Venusian Rotation: Retrograde & Axial Tilt 178° Explained

[DaveC426913]

This came up in https://www.physicsforums.com/showthread.php?t=119891", and now I'm not sure of my answers.

1] Venus' rotation is retrograde. Is that taking into account its revolution about the Sun, or is that still true from and external point-of-view?

i.e.: If the Sun were suddenly whisked out of existence, and Venus were left adrift on a straight path, it would continue to rotate slowly on its axis:
a] clockwise
b] counterclockwise


2] Venus' axial tilt is ~178 degrees. this is because

a] Axial tilt is always defined with North magnetic pole being at the top. Since on Venus, the South magnetic pole is at the top, the axial tilt is measured as upside down (i.e. ~180 from upright).

b] Axial tilt is always defined with North rotating counterclockwise. Because Venus is retrograde, its "bottom" pole is actually the one rotating counterclockwise, and thus is considered the North pole, which means Venus is upside down (i.e. ~180 from upright).


Here's the thing about 2b. If true, you can't say Venus is BOTH retrograde AND upside down; it can be considered one OR the other, but to count both is basically double-dipping.

 

[SpaceTiger]

I only have a few minutes, so I'll try to think about your other questions later, but here's my basic reasoning. Try drawing out the orbit of venus as seen from above the solar system's north pole. It will be going counter-clockwise around this orbit as you said. Now, supposing that the period of rotation is very close to the period of revolution, which direction would it have to rotate in order to have a very long (much longer than its rotation period) solar day? I believe the answer is that it would have to rotate counter-clockwise, the same as the direction of revolution. To see this, try moving venus about its orbit without rotating it. What direction would it have to turn to keep one point on the surface always pointing to the sun?

Anyway, we know that its solar day is not much longer than its sidereal period -- in fact, it's shorter. This suggests that it really is rotating clockwise and that both effects, the motion about the sun and about its axis, are acting to move the sun across the sky from west to east as seen from an observer on Venus.

 

[Janus]

From this site we see that Venus' sidereal period of rotation is -243 solar days which makes it retrograde.

http://www.nineplanets.org/data1.html

 

[Tide]

Does Venus even have a magnetic field?

 

[SpaceTiger]

Does Venus even have a magnetic field?

Not a measurable one, it seems. As you're probably implying, a planet's magnetic field doesn't determine whether or not its rotation is retrograde. My understanding is that its rotation is always compared to one direction perpendicular to the plane of the ecliptic. This should yield the following answers to Dave's questions:

1) a (clockwise)
2) b (no consideration of magnetic field)

In this context, "north" defined by rotation and has no terminological connection to the magnetic pole (which need not align with the rotation axis).

Here's the thing about 2b. If true, you can't say Venus is BOTH retrograde AND upside down; it can be considered one OR the other, but to count both is basically double-dipping.

If we're using an external reference to define "retrograde", I don't see why it can't have a ~180 degree axial tilt and a retrograde rotation. It is true, I believe, that the statements are redundant and "retrograde or prograde" can be determined directly from the axial tilt.

 

[DaveC426913]

... supposing that the period of rotation is very close to the period of revolution, which direction would it have to rotate in order to have a very long (much longer than its rotation period) solar day? I believe the answer is that it would have to rotate counter-clockwise, the same as the direction of revolution.

: slaps forehead : Of course! It HAS to be prograde for the day to match up with the year! What was deducible with the facts I already knew!

thus, it is only retrograde wrt the Sun.

Thanks!

 

[DaveC426913]

This should yield the following answers to Dave's questions:

1) a (clockwise)

No, it would be counter-clockwise. As we just determined, Venus' rotation about its axis from a PoV external to the SS is still CCW. If the Sun were whisked away, it would continue in a straight line, and contimue its slow CCW rotation.

 

[DaveC426913]

If we're using an external reference to define "retrograde", I don't see why it can't have a ~180 degree axial tilt and a retrograde rotation. It is true, I believe, that the statements are redundant and "retrograde or prograde" can be determined directly from the axial tilt.

Why do they list its axis as ~178 degrees? Why wouldn't they say ~2 degrees?

 

[SpaceTiger]

: slaps forehead : Of course! It HAS to be prograde for the day to match up with the year! What was deducible with the facts I already knew!

thus, it is only retrograde wrt the Sun.

Perhaps my wording was poor. The part of my post you quoted was followed soon by:

Anyway, we know that its solar day is not much longer than its sidereal period-- in fact, it's shorter. This suggests that it really is rotating clockwise...

We're looking down on the solar system from a direction above the plane of the solar system -- that is, an external reference frame. From this vantage point, the planets revolve counterclockwise around the sun and Venus rotates clockwise about its axis. Removing the sun would do nothing to the direction of Venus' rotation.

Why do they list its axis as ~178 degrees? Why wouldn't they say ~2 degrees?

Because Venus is rotating in the opposite direction of the other planets. A rotation that was perfectly prograde (or, prograde and aligned with the ecliptic) would have axial tilt 0 degrees and one that was exactly retrograde would have 180 degrees. The two things are redundant, not interdependent -- that is, a retrograde planet with ~180 degrees axial tilt is not the same as a prograde planet with ~0 degrees axial tilt.