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Thread: Moons of Uranus

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    Moons of Uranus

    IIRC it is believed that Uranus got hit by a protoplanet and had it's axis knocked sideways.
    But how did the moons' orbits get knocked over?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    IIRC it is believed that Uranus got hit by a protoplanet and had it's axis knocked sideways.
    But how did the moons' orbits get knocked over?
    Before they formed. An impact when there is still a protoplanetary disc can (in effect) take the disc with it. The hitch is that the satellites are observed to orbit prograde, whereas an impact that shoved Uranus through more than 90 degrees should have led to a re-formed protoplanetary disc that was in the equatorial plane, but orbiting retrograde. The way around this is to invoke two impacts, each of which tilted Uranus by less than 90 degrees, but which summed to the current tilt.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Before they formed. An impact when there is still a protoplanetary disc can (in effect) take the disc with it. The hitch is that the satellites are observed to orbit prograde, whereas an impact that shoved Uranus through more than 90 degrees should have led to a re-formed protoplanetary disc that was in the equatorial plane, but orbiting retrograde. The way around this is to invoke two impacts, each of which tilted Uranus by less than 90 degrees, but which summed to the current tilt.

    Grant Hutchison
    It has always been my understanding that the five major moons orbit prograde in the sense of orbiting in the same direction as the planet's rotation. Do you have any references that say otherwise?

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    Quote Originally Posted by Hornblower View Post
    It has always been my understanding that the five major moons orbit prograde in the sense of orbiting in the same direction as the planet's rotation. Do you have any references that say otherwise?
    As I said, they are indeed observed to orbit prograde. Whereas simulations of "one big hit", with Uranus tipping through >90 degrees in a single event, predict that satellite motion would end up equatorial but retrograde. The implication is therefore that "two lesser hits", each tipping Uranus <90 degrees, but summing to >90 degrees, are required to push Uranus and its protoplanetary disc through 90 degrees and into their current orientations.

    Grant Hutchison

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    I'm a little confused on the rotation directions. If Uranus' rotation was originally prograde but got bumped past 90 deg., then it would become retrograde, as is consistent with what I think is reported. However, I have heard it described as "rolling along" it's orbit, assumingly like a ball on a hard-surfaced ecliptic plane, which would require a retrograde rotation originally. Is this just another ambiguous (at best) description?
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    Quote Originally Posted by George View Post
    I'm a little confused on the rotation directions. If Uranus' rotation was originally prograde but got bumped past 90 deg., then it would become retrograde, as is consistent with what I think is reported. However, I have heard it described as "rolling along" it's orbit, assumingly like a ball on a hard-surfaced ecliptic plane, which would require a retrograde rotation originally. Is this just another ambiguous (at best) description?
    It's a completely useless description. Uranus's rotation axis remains relatively fixed in space. Sometimes its rotation is aligned approximately as it were rolling along the north "surface" of its orbital plane. Half an orbital revolution later, it's aligned as if rolling along the southern "surface". Between those times, its rotation is skewed relative to its direction of travel and doesn't resemble rolling at all.

    "Prograde" and "retrograde" are in any case problematic words when assigned to planetary rotation axes. If we define the north pole of a planet as being the end of the rotation axis from which rotation appears to be anticlockwise, then Uranus's axis is inclined at 98 degrees to its orbital plane, and its rotation is neither prograde nor retrograde. If we define the north pole of the planet as being the one that lies to the ecliptic north of its orbital plane, then Uranus rotates in a retrograde manner (clockwise when viewed from its north pole) and its axis is inclined at 82 degrees to its orbital plane. People mix and match these definitions and cause themselves immense confusion. (I actually enjoy the rare distinction of having corrected the orientation of Uranus in two different astronomical simulators.)

    Satellites, however, are prograde or retrograde according to whether their revolution is in the same sense as, or opposite to, the rotation of the parent planet, and that works however we define "north pole".

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    It's a completely useless description. Uranus's rotation axis remains relatively fixed in space. Sometimes its rotation is aligned approximately as it were rolling along the north "surface" of its orbital plane. Half an orbital revolution later, it's aligned as if rolling along the southern "surface". Between those times, its rotation is skewed relative to its direction of travel and doesn't resemble rolling at all.
    Agreed, though I don't mind analogies serving as a "quick fix" to assist in getting certain points across, like the direction of rotation. But, as in this case, they must get the context right so that the listener isn't in a different part of the orbit than they are, and they should also note the difference. Is it worth it? I think so because pictures can be very powerful to help "eschew obfuscation" even at the risk of greater confusion.

    My problem is that "rolling along" would normally be applied to when the North pole is pointing away from the Sun, so it would be an erroneous analogy, given no explanation to the contrary.

    "Prograde" and "retrograde" are in any case problematic words when assigned to planetary rotation axes. If we define the north pole of a planet as being the end of the rotation axis from which rotation appears to be anticlockwise, then Uranus's axis is inclined at 98 degrees to its orbital plane,...
    Yes, but it makes sense to simply assign north to the pole pointing "upward" -- like the rest -- from the ecliptic.
    ... and its rotation is neither prograde nor retrograde.
    I would expect only at 90 degrees would it make little sense. Once it goes past 90 then it switches, right? [Ok, you're probably pointing-out the ambiguity since the axis angle determines which.]

    If we define the north pole of the planet as being the one that lies to the ecliptic north of its orbital plane, then Uranus rotates in a retrograde manner (clockwise when viewed from its north pole) and its axis is inclined at 82 degrees to its orbital plane. People mix and match these definitions and cause themselves immense confusion. (I actually enjoy the rare distinction of having corrected the orientation of Uranus in two different astronomical simulators.)
    Okay, I see your point, I think -- the grade becomes dependent on which angle you choose. It is awkward and I would normally prefer the 82 deg version if it weren't for the prograde-retrograde question as it should be taken in the context of its history. It should be understood that it was once "normal", before the bump or double bump, IMO, thus with its north pole (the one above the ecliptic like all the rest) pointing away from the Sun, then it went from prograde to retrograde in the normal Sun-planet relationship.

    Satellites, however, are prograde or retrograde according to whether their revolution is in the same sense as, or opposite to, the rotation of the parent planet, and that works however we define "north pole".
    Yes, and planets are the same where their host's direction defines their grade as well.
    We know time flies, we just can't see its wings.

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    To me, the more natural approach is to define poles using right-handed rotation vectors. Intuitively, we turn a screw in the "same direction" to secure it, no matter which way we're pointed. We don't need to remind ourselves that screws go retrograde into the floor but prograde into the ceiling. The IAU definition of "planetary north pole" (north side of the invariable plane of the solar system) is also poorly transferable to other star systems - you can do it, but it seems pointless to tie the north poles of extrasolar planets to the happenstance of our solar system's formation. And it's inconsistent with their polar definitions for dwarf planets, minor planets and comets, all of which use the right-hand rule. Some precess so fast, and are orientated in such a direction, that they'd flip their north and south pole definitions frequently if using the planetary definition - in other words they'd suddenly go from being prograde to retrograde, with only an infinitesimal change in their angular momentum.

    (I once asked Larry Niven why the Ringworld in his novels lies to galactic north of Known Space, if the Puppeteer Fleet Of Worlds discovered it while on their way to the Magellanic Clouds. He told me they were "taking the scenic route". But actually that would have made perfect sense if they'd been travelling to the rotational north of the Milky Way, which lies in the southern sky. There's even a suitable G-type star at a suitable distance in that direction. I tried to tempt him into fixing that one up in a later novel, but he never did.)

    Grant Hutchison

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    Rotational North! I like that.

    Which star did you suggest, by the way? Was it Alpha Mensae?

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    Quote Originally Posted by grant hutchison View Post
    To me, the more natural approach is to define poles using right-handed rotation vectors. Intuitively, we turn a screw in the "same direction" to secure it, no matter which way we're pointed. We don't need to remind ourselves that screws go retrograde into the floor but prograde into the ceiling.
    Hmmm....I'm unclear how this works. Using the Sun's north pole as our reference, and using your screw analogy, looking "down" (from the north) on the ecliptic the Earth would have a left-hand threaded screw, Uranus a right-hand thread. But looking "up" toward the ecliptic, the opposite is true. The screwdriver would have to be turned differently for both cases. Are you maintaining only a "downward" view for your analogy?

    It seems to me that the projection onto the ecliptic looking "downward" is all that is needed, and only a perfect 90 deg. obliquity would be ambiguous. I assume that's the convention.

    The IAU definition of "planetary north pole" (north side of the invariable plane of the solar system) is also poorly transferable to other star systems - you can do it, but it seems pointless to tie the north poles of extrasolar planets to the happenstance of our solar system's formation.
    It seems we always start with anthropomorphism as a rule. It gets the ball rolling, if I may play with that phrase in a better way than others.

    (I once asked Larry Niven why the Ringworld in his novels lies to galactic north of Known Space, if the Puppeteer Fleet Of Worlds discovered it while on their way to the Magellanic Clouds. He told me they were "taking the scenic route". But actually that would have made perfect sense if they'd been travelling to the rotational north of the Milky Way, which lies in the southern sky. There's even a suitable G-type star at a suitable distance in that direction. I tried to tempt him into fixing that one up in a later novel, but he never did.)
    That had to be a treat!
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    Hmmm....I'm unclear how this works. Using the Sun's north pole as our reference, and using your screw analogy, looking "down" (from the north) on the ecliptic the Earth would have a left-hand threaded screw, Uranus a right-hand thread. But looking "up" toward the ecliptic, the opposite is true. The screwdriver would have to be turned differently for both cases. Are you maintaining only a "downward" view for your analogy?
    I'm rejecting the idea that there is a unique "downward" direction in space, and I'm also rejecting the idea that "downward" has the slightest relevance to driving screws (or rotating planets).
    We don't insert screws from a particular "master direction" - we use standard right-handed screws, point them in any direction necessary, and drive them with the same clockwise rotation, viewed from the head. (Which discriminates massively against the left-handed, but that's another story.)
    By this reasoning Uranus and Earth have standard, matching threads - but one is driven from below the ecliptic and one is driven from above; one is screwed into the ceiling, and one into the floor. And that's exactly the IAU convention for dwarf planets, minor planets and comets. The "ecliptic north" convention for planets is just an accident of history.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    I'm rejecting the idea that there is a unique "downward" direction in space, and I'm also rejecting the idea that "downward" has the slightest relevance to driving screws (or rotating planets).
    We don't insert screws from a particular "master direction" - we use standard right-handed screws, point them in any direction necessary, and drive them with the same clockwise rotation, viewed from the head. (Which discriminates massively against the left-handed, but that's another story.)
    Yeah I get screws, of course, but not how the analogy works. Taking Earth, looking upward (South pole; ceiling-like) it would be a right-handed thread screw, but looking downward it becomes a left-handed thread screw, right? The direction requires a reference frame, so what is your reference frame?
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    Yeah I get screws, of course, but not how the analogy works. Taking Earth, looking upward (South pole; ceiling-like) it would be a right-handed thread screw, but looking downward it becomes a left-handed thread screw, right?
    No, it remains a left-hand threaded screw - you're just looking at the head end when you look from ecliptic north, and the sharp end when you look from ecliptic south.

    Quote Originally Posted by George View Post
    The direction requires a reference frame, so what is your reference frame?
    The reference frame IS THE ROTATING EARTH. Its rotation breaks symmetry, in the same way a screw breaks symmetry. A screw doesn't suddenly reverse its threads and the position of its head and point when you look at it from the other end.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    No, it remains a left-hand threaded screw - you're just looking at the head end when you look from ecliptic north, and the sharp end when you look from ecliptic south.
    Okay, it's the view from the northern half, which is what I was calling "down" on Earth. The screw analogy matches also a projected rotational component onto the ecliptic plane; the Earth retrograde (left-handed) and Uranus prograde (right-handed).

    I like your suggestion to avoid using the prograde/retrograde terms in favor of the right-hand rule given a northern reference convention. But what label would you give them -- RH and LH rotations? I assume this would apply for all objects and not just planets, in lieu of the actual rotation of the planet serving as the reference for its satellites, right?
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    Okay, it's the view from the northern half, which is what I was calling "down" on Earth. The screw analogy matches also a projected rotational component onto the ecliptic plane; the Earth retrograde (left-handed) and Uranus prograde (right-handed).
    I like your suggestion to avoid using the prograde/retrograde terms in favor of the right-hand rule given a northern reference convention. But what label would you give them -- RH and LH rotations? I assume this would apply for all objects and not just planets, in lieu of the actual rotation of the planet serving as the reference for its satellites, right?
    I have absolutely no idea what you're talking about.
    I give up.

    Grant Hutchison

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    Here's Phil Plait explaining the same thing. I agree with everything he says.
    Basically, the IAU definition of a planetary north pole requires three pieces of information - two coordinates (confined to half the sky), and the word "prograde" or "retrograde". Defining the north pole by the right-hand rule requires two pieces of information - two coordinates (anywhere in the sky).

    Grant Hutchison
    Last edited by grant hutchison; 2019-Sep-11 at 09:21 PM.

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    Quote Originally Posted by grant hutchison View Post
    I'm rejecting the idea that there is a unique "downward" direction in space, and I'm also rejecting the idea that "downward" has the slightest relevance to driving screws (or rotating planets).
    We don't insert screws from a particular "master direction" - we use standard right-handed screws, point them in any direction necessary, and drive them with the same clockwise rotation, viewed from the head. (Which discriminates massively against the left-handed, but that's another story.)
    I'd never really thought about that (though I have about a lot of other things). It might be harder for us to get them in, but I think that means it's easier for us to take them out...
    As above, so below

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    Quote Originally Posted by Jens View Post
    I'd never really thought about that (though I have about a lot of other things). It might be harder for us to get them in, but I think that means it's easier for us to take them out...
    But, since more screws are driven than are ever removed (it would be remarkable if the reverse were true), the threading favours the right-handed, who use their more powerful movement (supination) for the more commonly performed action.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    But, since more screws are driven than are ever removed (it would be remarkable if the reverse were true), the threading favours the right-handed, who use their more powerful movement (supination) for the more commonly performed action.
    Yes, absolutely true. You have helped to renew my feelings of victimization.

    Seriously, though, as a lefty I have always understood that there are inconveniences, but on the other hand I have always liked the feeling of being different from the majority. As a a white male, itís sort of the one thing that allows me to claim I am not completely mainstream.


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    As above, so below

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    Quote Originally Posted by Jens View Post
    Yes, absolutely true. You have helped to renew my feelings of victimization.

    Seriously, though, as a lefty I have always understood that there are inconveniences, but on the other hand I have always liked the feeling of being different from the majority. As a a white male, it’s sort of the one thing that allows me to claim I am not completely mainstream.


    Sent from my iPhone using Tapatalk
    As a lefty , we learn early to use both hands so it is not really discrimination, but oft times an advantage. As for screws, hammer them in and screw them out using left hand. OK, joking a little, we have to separate the LH rule and the RH rule. From school; “generighter”
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by grant hutchison View Post
    I have absolutely no idea what you're talking about...
    Actually we agree. It's surprising how something so simple can get tangled-up.

    Thanks for Phil's link, which explains it fairly nicely. [Nit -- His statement, "The invariable plane is very close to being the same as the Sun's equator," seems a weak use of the phrase "very close" given the > 7 deg. tilt of the Sun's axis.]

    From a northern celestial hemisphere -- actually the northern hemisphere formed by the invariance plane (something I have apparently ignored) -- looking down upon any given rotating body, you and Phil prefer the right-hand rule where one imagines wrapping their right hand around the object. [If the object is spinning in the same direction the middle finger is pointing then the thumb represents the north pole, otherwise it's the south pole.] Thus, unlike the other planets, Venus and Uranus would have their north pole pointing in the direction of the southern celestial hemisphere.

    This is the same as what I meant by projection. If you project their rotation (the appropriate rotational vector component) onto the invariance plane then, looking from the northern hemisphere, it will either be clockwise or counterclockwise. Clockwise only for the planets Venus and Uranus.

    Below, to provide some relief with today's unusual work stress if nothing else, is my attempt to demonstrate the direction and relative rotation rates, with Earth having a 360 deg/day rate. Also, the planets' position (side view) above the invariance plane is included (in a color not associated with the Sun ).

    Scale model with rotations.jpg
    Last edited by George; 2019-Sep-12 at 07:27 PM.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by grant hutchison View Post
    But, since more screws are driven than are ever removed (it would be remarkable if the reverse were true), the threading favours the right-handed, who use their more powerful movement (supination) for the more commonly performed action.
    The screws were a bit confusing as I tend to be a little too literal at times. An odd left-hand screw would be needed to match the rotational direction of the planets if one is looking down from the northern hemisphere, with the exception of Venus and Uranus.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    Actually we agree.
    Actually, we don't. But this is irrelevant to the OP.

    Grant Hutchison

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    Quote Originally Posted by George View Post
    Actually we agree. It's surprising how something so simple can get tangled-up.

    Thanks for Phil's link, which explains it fairly nicely. [Nit -- His statement, "The invariable plane is very close to being the same as the Sun's equator," seems a weak use of the phrase "very close" given the > 7 deg. tilt of the Sun's axis.]

    From a northern celestial hemisphere -- actually the northern hemisphere formed by the invariance plane (something I have apparently ignored) -- looking down upon any given rotating body, you and Phil prefer the right-hand rule where one imagines wrapping their right hand around the object. [If the object is spinning in the same direction the middle finger is pointing then the thumb represents the north pole, otherwise it's the south pole.] Thus, unlike the other planets, Venus and Uranus would have their north pole pointing in the direction of the southern celestial hemisphere.

    This is the same as what I meant by projection. If you project their rotation (the appropriate rotational vector component) onto the invariance plane then, looking from the northern hemisphere, it will either be clockwise or counterclockwise. Clockwise only for the planets Venus and Uranus.

    Below, to provide some relief with today's unusual work stress if nothing else, is my attempt to demonstrate the direction and relative rotation rates, with Earth having a 360 deg/day rate. Also, the planets' position (side view) above the invariance plane is included (in a color not associated with the Sun ).

    Scale model with rotations.jpg
    As I understood, Phil said that according to the IAU definition the north pole of a particular planet is the one that is north of the invariable plane. He added that he does not like it, for reasons that may or may not be compelling physics.

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    Quote Originally Posted by Hornblower View Post
    As I understood, Phil said that according to the IAU definition the north pole of a particular planet is the one that is north of the invariable plane. He added that he does not like it, for reasons that may or may not be compelling physics.
    Yes, but I was stating the bold part in the context of what Phil favored, though not in agreement with the current IAU convention. His view does make better sense and it seems to be the convention for moons. Is this right? IOW, using the right-hand rule method, or as I prefer, simply projecting their rotation downward onto the invariable plane to determine the rotation direction, ignoring their host planet?
    We know time flies, we just can't see its wings.

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    Again, what star did you propose to Niven for his Ringworld?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Again, what star did you propose to Niven for his Ringworld?
    HD 512.

    Grant Hutchison

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    Ah yes. Right near the South Galactic Pole (the North Rotational pole). Thanks!

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    Quote Originally Posted by eburacum45 View Post
    Ah yes. Right near the South Galactic Pole (the North Rotational pole). Thanks!
    G3, rather than G2, and a parallax corresponding to 204 lightyears. But it's a pretty good fit, considering.

    Grant Hutchison

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    I've heard about ice giants spawning diamonds deep within--I wonder if some are scattered o the moons/rings after the impact.

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