but changed direction which created a kind of petal effect.
    What would cause that?
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    Quote Originally Posted by hannah40 View Post
    Thanks Gort.
    What I found fascinating was the planet Mercury which had an unusual rotation. It didnt follow the basic one track circling but changed direction which created a kind of petal effect.
    What would cause that?
    Try here.
    And it's not just Mercury that it happens to: Earth and Mars (at least) also do this, but much less noticeably.
    "[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
    Quote Originally Posted by hannah40 View Post
    Thanks Gort.
    What I found fascinating was the planet Mercury which had an unusual rotation. It didnt follow the basic one track circling but changed direction which created a kind of petal effect.
    What would cause that?
    Try here.
    And it's not just Mercury that it happens to: Earth and Mars (at least) also do this, but much less noticeably.
    Thanks Duck. I'll take a look now.
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  7. #6  
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    Quote Originally Posted by G O R T View Post
    Gravity is pulling the Earth toward the Sun.
    But the earth is moving in as straight a line as it can along the curvature of spacetime caused by the immense body of the Sun.
    In the case of Mercury it is so close to the Sun that its orbit is slightly different, which proved Einstein's GR to be more accurate than Newton's theory of gravity. The closer a planet is to the Sun the faster it needs to travel to avoid being pulled into it.

    If I can add to the strangeness of how it all behaves, consider the Earth and Moon. We assume that the Moon is orbiting the Earth, but in terms of GR it can be argued that the reverse is just as valid. When I take the train to Oxford I am always reminded of the old puzzle 'does the train arrive at Oxford or does Oxford arrive at the train?' Both are equally true.
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    If I can add to the strangeness of how it all behaves, consider the Earth and Moon. We assume that the Moon is orbiting the Earth, but in terms of GR it can be argued that the reverse is just as valid. When I take the train to Oxford I am always reminded of the old puzzle 'does the train arrive at Oxford or does Oxford arrive at the train?' Both are equally true.
    Don't tell that to a certain Geocentric crackpot on here...
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    Quote Originally Posted by ox View Post
    Quote Originally Posted by G O R T View Post
    Gravity is pulling the Earth toward the Sun.
    But the earth is moving in as straight a line as it can along the curvature of spacetime caused by the immense body of the Sun.
    In the case of Mercury it is so close to the Sun that its orbit is slightly different, which proved Einstein's GR to be more accurate than Newton's theory of gravity. The closer a planet is to the Sun the faster it needs to travel to avoid being pulled into it.

    If I can add to the strangeness of how it all behaves, consider the Earth and Moon. We assume that the Moon is orbiting the Earth, but in terms of GR it can be argued that the reverse is just as valid. When I take the train to Oxford I am always reminded of the old puzzle 'does the train arrive at Oxford or does Oxford arrive at the train?' Both are equally true.

    I was awake really early this morning watching documentaries and I was thinking how the forces that hold it all together must be so great. You can't help but marvel at it. That's what I want for breakfast and to wake up to on a morning, good entertainment and worldly satisfaction.
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    More amazing is how wimpy gravity really is. The whole mass of the Earth is pulling you into your chair, but the electromagnetic force between a few molecules of plastic keeps you from sinking through and it doesn't take much effort for most people to stand up. The other forces are incredibly strong, but because they all can be both positive and negative (unlike gravity) they cancel out on average. (Why gravity is so much weaker is still an open question AFAIK.)
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    Newton's theory that earth's momentum and sun's gravitational pull cause the earth to revolve around sun is a good approximation to what we normally observe. General relativity is accurate enough to predict even minor changes in their paths.According to GR, any massive body curves the space around it so that objects nearby travel in a straight line in curved space-time, which appears as circular or elliptical in 3 dimensions. Consider a bucket of water similar to empty space. When u put some heavy objects into it, obviously you can notice the water around the object is now wrapped on it. Similar case.
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    Quote Originally Posted by arun karthik View Post
    Newton's theory that earth's momentum and sun's gravitational pull cause the earth to revolve around sun is a good approximation to what we normally observe. General relativity is accurate enough to predict even minor changes in their paths.According to GR, any massive body curves the space around it so that objects nearby travel in a straight line in curved space-time, which appears as circular or elliptical in 3 dimensions. Consider a bucket of water similar to empty space. When u put some heavy objects into it, obviously you can notice the water around the object is now wrapped on it. Similar case.
    Yes. It explained that space was malleable and it can move and twist.
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    Quote Originally Posted by ox View Post
    Quote Originally Posted by G O R T View Post
    Gravity is pulling the Earth toward the Sun.
    But the earth is moving in as straight a line as it can along the curvature of spacetime caused by the immense body of the Sun.
    In the case of Mercury it is so close to the Sun that its orbit is slightly different, which proved Einstein's GR to be more accurate than Newton's theory of gravity. The closer a planet is to the Sun the faster it needs to travel to avoid being pulled into it.

    If I can add to the strangeness of how it all behaves, consider the Earth and Moon. We assume that the Moon is orbiting the Earth, but in terms of GR it can be argued that the reverse is just as valid. When I take the train to Oxford I am always reminded of the old puzzle 'does the train arrive at Oxford or does Oxford arrive at the train?' Both are equally true.
    The correct way to talk about orbits is that any system such as the Earth and moon orbit a center of gravity. In the case of Earth/moon the center of gravity is still below the surface of the Earth (which has the greater mass) so it appears that the moon is orbiting the Earth, and when we talk about it, that's the way it is mostly used. The moon orbits the Earth, but scientifically that's wrong.
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    The Sun must be quite wobbly, having to orbit around all those centres of gravity all at the same time...
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    The sun is so massive compared to the planets that they are all roughly in the same place...
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    I found a couple of pictures to illustrate what I said about the Earth/moon orbit. A similar picture of the planets and sun could be found, it would show the barycenter very close to the suns core. It actually does wobble a bit, just not enough so you could tell.

    Where is the barycenter of the Earth-Moon system located?

    The center of mass of the Earth-Moon system, is a point in space about which the Earth and Moon appear to orbit as they travel around the Sun. It is located exactly along the line that connects the center of the Earth with the center of the Moon. The average distance between the centers is 384,405 kilometers. The distance from the Earth's center to the barycenter is

    D = M(moon)d(moon)/(M(earth) +M(moon)
    0.012 x 384405/(1.00 + 0.012
    4641 kilometers

    where the mass of the Earth is 1.0 and the mass of the Moon relative to the earth is 0.012. Now, the radius of the Earth is 6,378 kilometers, so that means that the barycenter is located INSIDE the Earth about 1707 kilometers below its surface. Does anything weird happen there? Not that anyone can tell.


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  17. #16  
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    Yes indeed, the barycentre of the earth-moon system is about a thousand miles beneath the surface of the earth, and not at the earth's centre. However lunar dwellers might be of the opinion that it is the earth that is in orbit and spinning, while their world keeps the same face, as the earth moves in relation to the night sky.
    Must always remember that not only does a large body pull on a smaller one but the smaller one is also pulling on the larger one, hence the elliptical orbits. In the case of Mercury in a very strong gravitational field the orbit is a bit squashed, while the earth's is nearly circular, but still allowing for a precession. Elliptical orbits were discovered by Kepler and confirmed by Newton's maths. Then Le Verrier noticed this small difference in Mercury's orbit which could not be explained until Einstein's GR.
    Last edited by ox; June 17th, 2014 at 08:17 AM.
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    Quote Originally Posted by ox View Post
    Yes indeed, the barycentre of the earth-moon system is about a thousand miles beneath the surface of the earth, and not at the earth's centre. However lunar dwellers might be of the opinion that it is the earth that is in orbit and spinning, while their world keeps the same face, as the earth moves in relation to the night sky.
    Must always remember that not only does a large body pull on a smaller one but the smaller one is also pulling on the larger one, hence the elliptical orbits. In the case of Mercury in a very strong gravitational field the orbit is a bit squashed, while the earth's is nearly circular, but still allowing for a precession. Elliptical orbits were discovered by Kepler and confirmed by Newton's maths. Then Le Verrier noticed this small difference in Mercury's orbit which could not be explained until Einstein's GR.
    All orbits are elliptical. In our solar system the orbits of all the planets are very close to circular and that does appear to be an exception to most solar systems. The picture below is a bit exaggerated to better illustrate ellipticalness of the orbit of Earth. But if your interested it's an easy search to find the actual orbits of all the planets.

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    Quote Originally Posted by Daecon View Post
    The Sun must be quite wobbly, having to orbit around all those centres of gravity all at the same time...
    One center of gravity predominates, and that is the Sun-Jupiter barycenter. The inner planets can be ignored as they are so small compared to Jupiter and so close to the Sun that they induce a very small wobble. The outer gas giants are more significant( but still less so than Jupiter,) but because of their distance have such long orbital periods that its more like a long term shift in the wobble than an additional wobble in itself.

    The Sun-Jupiter barycenter actually falls ~82,500km above the surface of the Sun.
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    And what happens if the barycenter is outside the surface of the central body?

    Well, nothing different actually "happens"; that is, both bodies will orbit around the barycenter as usual. However, our thinking about the arrangement changes. We might think of it as a binary system instead of a central body and a satellite. We see such a barycenter with binary stars. It also played a part in why astronomers stopped thinking of Pluto as a planet, that is, Pluto does not have such a "command presence" within its little system as other planets do in theirs Pluto's barycenter is outside its body, somewhere between Pluto and Charon.
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    Quote Originally Posted by jrmonroe View Post
    And what happens if the barycenter is outside the surface of the central body?

    Well, nothing different actually "happens"; that is, both bodies will orbit around the barycenter as usual. However, our thinking about the arrangement changes. We might think of it as a binary system instead of a central body and a satellite. We see such a barycenter with binary stars. It also played a part in why astronomers stopped thinking of Pluto as a planet, that is, Pluto does not have such a "command presence" within its little system as other planets do in theirs Pluto's barycenter is outside its body, somewhere between Pluto and Charon.
    It almost sounds like you are making a case to call our solar system a binary system because of Jupiter who's barycenter is outside the suns surface. Well maybe it could but I've never of a star and its planet being referred to as a binary before.
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    Hmm, I wasn't trying to make such a case, I didn't know that the Sun-Jupiter barycenter was outside of the Sun's body. I'm really not knowledgeable about astronomy. For myself, I see a binary system as homogeneous, and so, binary stars, binary plutoids, etc. Some websites talk about "binary planets", but this seems a contradiction of terms.
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    Quote Originally Posted by jrmonroe View Post
    Hmm, I wasn't trying to make such a case, I didn't know that the Sun-Jupiter barycenter was outside of the Sun's body. I'm really not knowledgeable about astronomy. For myself, I see a binary system as homogeneous, and so, binary stars, binary plutoids, etc. Some websites talk about "binary planets", but this seems a contradiction of terms.
    I decided to search on "binary planets" and found the following. Seems like there is is a bit of controversy on how to decide what what defines a "double planet and binary planet" Before right now I never even considered that the Earth/moon system might be a double planet system.

    In astronomy, double planet and binary planet are informal terms used to describe a binary system where both objects are of planetary mass. Though not an official classification, the European Space Agency has referred to the EarthMoon system as a double planet.

    Definition of a double planet

    There has been debate on where to draw the line between a double planet and a planet–moon system. In most cases, this is not an issue because most satellites have small masses relative to their planets. In particular, with the exception of the Earth–Moon system, all satellites of planets in the Solar System have masses less than 0.00025 (14000) the mass of the host planet. The Moon-to-Earth mass ratio is 0.01230 (≈ 181). In comparison, the Charon-to-Pluto mass ratio is 0.117 (≈ 19).
    Co-accretion definition

    The now-abandoned co-accretion hypothesis of the origin of the Moon is also called the double-planet hypothesis. The idea is that two bodies should be considered a double planet if they accreted together directly from the proto-planetary disk, much as a double star typically forms together.
    Once it was realized that both the Earth's Moon and Pluto's Charon likely formed from giant impacts, this parallel was noted when calling them double planets. However, an impact may also produce tiny satellites, such as the small outer satellites of Pluto, so this does not determine where the line should be drawn.

    Center-of-mass definition


    Two fictional planets orbit their common center of gravity (red).

    A common proposal for a double planet is a system where the center of mass lies outside the primary. This was the basis for the argument that the Pluto–Charon system be considered a double planet when the IAU was debating whether dwarf planets should be considered a class of planet. Under this definition, the Earth–Moon system is not a double planet. However, the center of mass varies with the distance between the bodies. As the Moon migrates outward from the Earth, the center of mass of the system will migrate outward as well, until in a few hundred million years Earth will fit the definition. It has been suggested that such a definition would call into question Jupiter's status as a planet, as the center of mass of the Jupiter–Sun system lies outside the surface of the Sun.

    Tug-of-war definition

    Isaac Asimov suggested a distinction between planet–moon and double-planet structures based in part on what he called a "tug-of-war" value, which does not consider their relative sizes. This quantity is simply the relationships between the masses of the primary planet and the Sun combined with the squared distances between the smaller object and its planet and the Sun:
    tug-of-war value = m1m2 (d1d2)2 where m1 is the mass of the larger body, m2 is the mass of the Sun, d1 is the distance between the smaller body and the Sun, and d2 is the distance between the smaller body and the larger body. Note that the tug-of-war value does not rely on the mass of the satellite or smaller body.
    This formula actually reflects the relation of the gravitational effects on the smaller body from the larger body and from the Sun. The tug-of-war figure for Saturn's moon Titan is 380, which means that Saturn's hold on Titan is 380 times as strong as the Sun's hold on Titan. Titan's tug-of-war value may be compared with that of Saturn's moon Phoebe, which has a tug-of-war value of just 3.5. So Saturn's hold on Phoebe is only 3.5 times as strong as the Sun's hold on Phoebe.

    Asimov calculated tug-of-war values for several satellites of the planets. He showed that even the largest gas giant, Jupiter, had only a slightly better hold than the Sun on its outer captured satellites, some with tug-of-war values not much higher than one. Yet in nearly every case the tug-of-war value was found to be greater than one, so in every case the Sun loses the tug-of-war with the planets. The one exception was Earth's Moon, where the Sun wins the tug-of-war with a value of 0.46, which means that Earth's hold on the Moon is less than half the Sun's hold. Since the Sun's gravitational effect on the Moon is more than twice that of Earth's, Asimov reasoned that the Earth and Moon form a binary-planet structure. This was one of several arguments in Asimov's writings for considering the Moon a planet rather than a satellite.

    We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth. From within the Earth–Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun. It is always "falling toward" the Sun. All the other satellites, without exception, "fall away" from the Sun through part of their orbits, caught as they are by the superior pull of their primary planets – but not the Moon.
    — Isaac Asimov
    See the Path of Earth and Moon around Sun section in the "Orbit of the Moon" article for a more detailed explanation.
    Note that this definition of "double planet" depends primarily on the two-body structure's distance from the Sun. If the Earth–Moon system happened to orbit farther away from the Sun than it does now, then Earth would win the tug of war. From the orbit of Mars, the Moon's tug-of-war value would be 1.05, so the Sun would no longer win the tug of war with Earth. Also, several tiny moons discovered since Asimov's argument would qualify as double planets. Neptune's small outer moons Neso and Psamathe, for example, have tug-of-war values of 0.42 and 0.44, less than that of Earth's Moon. Yet their masses are tiny compared to Neptune's, with an estimated ratio of 1.510−9 (1700,000,000) and 0.410−9 (12,500,000,000).
    Double planet - Wikipedia, the free encyclopedia
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