Uranus Retrograde Rotation

[Sigma88]

Why is that Uranus is considered having a 97° tilt and retrograde rotation?

couldn't it be that the planet has a 83° tilt and prograde rotation?

Is there a reason why they chose the North pole to be that one and not the other one?

I feel like I'm missing something, but I can't find any explanation

[Starwhip]

Beeeecause the North pole is the north end of the magnetic field, perhaps?

We've flown by Uranus, I'm sure we conducted some experiments.

[Bill Phil]

I think its thd magnetic poles...

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Beeeecause the North pole is the north end of the magnetic field, perhaps?

We've flown by Uranus, I'm sure we conducted some experiments.

Our geographic north is near the magnetic south...

[Camacha]

The magnetic poles, it seems, are messed up in their own ways.

Uranus is the tilted planet. The axis through its North and South Poles is tilted by 98°. In other words, Uranus is lying on its side! That's not all - the magnetic field of Uranus is tilted, too. The planet's magnetic field is tilted 59°. A compass wouldn't be much help in finding North on Uranus!

[Sigma88]

I think its thd magnetic poles...

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Our geographic north is near the magnetic south...

well, magnetic poles around earth have switched back and forth many times, I doubt that would be considered a good reason (but what do I know? ;D )

I can understand why venus is considered having a retrograde rotation, but for uranus to me seems weird

[Kryten]

The north pole is the one which the planet rotates counterclockwise relative to, the same as Earth's north pole.

[Sigma88]

The magnetic poles, it seems, are messed up in their own ways.

exaclty what I meant.

Also, magnetic poles can have weird angles between one another. geographic poles are always 180° apart

[ZetaX]

Why is that Uranus is considered having a 97° tilt and retrograde rotation?

couldn't it be that the planet has a 83° tilt and prograde rotation?

Unlike some here claimed this has nothing to do with magnetic poles. See http://en.wikipedia.org/wiki/Axial_tilt. We are simply measuring the tilt in regard to the direction of orientation (i.e. prograde; you also mixed that one up)

[Sigma88]

The north pole is the one which the planet rotates counterclockwise relative to, the same as Earth's north pole.

so why is it considered retrograde?

[Kryten]

Because it's clockwise seen from above the ecliptic, i.e. the north pole of the solar system as a whole.

[Sigma88]

Unlike some here claimed this has nothing to do with magnetic poles. See http://en.wikipedia.org/wiki/Axial_tilt. We are simply measuring the tilt in regard to the direction of orientation (i.e. prograde; you also mixed that one up)

ok, 97° is fine, I've got it.

so its considered retrograde because it's like a ball rolling in one direction while actually moving in the other?

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Because it's clockwise seen from above the ecliptic, i.e. the north pole of the solar system as a whole.

this is what confuses me, how do you define clockwise on a thing which has it's axis on the same plane of the exliptic (more or less)

seems to me than in some positions of the orbit uranus will rotate against its revolution and in the opposite side of the orbit it will rotate in the same direction it revolves.

here's a picture

in 1986 it was spinning in the same direction of its revolution (or at least this is how it looks to me)

Edited April 7, 2015 by Sigma88

[Kryten]

'More or less' is important. It would only be impossible if it was exactly on the plane.

[Sigma88]

'More or less' is important. It would only be impossible if it was exactly on the plane.

Ah, so since the n pole is slighly under that means retrograde.

Got it.

It still feels wrong to be honest

[NovaSilisko]

Meanwhile, in an alternate universe...

Why is that Uranus is considered having a 83° tilt and prograde rotation?

couldn't it be that the planet has a 97° tilt and retrograde rotation?

[Sigma88]

Meanwhile, in an alternate universe...

Ä™

I will answer when i get my hands on a pc, my phone sucks

[ZetaX]

To clarify further what that angle actually measures:

There is a unique direction A from which, if looking at the planet, we look directly at the pole and such that it is rotating counterclockwise (from the opposite side, it would rotate clockwise).

Similiarly, there is a unique direction B from which the planet orbits counterclockwise around the sun (more accurately, we take this one to be orthogonal to the plane of ecliptic).

Now the tilt is the angle between A and B.

Thus the tilt is any angle from 0° to 180°.

As a special case: if the axis lies in plane of ecliptic, that angle is 90°. We don't need to distinguish between pro- and retrograde in that case and/or it would make no sense.

Edited April 8, 2015 by ZetaX

[pincushionman]

It seems to me the statement "has a retrograde rotation and a 97 degree tilt" is somewhat inaccurate, and instead should be "has a retrograde rotation because it has a 97 degree tilt." Subtle difference, but huge in meaning.

[Yasmy]

this is what confuses me, how do you define clockwise on a thing which has it's axis on the same plane of the exliptic (more or less)

Clockwise assumes you have defined some oriented reference plane, for example, the direction pointing away from a wall clock. If the wall and clock are transparent, then looking from "behind" the wall, the hands are moving counterclockwise. Normal clocks only go clockwise by convention of looking at them from the front.

The same goes for planets.

1) Find the axis of rotation. (Nothing to do with magnetic fields. Look at the rotation of the body).

2) You can look at the planet from either end of the spin axis. Looking back along one pole, the planet rotates clockwise. Looking back along the opposite pole, the planet rotates counterclockwise. Clockwise is a matter of perspective.

3) The oriented reference plane mentioned above is the plane through the equator of the planet, and by convention, the up direction (the orientation) from the equatorial plane is the direction along the spin axis from which, if you looked back down at the planet, the planet rotates counterclockwise. For the Earth and the Sun and most planets, we call that direction North. Not magnetic north, but true north.

4) If the angle between a planet's north spin axis is within 90 degrees of its north orbital axis, then the planet orbits and rotates in the same sense. If you looked at Uranus from north of the ecliptic, the surface would appear to (very modestly) rotate counter to the direction of its orbit.

[Sigma88]

Thank you everybody, I think it's clear now why uranus has 97 tilt and why it rotates retrograde

Even tho I understand it now, I think this can be misleading, and honestly I don't like it very much.

Basically you have 2 "variables" that are redundant ( tilt>90 equals retrograde rotation and vice versa)

And you lose the information about how much the axis got actully tilted overtime.

To explain:

When you say earth has a 23 degrees tilt most people will imagine the axis being straight some time in fhe past and then it got tilted progressively untill it reached a 23 degrees angle.

And this is consolidated when you explain the axial precession comparing earth to a spinning top.

By this logic, if you say venus has a 177 degrees tilt people will imagine the planet having flipped upside down between the time it was born and now. And if my knowledge about venus is correct, this is not the case.

To me it would look more normal to define the N pole as "the original pole around which the (proto)planet rotated counterclockwise" and defining the tilt as the "inclination between that axis (in its current position) and the ecliptic"

The rotation would be prograde if the planet is still rotating counterclockwise around the N pole, retrograde if clockwise.

With this rule, venus would be a planet with a 3 degrees tilt and retrograde rotation

(the most common theory is that it was rotating prograde, than slowed and turned retrograde)

And uranus would be a planet with 97 degree tilt and prograde rotation

Either because the planet formed from a weird cloud that was rotating that way already or because it was knocked around by an impact.

Anyway since all the major moons are orbiting prograde to the planet I think it could mean that at some point there was an accretion disc rotating that way.

Maybe there's something wrong in this method, I'm no astronomer/physicist.

But to me looks more intuitive

Edited April 8, 2015 by Sigma88

[Darnok]

Why is that Uranus is considered having a 97° tilt and retrograde rotation?

couldn't it be that the planet has a 83° tilt and prograde rotation?

Is there a reason why they chose the North pole to be that one and not the other one?

I feel like I'm missing something, but I can't find any explanation

From what I read about Earth its tilt is changing over time, so maybe Uranus is still in prograde rotation just its tilt is changing faster.

[Perry Apsis]

It still feels wrong to be honest

I know, but you should be honest anyway. You get used to it, eventually.

Commas are your friend. :-)

[YNM]

Axial tilt are measured from the angle between normal vector of ecliptic plane and the rotating body angular momentum vector. A 97 degree tilt means it's more than just being flat to the ecliptic plane, it means that it's a tad retrograde (south pole on the north side / hemisphere of ecliptic) - mind that wrt the equator of the planet the planet is always rotating prograde (angular momentum vector always points to / matches the rotational north pole, in other words the normal vector of the equator).

[Sigma88]

Axial tilt are measured from the angle between normal vector of ecliptic plane and the rotating body angular momentum vector. A 97 degree tilt means it's more than just being flat to the ecliptic plane, it means that it's a tad retrograde (south pole on the north side / hemisphere of ecliptic) - mind that wrt the equator of the planet the planet is always rotating prograde (angular momentum vector always points to / matches the rotational north pole, in other words the normal vector of the equator).

I still think it's a bad way to define it.

well, not necessary bad. But it can lead to confusion

[YNM]

Probably due to my less competent english skills...

Here's some images for you, hope it helps :

[Sigma88]

Probably due to my less competent english skills...

Here's some images for you, hope it helps :

https://dl.dropboxusercontent.com/s/j3miixke548vx6o/Uranus%2C%20prograde%20-%20retrograde.png?dl=0

I wasn't implying you explanation was bad, all I'm saying is that this method can lead to confusion.

and when I say confusion, I mean in the general public, not for those who study this stuff

let me give you an example.

If you say earth axial tilt is 23°, this kinda gives me the idea of a "movement" of the axis from the vertical position to the 23° tilt

if earth was to stop rotating and then started rotating backwards (some say venus did just that) suddently you will say that earth axial tilt is 203°

which kinda makes me go "whoa, when did we turn upsidedown?"

same goes for uranus, let's say the axial tilt of uranus went from 83 to 97 overtime

this would have been the situation:

83<tilt<90 rotating prograde

tilti = 90 rotating, neither prograde nor retrograde (...?)

90<tilt<97 now we are rotating retrograde

this can give the wrong idea that uranus inverted it's rotation, when actually it's just (what I consider) a bad reference frame.