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.