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Is Earth orbiting Sun this way?

  1. Apr 7, 2013 #1
    Dear Sirs , Could someone please tell me this.
    See the attached picture, is Earth orbiting Sun this way or other way around, or changes dynamically?
    And why does only Moon orbiting Earth while Earth doesn't orbiting the Moon? Don't they both have relative gravitational force to each other?

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  2. jcsd
  3. Apr 7, 2013 #2


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    Its all about mass. Earth and moon orbit each other around their center of mass, which happens to be much closer to earth than the moon. Similarly earth and the sun orbit each other around their relative center of mass, which happens to be like inside the sun.
  4. Apr 7, 2013 #3
    Thanks,But that isn't moon has 1/6 of gravitational force of the Earth(I'm sure I have read this somewhere,or it appears to be only the gravitational force on the ground of the moon not between Earth-Moon system?) and at what point of relative mass differences to create a binary star system?

    and could you please also tell me the Earth orbit towards the Sun is it also flat on Sun's orbit, or not? Please see the attached pictures.
  5. Apr 7, 2013 #4


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    Acceleration on the Moon's surface is roughly 1/6th of than on Earth's. This is not the same as gravitational force between the two bodies, which is equal.(Newton's law of gravity)
    But since the Moon is much less massive than the Earth, it gets swinged around more than the Earth ever does(Newton's second law of motion), hence it looks like it's the Moon orbiting Earth.

    I'm not sure what you're asking here.

    Your pictures seem to depict a planet with a moon, not the planet-star system(the bigger body has an orbit of it's own, after all). When you say "Sun's orbit", what do you mean exactly by that?

    But to try and answer your question, Earth and Sun orbit each other(or more precisely, their mutual centre of mass - i.e.,the barycentre) in the same plane. So, flat.

    I recommend playing with planetarium software, like Celestia(http://www.shatters.net/celestia) to help you visualise the way orbits work.
  6. Apr 7, 2013 #5
    I mean at what point of mass differences between two different body to get a mutual centre of mass at the point between the two body in the space,not inside one body. I mean to make two body orbiting each other, not one orbiting another.

    Isn't the Sun having an orbit of its own orbiting the centre of the galaxy? And while all the planets orbiting the Sun at the same plane of the Sun's orbit towards the centre of the galaxy,and while the sun is always moving, and then all 8 planets could be even moving faster(to catch up with the Sun and get to its front) and then slower than the Sun(gets to the behind)? and is that why planets orbit towards sun becoming ellipse?

    Thanks , that's really helpful, although those planets looked so smaller than I've imagined.
  7. Apr 7, 2013 #6


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    It depends not only on the ratio of masses, but also on the size of the bodies.
    The centre of mass is easy to find. If a body is x times more massive than the other, then the centre of mass lies x times closer to it.
    Let's take the Sun-Jupiter case.
    The Sun weighs roughly 1000 times as much as Jupiter, so the barycentre lies 1/1000th of the Sun-Jupiter distance (~5AU ~=5*150 000 000 km =750 million km) away from the centre of the Sun, which is roughly 750 000 km.
    That's more or less the radius of the Sun.
    And indeed, the Sun-Jupiter barycentre lies outside the Sun!

    Now, if the Sun were in its last throes, and swelled to become a red giant, then its radius would be much larger than what it is now(while the mass stays nearly the same), enough to encompass the barycentre it shares with Jupiter.

    Ah, o.k. That makes it clearer.
    If you think of the plane in which the Earth orbits the Sun, then that plane is inclined 60deg to the plane of the galactic disc. So the Earth orbit looks like some two thirds between the two pictures you've provided.
    (Note that the position of the nodes - the points where the two planes intersect - changes as the Solar system travels around the galactic centre.)

    It's easy to see for yourself. Go out at night somewhere where you can see the Milky Way band in the sky. You'll notice that it is running at a different angle to the path that the sun follows in the sky as the year changes(that path lies close to the daily path. It's where all the zodiacal signs lie).

    Alternatively, turn on galactic coordinate grid in Celestia. It'll show you where the galactic "poles" are.
    The direction of the Sun's movement in it's galactic orbit lies roughly where the star Vega is currently.
    The galactic centre is in the constellation of Sagittarius.

    I know! One gets so bombarded by exaggerated scales in the pop-sci programs and sf films that it's sometimes hard to shake off.
  8. Apr 7, 2013 #7
    Thanks, while I was in school,being forced to learn physics, I never knew it is used this way,not to blame my teacher though, It was the education system sucked at that time.

    And could you please explain a little more about did We get the positions(running path) of planets&stars first to calculate the weight? If that's true, that would be amazingly accurate by those ancient people with small telescopes.

    I'm living in a city,can't get a chance to see that yet,but I'm sure I'll get a telescope asap, but saying that the Milky Way band running at a different angle,isn't that the earth's angle changes too due to its own orbit towards sun? Since nothing is stationary in the space, did we at least have a stationary formula or equation for this?

    This is amazing!! I've been hearing this kind of stuff only in my ears for years and then in my own imagination, but now I could get to see it in my naked eyes. I don't know how to thank you.

    and one more question, while our earth orbiting the sun,does it perfectly match with our clock time? I mean ideally We thought Earth would be perfect sphere while in reality is only close to it, would that happen earth running slower or faster in its path than our clock time? since I've been hearing about people saying this winter is coming early or this spring comes sooner.

    Thanks, you made my day!!
  9. Apr 7, 2013 #8


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    I'm not that well versed in the history of science, but from what I read in Resnick & Halliday(a very good college-level introductory book to physics), positions of planets were measured reasonably accurately even before the invention of telescope some four centuries ago.
    In fact, Kepler synthesised his laws of planetary motion using data gathered by Tycho Brache, who never used a telescope.

    I must confess I'm having a somewhat hard time understanding what is "this". If you could try and formulate your questions as clear and to the point as possible, that'd help with getting the right answer.

    I'm guessing, though, that you might be asking if the fact that the Earth is tilted on it's axis has any effect?
    It has no effect on the angle between the ecliptic(the yearly orbital plane of Earth) and the galaxtic plane.
    What it does, is to make the whole sky shift a bit up and down over the year.

    I'm glad I could help!

    Again, I'm not sure what is the meat of the question.
    If you're asking whether a full orbit takes exactly a year, then no. That's why we get leap years every now and then to compensate for the difference.
    If you're asking whether the Earth moves in its orbit with constant speed, then no again. This is what Kepler explained with his laws - orbits are(usually) ellipses, and the closer to the central body(e.g.the Sun), the faster the orbiting one goes.
    And if you're asking whether all the speeds and angles and other parameters change over time, then yes.
    For example, which star is the northern star changes over ~26000 years(it was Thuban in the constellation Draco 2000 years ago). Planets(esp. Jupiter) tug on each other, constantly perturbing the orbits. The Sun loses mass while planets and moons exchange angular momentum via tidal forces, causing the orbits to get longer, and sometimes shorter. The point of the closest approach to the Sun on Earth's orbit shifts as well(these days it's in January).
    It's all in a constant state of flux, but the changes are minuscle when compared to the human history.
    Furthermore, it's all governed by very simple laws - notably the universal law of gravitation.

    I don't think it's got much to do with short-term climatic changes. It's all very much too slow a process to credit/blame for it.
  10. Apr 7, 2013 #9
    Thanks, Despite I've asked questions inaccurate and purely in common sense,but I think I've got all the answers, For Now!!!
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