Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
Dane B
super member
Reged: 02/23/08
Posts: 120
Loc: Seattle, Wa
|
|
Here's a barrage of questions related to how orbiting bodies move. If you have the answers I'd love to hear your explanations.
Galaxies, planetary systems, moons, rings - they all have a plane in which things orbit. Why do the orbiting bodies "organize" themselves into this plane, rather than each object orbiting on it's own plane?
Why is Pluto's orbit out-of-plane, and why does it intersect Neptune's orbit?
Why are the rings of Uranus out of plane with the planet’s orbit?
If all orbits are ellipses with the object being orbited at one of the foci, what determines the size and proportions of the ellipse?
Saturn's rings appear to be centered around the globe - are they actually orbiting in an off-center ellipse?
Earth speeds up as it gets closer to the foci occupied by the sun, and slows down as it gets farther away. Does this acceleration (and change in acceleration) result in any measurable effects on Earth (such as a change in ocean tides)?
What determines the average velocity of a planet throughout its orbit?
Why do planets rotate as they orbit, giving rise to days?
Why is this rotation around an axis perpendicular to the plane of orbit?
Do they all rotate in the same direction?
What determines the rate of rotation?
Why doesn't our moon rotate?
Does the sun rotate with respect to the galactic center? If so, how long is the period of rotation?
Do planetary rings rotate?
Why do all planets orbit in the same direction - why doesn't one planet go clockwise and another counter-clockwise relative to a certain perspective?
Why do spiral galaxies have arms, and what determines their proportions?
Do elliptical and irregular galaxies rotate?
Say from a certain perspective outside the galaxy we saw the Sun orbiting the galactic center in a clockwise direction. If we zoomed in would we see the planets also orbiting clockwise? Would the planets' moons and rings orbit clockwise? Would the planets themselves rotate clockwise? Would this be the case for all planetary systems in the galaxy? If so, what is it about our galaxy that makes things rotate in the same direction?
Why does Earth wobble in its daily rotation – why does the axis of rotation not remain perfectly perpendicular to the plane or orbit?
Over greater time periods, the average angle of the axis of rotation changes – why?
Do all planets wobble in their daily rotation?
Does the sun wobble?
Okay that's probably enough for now.
Dying of curiousity,
Dane
--------------------
Meade DS-2130AT w/ GOTO
Nikon 12x50AEs
Without judgement what would we do? We would be forced to look at ourselves...
-Death
|
Pess
(Title)
Reged: 09/12/07
Posts: 1910
Loc: Toledo, Ohio
|
|
I'm not going to wade through all that since most can be answered fairly easily in the same manner.
The law of conservation of angular momentum.
Everything in our neighborhoof started out as a giant gas cloud left over from a number of previous supernova explosions. Over eons this gas (usually referred Io as a Nebula) became denser as it spun toward and contracted around its own gravitational nexus.
Just like a skater who pulls her arms in, the cloud spun faster and faster as the cloud contracted.
Of course, as the cloud accredited into planets, moons, comets and other debris of substantial size they collided with one another altering the local angular momentum of the individual players. Case in point: Venus.
The Ring systems nobody knows for sure. Guesses include they come from moons spewing ice and whatnot. We know they would not last very long without some source of constant renewal but we are not sure where that comes from as of yet.
Hope this gives a start.
Pesse (yawn) Mist
|
LivingNDixie
Lord of Ferrets
Reged: 04/23/03
Posts: 15790
Loc: Hoover, AL
|
|
Regarding the Pluto questiosn about it being out of the plane of the solar system and crossing the orbit of Neptune.
Pluto is a KBO and is part of the leftovers of the solar system's formation. Their orbits are not same as the planets. That is one of the reasons why it was (appropriately) demoted from planet status.
--------------------
Preston
Celestron 11" Nexstar GPS XLT
Lunt LS60T/Ha 60mm f/8.33 (on order)
It’s not finishing something when your tank is empty that makes you a stronger person. It’s brushing yourself off and refacing the foe that defeated you with the same determination and willingness to fight that you had when you began your journey.
|
llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10475
Loc: Amargosa Valley, NV, USA
|
|
Quote:
Galaxies, planetary systems, moons, rings - they all have a plane in which things orbit. Why do the orbiting bodies "organize" themselves into this plane, rather than each object orbiting on it's own plane?
Not all galaxies have a preferred orbital plane. Elliptical galaxies such as M-87 don't. Neither do globular clusters, for that matter.
As for the others, it's as Pess stated -- conservation of angular momentum. In this case, the angular momentum is an artifact of the gravitational collapse of the parent gas and dust cloud from which the system formed.
Quote:
Why is Pluto's orbit out-of-plane, and why does it intersect Neptune's orbit?
Again, Pluto is a KBO -- the smaller a body is, the more vulnerable to random gravitation perturbations by other objects. Beyond the major planets, many of the small bodies have been orbitally disturbed -- many have indeed been ejected from their original orbital locations altogether.
Quote:
Why are the rings of Uranus out of plane with the planet's orbit?
Because its equator is also tilted. The rapid spin of the giant planets creates a significant tidal bulge around their equators, and this bulge affects the shapes of their gravitational fields. Objects orbiting very close to the planet will be "herded" into zones of increased orbital stability, and with the gravitational field other than spherical, the stable zones will reflect that bulge.
Quote:
If all orbits are ellipses with the object being orbited at one of the foci, what determines the size and proportions of the ellipse?
The instantaneous velocity and direction vectors of the gravitating bodies with respect to one another.
Quote:
Saturn's rings appear to be centered around the globe - are they actually orbiting in an off-center ellipse?
Probably not for the most part. The bodies that make up the ring have had their orbital energies "smeared out" by an averaging effect produced by the billions of other ring particles and the other major and minor moons. However, I think that some of the minor rings might be more directly perturbed into non-circular orbits.
Quote:
Earth speeds up as it gets closer to the foci occupied by the sun, and slows down as it gets farther away. Does this acceleration (and change in acceleration) result in any measurable effects on Earth (such as a change in ocean tides)?
I think the only measureable tidal effect is due to the change in distance from the Sun, not the orbital velocity. It's rotation, not orbital acceleration, that primarily powers the tides. Any such effect would be miniscule compared to the direct gravitational effects of the Sun and Moon.
Quote:
What determines the average velocity of a planet throughout its orbit?
Primarily its average distance from the Sun.
Quote:
Why do planets rotate as they orbit, giving rise to days?
If planetary growth is by condensation or accretion from a particle-rich zone centered on its orbit, then the collection of material from both inside and outside its orbit will likely result in a coriolis effect of sorts. The rate of rotation will be partly determined by the width of the accretion zone, and also significantly influenced by random impacts of massive planetesimals.
Quote:
Why is this rotation around an axis perpendicular to the plane of orbit?
It's not, completely. Most planets, and most dramatically Uranus, are tilted with respect to the plane of orbit. The fact that most accreting bodies are within the thin pre-solar disk tends to drive rotation axes perpendicular to that disk, but the randomness of individual impacts superimposes random deviations from that orientation.
Quote:
Do they all rotate in the same direction?
No. Venus and Uranus are retrograde.
Quote:
What determines the rate of rotation?
Se earlier response: "The rate of rotation will be partly determined by the width of the accretion zone, and also significantly influenced by random impacts of massive planetesimals." I should also add, with respect to your next question, that tidal forces also exert a significant influence on bodies that orbit close to a massive parent.
Quote:
Why doesn't our moon rotate?
It does. It rotates with the same period as its orbit. Tidal locking to Earth is the explanation.
Quote:
Does the sun rotate with respect to the galactic center? If so, how long is the period of rotation?
Yes. 25.38 days at the equator, more slowly near the poles.
Quote:
Do planetary rings rotate?
With respect to what? They are essentially a fluid of small particles. The term "rotation" is kind of meaningless in that context. The individual particles orbit the planet, but the ring as a whole is not a solid surface that can rotate.
Quote:
Why do all planets orbit in the same direction - why doesn't one planet go clockwise and another counter-clockwise relative to a certain perspective?
Once again -- angular momentum from the pre-solar disk is being conserved.
Quote:
Why do spiral galaxies have arms, and what determines their proportions?
The jury is still out on that one. General explanations such as "density waves" and "gravitational interactions" are out there, but a lot of the details still need to be worked out. The dynamics are fightfully complex.
Quote:
Do elliptical and irregular galaxies rotate?
Probably not as a whole, since their stars circumscribe individually unique orbits within them. If they have degrees of flattening, then there might be a sense in which there is a preferred orbital direction among their stars.
Quote:
Say from a certain perspective outside the galaxy we saw the Sun orbiting the galactic center in a clockwise direction. If we zoomed in would we see the planets also orbiting clockwise? Would the planets' moons and rings orbit clockwise? Would the planets themselves rotate clockwise? Would this be the case for all planetary systems in the galaxy? If so, what is it about our galaxy that makes things rotate in the same direction?
No, the solar system is tilted with respect to the plane of the galaxy, and I'm under the impression that other stars rotate in directions that are completely random with respect to the overall galactic rotation. The rotation orientation of a star system is controlled by the local circumstances of its birth within a large, turbulent cloud, not by the direction of its orbit with respect to the galactic center.
Quote:
Why does Earth wobble in its daily rotation? Why does the axis of rotation not remain perfectly perpendicular to the plane or orbit?
Primarily because of gravitational disturbances caused by the Moon and other planets, since none of them orbit in exactly the same plane.
Quote:
Over greater time periods, the average angle of the axis of rotation changes: why?
Same as above. The dynamic influence of the other planets is ever-changing.
Quote:
Do all planets wobble in their daily rotation?
Probably.
Quote:
Does the sun wobble?
Probably -- but very slowly, I would think.
--------------------
"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
|
Dane B
super member
Reged: 02/23/08
Posts: 120
Loc: Seattle, Wa
|
|
Thanks for that detailed response, Dave.
I had the feeling much of it could be explained with a single concept (turned out to be angular momentum), but I thought I'd ask them all so I wouldn't miss out on an interesting answer.
I mixed up my terminology when I asked if planetary rings rotate - I meant do they move around the planet in an orbit, but thinking about it now reveals what a silly question it is. I have to learn to think of the rings as billions of individual objects in their own orbits instead of treating the rings as a unit.
"If planetary growth is by condensation or accretion from a particle-rich zone centered on its orbit, then the collection of material from both inside and outside its orbit will likely result in a coriolis effect of sorts. The rate of rotation will be partly determined by the width of the accretion zone, and also significantly influenced by random impacts of massive planetesimals."
That's fascinating!
--------------------
Meade DS-2130AT w/ GOTO
Nikon 12x50AEs
Without judgement what would we do? We would be forced to look at ourselves...
-Death
|
Jarad
Post Laureate
Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
|
|
Just to add on a bit, you can do all the calculations for newtonian orbits using conservation of angular momentum, conservation of energy, and gravity. Energy is conserved - as objects move towards each other, they speed up due to gravity, as they move away, they slow down. Combine that with angular momentum, and you get newtonian orbital mechanics.
There is some good info here if you want to dabble in the math: Wiki Orbital Mechanics page
There are relativistic effects that will cause deviations from the newtonian predictions, but they only become significant with massive objects close together (Mercury deviates measureably from newtonian predictions because it is so close to the sun, but the other planets don't - GR and newtonian predictions are too close to distinguish).
Jarad
--------------------
|
Pess
(Title)
Reged: 09/12/07
Posts: 1910
Loc: Toledo, Ohio
|
|
Quote:
....., but they only become significant with massive objects close together (Mercury deviates measureably from newtonian predictions because it is so close to the sun, but the other planets don't - GR and newtonian predictions are too close to distinguish).
Jarad
Pesse (Unless, just maybe, you are a Pioneer spacecraft.) mist 
|
Jarad
Post Laureate
Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
|
|
Nope - GR doesn't explain the Pioneer anomaly. If it isn't some minor physical glitch (outgassing, solar wind, etc.), then it would mean that GR needs to be modified as well.
Jarad
--------------------
|
|
0 registered and 1 anonymous users are browsing this forum.
Moderator: llanitedave
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 265
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 