Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question About Elliptical Orbits

  1. Oct 23, 2003 #1
    The orbits of celestial bodies are described as elliptical.

    If we look at the well known means of drawing an ellipse with two tacks and a string, you can keep moving your pencil around and around and it will draw over and over the same line.

    The same cannot be said of an orbit. The center of all orbits is constantly traveling. The orbiting body will never retrace a previous path.

    My question is: are there some orbiting bodies that would end up describing circles if the centers of their orbits would just stay still, and others that would actually describe ellipses, or do all orbits always describe ellipses because of the fact all centers of orbit are continuously moving?
     
  2. jcsd
  3. Oct 23, 2003 #2

    wolram

    User Avatar
    Gold Member

  4. Oct 23, 2003 #3
    Thanks Wolram,

    I have this primitive system called "WebTv", not a computer, which does not allow me to use java, or pdf/adobe. I couldn't view the animation at the link.
    If the text there had the answer to my question, I did not recognise it behind the concepts and formulas I don't grasp yet.
     
  5. Oct 23, 2003 #4

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Do you want to clarify this statement? In terms of the orbit, the focus remains fixed.
     
  6. Oct 23, 2003 #5
    Here is what I mean using the earth, sun, and moon as examples.
    The earth orbits the sun. The earth is, therefore, not standing still, but constantly traveling.
    While it is doing this, the moon orbits the earth.

    Each time the moon comes round the earth, the earth has moved from where it was the last time the moon came round. The moon will never, therfore, retrace the exact path of a previous orbit.
     
  7. Oct 23, 2003 #6

    wolram

    User Avatar
    Gold Member

    http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

    It fell to Kepler to provide the final piece of the puzzle: after a long struggle, in which he tried mightily to avoid his eventual conclusion, Kepler was forced finally to the realization that the orbits of the planets were not the circles demanded by Aristotle and assumed implicitly by Copernicus, but were instead the "flattened circles" that geometers call ellipses (See adjacent figure; the planetary orbits are only slightly elliptical and are not as flattened as in this example.)
    ----------------------------------------------------------------------
    try this one zooby.
    as long as a two body system is not perturbed the orbit remain
    the same, over a reasonable period, "eliptical".
     
  8. Oct 23, 2003 #7

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Question About Elliptical Orbits

    In a simple two-body system, both bodies orbit in ellipses with the center of mass at once focus. Excepting for general relativistic precession, the two bodies will follow the same two ellipses together, forever, in a resonance.

    - Warren
     
  9. Oct 23, 2003 #8

    russ_watters

    User Avatar

    Staff: Mentor

    Re: Re: Question About Elliptical Orbits

    I think you are mixing two separate principles. Precession of an orbit is not due to the motion of the body being orbited. It happens even without factoring in other sources of motion.

    Pick a stationary frame of reference at the center of mass of two objects and both objects will orbit the center of mass in ellipses with precession.

    To answer your first question though, YES, it is possible to have a circular orbit, as a circle is simply a special case of an ellipse with a distance of zero between the two foci. This type of perfection is however, unlikely to happen naturally.
     
    Last edited: Oct 23, 2003
  10. Oct 24, 2003 #9
    OK. Each has contributed a piece of the puzzle and the question is answered:

    My question is: are there some orbiting bodies that would end up describing circles if the centers of their orbits would just stay stillAffirmative: special case ellipse.

    and others that would actually describe ellipses
    Yes, all orbits are elliptical.

    or do all orbits always describe ellipses because of the fact all centers of orbit are continuously moving?
    Answer is No. I guess this one threw people off the most because I didn't know how to explain what I was wondering about explicitly enough.

    Thanks everyone for your imput.

    Wolram: That keppler/Brahe site looks very readable and easy to understand. Thanks for finding it.

    -Zooby
     
  11. Oct 24, 2003 #10

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think that by the laws of large numbers, assuming of course that planets are in fact common, we can safely assume that circular orbits do occur. However, due to the drag created by the molecules of hydrogen, dust, and any other debris floating around in space, no circular orbit could remain stable.
     
    Last edited: Oct 24, 2003
  12. Oct 24, 2003 #11

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Question About Elliptical Orbits

    Weather or not you consider the focus of the orbit as moving or not has no effect on the shape of the orbit around that focus.
     
  13. Oct 24, 2003 #12
    Re: Re: Re: Question About Elliptical Orbits

    This is one of the things I was, clumsily, trying to figure out: do astronomers say orbits are elliptical from the perspective of the focus or from some larger perspective; looking down on the solar system, for instance, and imagining each planet and moon left a trail. I figured out the latter option had to be impossible when I was able to imagine the trail left by the moon over one year: it would look over all like a circle, but made of 12 shallow scallops.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Question About Elliptical Orbits
  1. Elliptical Orbits (Replies: 2)

Loading...