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  1. #1
    Jonas Cross's Avatar
    Jonas Cross Guest

    Default What is "retrograde orbital motion?"




    I once read that some of Jupiter's moons have retrograde orbital
    motion. I understand what that means. It means that, when the Jovian
    "system" is viewed from Jupiter's North Pole, the moons appear to be
    orbiting Jupiter in a clockwise direction. But, why do we describe such
    motion as "retrograde?" Is it because:

    (a) those moons are orbiting Jupiter in the direction opposite from the
    way that Jupiter rotates on its axis;

    (b) those moons are orbiting Jupiter in the direction opposite from the
    way that Jupiter orbits the Sun;

    (c) those moons are orbiting Jupiter in the direction opposite from the
    way that Earth orbits the Sun (the idea being that Earth sets the standard
    for the entire Solar System); or

    (d) other (please explain).

    Thanks.

  2. #2
    Brian Tung's Avatar
    Brian Tung Guest

    Default What is "retrograde orbital motion?"

    Jonas Cross wrote:

    I think the canonical answer is (a), although both (b) and (c) also
    happen to be true, and of course, that isn't coincidental, since both
    the Earth and Jupiter were formed in the same primordial cloud of gas
    and dust. (I think there's a law that says you have to use the word
    "primordial" there.)

    --
    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

  3. #3
    Davoud's Avatar
    Davoud Guest

    Default What is "retrograde orbital motion?"

    Jonas Cross:


    Brian Tung:

    1. Your eloquence never ceases to amaze me. I was going to answer Mr.
    Cross's question in this way:


    *Bingo*

    But you made that impossible.

    2. I think that "primordial" (literally, "existing at or from the
    beginning of time") is superfluous here, even in the figurative sense
    in which you and I understand it -- very old. Why does it have to be
    said that the cloud of dust and gas is primordial? Whether it dates
    from perhaps 6 billion years BP or three weeks BP, the essential
    information in this context is that, for the Earth and Jupiter, it is
    the /same/ cloud.

    Just killing time here, waiting for my lunch to cook. Move on, nothing
    to see here, etc.

    Davoud

    --
    usenet *at* davidillig dawt com

  4. #4
    Brian Tung's Avatar
    Brian Tung Guest

    Default What is "retrograde orbital motion?"

    Davoud wrote:

    Sorry about that.


    That was my point--it's one of those things that seems almost de rigeur,
    for no particular reason. It's rare that a science program refers to
    that cloud and doesn't *somewhere* use the word "primordial" to describe
    it.


    What's your lunch cooking? Itself? Isn't that a little macabre?

    (Bump, set, ...)

    --
    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

  5. #5
    Davoud's Avatar
    Davoud Guest

    Default What is "retrograde orbital motion?"

    Davoud:

    Brian Tung:

    Uh-uh! I was ready for that one. "cook: verb, intrans. (of food) be
    heated so that the condition required for eating is reached."

    Macabre? That would be the sentient food animal at Milliways. He
    described to the patrons the virtues of eating his various parts, then
    said "I'll just nip off and shoot myself..." or words to that effect.

    I'm having boiled fresh* white corn and boiled potatoes for lunch. They
    are cooking themselves now.

    Davoud

    *fresh: adj. picked on my wife's farm within the past hour.

    --
    usenet *at* davidillig dawt com

  6. #6
    William C. Keel's Avatar
    William C. Keel Guest

    Default What is "retrograde orbital motion?"

    Jonas Cross <nope@isp.invalid> wrote:






    The most comon meaning of retrograde would be opposite the direction of
    the parent body's rotation, because this has dynamical consequences
    (affects tidally driven decay of orbits, for example). In galaxy
    pairs, this is the meaning. In Jupiter's case, senses a and b coincide.
    (b) also matters, because for moons which are just barely bound to
    a planet in view of perturbations by the Sun, retrograde orbits
    are more stable and longer-lasting than direct (the astronomical
    opposite of retrograde, no matter what the flight-dynamics people
    think by inventing "posigrade"; prograde is only a bit better) orbits.
    Just as well Venus has no dynamical room to fit satellites, because
    we'd have a hard time telling whether they were direct or retrograde
    since it would depend on he context. And Pluto and Uranus skip that
    problem by being polar with respect to the orbital planes.

    Bill Keel

  7. #7
    oriel36's Avatar
    oriel36 Guest

    Default What is "retrograde orbital motion?"


    William C. Keel wrote:


    There is only one astronomical meaning for retrogrades and it is
    very,very simple.

    The plotted positions of planetary orbits against the stellar
    background generate a particular effect reflecting the appearance of a
    periodic backward motion against the stars,this is retrograde.This was
    common knowledge to both Ptolemaic and Copernican astronomers however
    the conclusions to what those motions were are entirely different.

    http://www.opencourse.info/astronomy...turn_retro.gif

    Looking at the time lapse footage you can see how the Ptolemaic
    astronomers guessed that periodic looping motions occured in planetary
    orbits against a stationary Earth.

    Looking at the same time lapse footage you can see the orbitally moving
    Earth overtaking the slower moving outer planets hence a common orbital
    motion around the Sun.




    In galaxy

    Newtonian junk !,without time lapse footage of the motion of Jupiter
    and Saturn it would be difficult to explain how the Ptolemaic and
    Copernican astronomers thought and why they thought the way they
    did.With time lapse footage it is extremely easy ,with a little
    intuitive intelligence,to see the reasoning behind heliocentricity
    unfold is such a wonderful way.It was the way the first heliocentric
    astronomers understood it.

    Newton did not even rise to the level of heliocentricity and misjudged
    what retrogrades are,how the solution is provided by considering the
    observed motion of the planets from an orbitally moving Earth ~

    "For to the earth they appear sometimes direct, sometimes stationary,
    nay, and sometimes retrograde. But from the sun they are always seen
    direct, " Newton

    So.the original poster can come to the actual Copernican understanding
    and bypass Newton by recognising that planetary orbits are seen
    directly from Earth,what retrogrades are and how to distinguish plotted
    positions from conclusions as the Ptolemaic and Copernican astronomers
    did.





    You are just another Newtonian blusterer in a world that has known too
    many,it is why there are no real astronomers ,not even people who can
    rise to the level of appreceating the Ptolemaic system even though it
    is easy enough to see where they were drawing their conclusions.


  8. #8
    Bill Owen's Avatar
    Bill Owen Guest

    Default What is "retrograde orbital motion?"

    Brian Tung wrote:

    The canonical answer is indeed (a). The inner satellites of Uranus and
    the three known satellites of Pluto are considered to be in direct
    orbits, not retrograde, even though the planets themselves are in
    retrograde rotation.

    -- Bill Owen

  9. #9
    Bill Owen's Avatar
    Bill Owen Guest

    Default What is "retrograde orbital motion?"

    William C. Keel wrote, inter alia:

    THANK YOU, thank you, thank you for that parenthetical remark. I've
    been trying to convince the "flight-dynamics people" here at JPL of
    this for years. Even some of the astronomy types have succumbed to
    "prograde". Sigh.

    Incidentally, equatorial orbits about irregularly shaped bodies tend
    to be stable only if they're retrograde. Dan Scheeres (U of Michigan)
    has done a lot of work in this area.

    -- Bill Owen

  10. #10
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default What is "retrograde orbital motion?"

    On Tue, 11 Jul 2006 11:49:37 -0700, Bill Owen <wmo@jpl.nasa.gov> wrote:


    Nothing wrong with "prograde". I'm an orbital dynamicist, and that's
    what I use. The pairing of "prograde" and "retrograde" works very
    nicely, much better than using "direct", IMO.

    Word usage changes, and you can spend all the time you want trying to
    convince people to use your choice of words, but language will still
    evolve. Prograde has been with us about 40 years now, and it looks like
    it will be with us for a while.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

 

 
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