## / Planetary motion

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Is this a fair description of it?

I'm hoping Coel will wade in on this one.
In reply to Tony Naylor: Not really, I mean everything revolves around the Earth for a start.
> ... everything revolves around the Earth for a start.

or Simon Cowell

or Simon Coel.

That raises two questions in my mind...

Why do the planets orbit on a track perpendicular to the direction of travel of the sun?

The sun appears to be moving aong a straight line, which I,m sure at the scale of the planets orbits is correct, but presumably the suns path is more complex when looked at in a grander scale.

I recall having a late night drunken conversation on this site about gravity. The discussion went along the lines of the fact that we were not revolving around the sun but falling towards it. My brain went pop over this.

But it makes more sense with this concept of the sun spiralling around something bigger than it, and we are being dragged along in its wake. Our perception is that the sun and celestial objects revolve around us. Our knowledge says that we revolve around the sun.

Surely its not so hard a leap to use computers to plot a path for the sun and how the planets would create little breadcrumb trails travelling around it?

You telling me that the sun moves in a straight line as well?
In reply to Tony Naylor: The video is basically rubbish. The problem comes in that it has assumed that there is one frame of reference that is correct, i.e. the sun must be moving at 70,000mph around the stationary centre of the galaxy and all the others are wrong.

However this is not the case, physics is invariant under changes in frame of reference. Think of a frame of reference like a coordinate grid, you can pick the orientation of the lines of that grid, and you can pick the reference point where the zero of the grid is defined (Greenwich for example), you can also pick how fast that grid is moving in any particular direction and physics still holds (you can even pick a non-flat grid, the latitude longitude grid for example is on a sphere).

The only thing that changes with differing frames of reference is that the paths look different, if you're on a train you perceive all the trees as moving (or not depending on how many engineering works are going on), if you're standing on the platform they look still. That's exactly the same as what's happening in this video, they've picked a different frame of reference so the paths look different.

Utter rubbish! Even the Druids, Geeks, Ancient Egyptians, Romans, Mayans, and Incas, knew without any communication or collusion between them, that the sun rose in the morning and went down in the evening. It rose in a diagonally opposed location (in the East) to where it set (in the West). How it got back from the East to the West was a bit of a mystery but as the Sun was a God it was entitled to be mysterious. It had probably got something to do with it sneaking back under cover of darkness.

The other proven fact was that the Sun needed sacrifices, sometimes human, to keep it happy to go on with this cycle. I say proven, because the blood sacrifices kept it going. As the priests assured them, any interruption of these sacrifices would have resulted in the End of the World, which would not have been a good thing.

QED
In reply to The Lemming: google kepler's laws. We are being pulled towards the sun by mutal gravitational attraction but we are kept from getting closer by the fact ths attraction is balanced by the outward force wamting to eject us into space. Imagine a weight on a string, your hand is the sun and the mass is the earth. The string is the gravitatinal attraction (centripetal force). If you spin the mss and were to cut the string the momentum of the mass is perpedicular to the centrepetal force.
In reply to highclimber: The outward force (centrifugal force) that you talk about wanting to eject us into space is really just an artefact of choosing an accelerating frame of reference and illustrates my point above quite well.

Think of it this way, if you're in a car travelling down the motorway and the car turns you feel yourself being thrown outwards (i.e. a centrifugal force) and that this is balanced when you are pushed back on by the car door. In this interpretation you've picked yourself being stationary as your frame of reference.

Another way of thinking about this is from the point of view of someone standing by the road. They will see you travelling straight in one direction, then the car door pushing on you as it turns so that you turn as well, no centrifugal force because the 'stationary' observer's frame of reference has not accelerated (changing direction counts as accelerating).

In fact a lot of the fundamental laws of physics are derived from considering these invariances under certain transformations. Conservation of momentum is a direct consequence of invariance under translations in space for example, so all of this changing frames is really built into the equations.
In reply to kestreslspl: centripetal force, i said centripetal force. The force you feel in a car goung around a bend is the centripetal force (inwards) being overcome by the lack of friction between you and your seat. Ergo centrifugal force doesnt really exist in the same manner as you have to set up complex coorinate systems to incorporate it.
In reply to highclimber: I know you said centripetal force, but you then went on to describe a centrifugal force (even if you didn't call it that) when you talked about the force that the centripetal force is balanced by that wants to throw the ball out into space.

The force you feel throwing you outwards when you turn is a centrifugal force and it is an artefact of setting up the 'complex' (although from the point of view of the person in the car most relevant) frame of reference, but it does 'exist'.

If you restrict yourself to inertial reference frames then you can inded get rid of centrifugal forces, but just because accelerating frames are complicated to deal with doesn't mean that the results that you get in them are not valid, as I was saying above there is no privileged frame, they're all equally valid.
In reply to Tony Naylor: i'm more concerned about the supermassive black hole at the centre of our ultraworld..
'The widening gyre'

No. Not a fair description at all.

Movement is space is relative to other objects. The sun is orbiting around the center of our galaxy. Galaxies are moving away from each other but as there isn't a single known fixed point in space that isn't moving anywhere we can't accurately state the velocity of any object. The best we can do is to calculate a velocity relative to another object. As this object,and all others, are also moving you can chose an arbitrary point and make any object appear to move in a straight line simply by moving the arbitrary point in a straight line.

Silly question.

How come gravity makes things move towards each other, yet galaxies move away from each other?
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> Silly question.
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> How come gravity makes things move towards each other, yet galaxies move away from each other?

Sensible question, I'd say. Space is still expanding from the Big Bang. Best analogy I've heard is of a currant cake in the oven. The cake is the universe, the currants are galaxies. As the cake bakes, it expands and the galaxies move away from each other.

Re other peoples points about frames of reference, thank you, I hadn't thought of that. But if you took the galactic centre as a reference point (arbitrary, I know) am I right in thinking that would give the appearance of the helical motion described in the video?
In reply to kestreslspl and highclimber:

http://www.xkcd.com/123/
In reply to The Lemming: Not a silly question at all.

The universe has been expanding since the big bang, so on a large scale things are getting further away from each other, however as you say gravity attracts things to each other, the question is whether the attraction of gravity is greater than the expansion of the universe. In the case of things like our solar system (quite small on cosmological scales) the gravitational attraction dominates so the planets orbit around the sun. In the case of most galaxies which are much further away from each other (gravity falls off as the inverse square of distance so is much less dominant) the expansion of the universe dominates so they get further apart.

We're still not sure if overall the gravitational attraction will win out over the expansion in the end ( see http://en.wikipedia.org/wiki/Fate_of_the_universe#Role_of_the_shape_of_the_universe), but one of the great unanswered questions in physics is why the expansion of the universe is much faster than we would naively expect, the so called 'dark energy' problem.
In reply to GraemeDiack: I do love xkcd
In reply to Tony Naylor: You are right that if you took the galactic centre as a point of reference (in fact any point of reference where the sun was moving in a straightish line) would give the appearance of helical motion. Where the video goes into cloud cuckoo land is where it tries to attach some great significance to this fact.
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> Silly question.
>
> How come gravity makes things move towards each other, yet galaxies move away from each other?

Not a silly question at all, try listening to some of the Dr Karl or Naked Scientist podcasts, really good at explaining some of these things in an easy to understand (for me) way.
> (In reply to Tony Naylor) You are right that if you took the galactic centre as a point of reference (in fact any point of reference where the sun was moving in a straightish line) would give the appearance of helical motion. Where the video goes into cloud cuckoo land is where it tries to attach some great significance to this fact.

Cheers, I thought I'd grasped it right. Re the rest of the video spiralling off into over-generalisation, I took that with a pinch of salt. It is sort of interesting, though, that you get spiral patterns a lot in nature.

In reply to cfer: The galaxies aren't moving away from each other per se. the space between them is expanding. if you draw some dots on a balloon and blow it up you get a good analogue for whats happening (though you have to disregard the fact that the dots get bigger as this isn't what's happening).
It's all just semantics though, the distance (I know you can define distance measures which don't increase, but they're quite different from the normal concept of distance) between them has increased so they could be said to be moving apart.

What's important is, as you say, the reason for this, which is the universe expanding. Once you know that most people ask why aren't the planets in the solar system getting further away from each other, and the reason for this is that gravity dominates over the expansion of the universe at short distances (because of the to a first approximation 1/distance squared dependence), so we only really see the effects of gravity in our solar system.

The other interesting thing which was mentioned in the wikipedia link I posted is that the expansion of the universe isn't just carrying on irrespective of what's inside the universe. The gravitational forces acting to pull things back together slow the rate of expansion, so the balance between gravity and the expansion is really what people are trying to work out when they study the eventual fate of the universe.
The planets are (close to) fairly boring Keplerian motion - i.e. most of the mass is in the Sun, at the centre, with only very weak tidal forces and roughly circular/elliptical orbits. The tidal forces on the Solar system from things outside it can be ignored for these purposes as they are very small.

What is quite fun is the path the Sun takes around the Galaxy. The Galaxy is not a Keplerian system - the mass is not concentrated at the centre, and the distribution is not spherically symmetric. This means that there is no reason why the Sun's orbit should be circular or elliptical...

Instead the Sun, while mostly still going around the centre every 200 million years or so in the plane of the galaxy, its orbit will be distorted by the spiral arms where there are local concentrations of mass. It is also bouncing up and down through the disk of the galaxy on an epicyclic frequency, because as you go above the disk the force of gravity from the disk below you pulls you back down into the disk, and then you overshoot and go out the other side of the disk.

Physics!

Equally silly answer. Flip a coin. For the period of time that it is going up gravity isn't making it move towards the earth .Initially it will accelerate upwards, slow, stop and then start to accelerate downwards.

Maybe jesus is pushing the galaxies away from each other.. (oops wrong thread there)
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> Instead the Sun, while mostly still going around the centre every 200 million years or so in the plane of the galaxy, its orbit will be distorted by the spiral arms where there are local concentrations of mass. It is also bouncing up and down through the disk of the galaxy on an epicyclic frequency, because as you go above the disk the force of gravity from the disk below you pulls you back down into the disk, and then you overshoot and go out the other side of the disk.
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> Physics!

So that's why I feel permanently travel sick!

If each and everyone of us is the centre of the universe...
any point can be reference point to relate motion to?!?!
Eeek, too heavy now, I'm out!
> (In reply to The Lemming)
> [...]
But if you took the galactic centre as a reference point (arbitrary, I know) am I right in thinking that would give the appearance of the helical motion described in the video?

Again the answer would be no. The sun is orbiting around the centre of the galaxy (about 26,000 light years away) and we complete 1 orbit every 200-250 million years. That means we are moving relative to an assumed fixed centre at 250 km per second.

The orbital plane of our own galaxy is tilted at about 60 degrees to our own planets orbital plane so the apparent planetary orbit of the earth in relation to the galactic centre is more like motion of a point on the earth in relation to the sun (but tilted more)

> (In reply to The Lemming)
>
> Equally silly answer. Flip a coin. For the period of time that it is going up gravity isn't making it move towards the earth .Initially it will accelerate upwards...

No, once it leaves your hand the only force (assuming no air resistance) acting on the coin is the downward force of gravity. Hence it will accelerate in the direction of this force i.e. the magnitude of the initial upwards velocity will decrease at a rate of 9.8 m/(s^2) (g) until it reaches 0. Then it will fall with an acceleration of g i.e. equal to the magnitude of deceleration it experienced on the ascent.

Once it has left your hand it will not 'accelerate upwards'.

In reply to Tony Naylor: now take that vid make it bigger and include the comets that seem to visit on a regular cycle, its all moving and still they meet us after encountering all the rest of the milky ways gravitational pulls