# I How to define the rotation of a planet in retograde orbit?

1. Jul 6, 2016

### Alltimegreat1

If a planet is in retrograde orbit, what direction would it rotate to also have retrograde rotation?

2. Jul 6, 2016

### Bujon

If it is orbiting anticlockwise around an object which is rotating clockwise, then assuming by retrograde rotation in relation to the motion around that central object, I would suggest it too would rotate clockwise.

Last edited: Jul 6, 2016
3. Jul 6, 2016

### Beduino

Actually this is a bit tricky question because it depends on the observer, if you are above the "north" pole looking below, or below the south pole looking above, but I suppose it would the reverse or negative of whatever you consider prograde rotation.

4. Jul 6, 2016

### Bujon

Assuming the OP might be assuming the axis of rotation of the planet is in the same plane as that of the central object? And that the perspective is from a relative viewpoint to both.

5. Jul 6, 2016

### Alltimegreat1

The sun rotates counter-clockwise as viewed from above looking down at its north pole. Let's assume one of its planets orbits the sun clockwise (retrograde) and also rotates clockwise. Is this planet's rotation prograde or retrograde?

6. Jul 6, 2016

### Bujon

I would say its rotation would be retrograde in relation to the rotation of the sun, but prograde in relation to its own orbital rotation of the sun

7. Jul 6, 2016

### Beduino

Actually lets compare with the Earth, from above north pole it rotates counter clock wise around itself and around the sun. So the retrograde is the opposite of both.

8. Jul 6, 2016

9. Jul 6, 2016

### Alltimegreat1

So does that mean that retrograde/prograde rotation refers to the object's rotation compared to its own orbital direction, regardless of whether that orbit is retrograde or prograde?

10. Jul 6, 2016

### Beduino

See it depends on several things, is Earth your reference frame? How do you define the north pole of a planet? so on...
Also this notion of prograde or retrograde around itself gets really confusing if the planet axis inclination goes to 90 degrees compared with everything else, but again retrograde is going to be the negative of whatever you have considered prograde.

11. Jul 6, 2016

### Bujon

I could be wrong, but my common sense approach is you have to define with what you are relating the observation. You could be retrograde to the spinning rotation of an object, but prograde to your own orbital path around that object, irrespective of the central objects spin?

12. Jul 6, 2016

### Alltimegreat1

Yes it is a bit confusing, but we should try to get to the bottom of this. To make things easier, let's assume all objects in the solar system have no axial tilt. If our Earth, which has both prograde orbit and rotation, suddenly started orbiting in the opposite direction but retained its current rotational direction, it would have a retrograde orbit but a rotation that is prograde compared to the sun but retrograde compared to its own orbit.

13. Jul 6, 2016

### Alltimegreat1

14. Jul 6, 2016

### Bujon

Again it brings you back to relativity in the sense of where you are looking and from what you are measuring against.