Astronomy: Orbit Terminology

[Hornbein]

The trouble is if you look at the earth from above the north pole then counterclockwise, from above the south pole clockwise. Same with the solar system.

 

[Bandersnatch]

The trouble is if you look at the earth from above the north pole then counterclockwise, from above the south pole clockwise. Same with the solar system.

Yet, the right-hand rule provides an unambiguous direction towards the rotational north pole. This isn't as much of a problem as you make it out to be.

 

[Hornbein]

Yet, the right-hand rule provides an unambiguous direction towards the rotational north pole. This isn't as much of a problem as you make it out to be.

But then all heavenly bodies rotate counterclockwise. Is that useful?

 

[Bandersnatch]

But then all heavenly bodies rotate counterclockwise. Is that useful?

Why wouldn't it be? The direction of the pseudovector in whatever reference frame we pick provides all the information about the direction of the rotation, and one is still free to describe rotation of bodies in relative terms using words like clockwise/anticlockwise.

The point is to remove ambiguity from statements like 'Venus rotates clockwise when viewed from the North pole of the invariable plane of the solar system'.

And in any case, if it's good enough for the IAU, it's maybe good enough for us.

 

[Hornbein]

The point is to remove ambiguity from statements like 'Venus rotates clockwise when viewed from the North pole of the invariable plane of the solar system'.

All you can do is compare the rotational or orbital planes of two heavenly bodies and conclude that their rotational direction is either the same or differs. In the case of Uranus this does not work, as the planes are perpendicular.

 

[Bandersnatch]

All you can do is compare the rotational or orbital planes of two heavenly bodies and conclude that their rotational direction is either the same or differs.

I'm not sure why you're framing this as an objection. Yes, that's what you do, and that's what makes it unambiguous.
Wasn't the perceived ambiguity in picking the reference 'north' direction from which to judge the rotation the point you initially took issue with? We may be talking at cross purposes here.

As for Uranus. It's plane of rotation is not exactly perpendicular to the invariant plane, or any other obvious reference frame you may pick, be it solar equatorial plane, or the ecliptic. The few degrees of deviation from perpendicularity lets you say that when its rotation vector points below the plane, i.e. towards the southern hemisphere of the reference - that's clockwise rotation. Above that plane - anticlockwise.

 

[sophiecentaur]

how is the prograde or retrograde direction determined if the body in question is not rotating?

IMO this thread has taken us down a rabbit hole. The apparent magic of the retrograde orbit of Mars was a huge stumbling block for the Geocentric Universe model. Several hundred years ago, the Heliocentric model solved the 'anomaly. The term "retrograde" its really pretty irrelevant now. because celestial objects go as you'd expect and they are (afaik) consistent.
It does have a place when discussing some situations, though. When placing a craft in lunar orbit, it is less costly in energy if you choose a 'figure of eight' with retrograde lunar path because the net orbital energy change is a lot less than if you went for a prograde lunar orbit. You would need a huge retro thrust to get the ship to slow down enough to be captured into a lunar orbit of the same sense as its orbit round the Earth (which is there all the time when hooked on to the Moon).

 

[sophiecentaur]

how is the prograde or retrograde direction determined if the body in question is not rotating?

IMO this thread has taken us down a rabbit hole. The apparent magic of the retrograde orbit of Mars was a huge stumbling block for the Geocentric Universe model. Several hundred years ago, the Heliocentric model solved the 'anomaly. The term "retrograde" its really pretty irrelevant now.
It does have a place when discussing some situations, though. When placing a craft in lunar orbit, it is less costly in energy if you choose a 'figure of eight' path because the net orbital energy change is a lot less. You would need a huge retro thrust to get the ship to slow down enough to be captured into a lunar orbit of the same sense as its orbit round the Earth (which is there all the time when hooked on to the Moon).
View attachment 371084

 

[Bandersnatch]

The apparent magic of the retrograde orbit of Mars was...

You're thinking of 'apparent retrograde motion', on the celestial sphere. Where this thread talks about retrograde, it's in the context of directions of orbits. A different thing.

 

[sophiecentaur]

You're thinking of 'apparent retrograde motion', on the celestial sphere. Where this thread talks about retrograde, it's in the context of directions of orbits. A different thing.

I take your point but why would there her any significance in this? In an early solar system there would have been a mix of small objects with an assortment of angular momenta; only a small majority being one way. Collisions and partial captures would end up producing a mature solar system with most large objects with prograde orbits.

The same set of interactions would produce a disc shaped system with an angular momentum vector more or less the same a that of the original cloud.

 

[Bandersnatch]

I take your point but why would there her any significance in this?

I suppose the significance could manifest when you're in a spaceship and Houston tells you to rotate the craft for a retrograde burn. It might be nice to know what they're talking about.
And in any case, it's the context the OP asked about.

 

[sophiecentaur]

rotate the craft for a retrograde burn.

Maybe but most navigation instructions tend to be numerical - eg Heading and thrust. "Rotating the craft " would not be sufficient and they have to get it right. But I have never driven one of those things so don't listen to me.
Edit: Then again; if you were in a polar orbit, which would ground control tell you to do? It's like the old problem when people insist on using acceleration and deceleration when the variable acceleration would have a sign. It's hard to do calculations with signless variables.