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fagricipni
2012-Oct-11, 09:10 PM
When looking at Earth from above its north pole, it appears to rotate counterclockwise (assuming that the observer is not rotating emself). Earth is also said to rotate west to east. (Thus causing the stars to appear to travel east to west to an Earth-bound observer.)

However, Venus is said to rotate retrograde.

Does this mean that the ray from the center of Venus to its north pole is in approximately the same direction as the ray from the center of Earth to its north pole, and thus that Venus appears to rotate clockwise to an observer above its north pole, or east to west?

Or does it mean that by definition planets orbit west to east, and the pole above which the planet appears to rotate counterclockwise is by definition the north pole, and thus that the ray from the center of Venus to its north pole is in approximately the opposite direction as the ray from the center of Earth to its north pole?

If Venus had a significantly shorter sidereal day than its sidereal year, then this question could easily be answered by asking whether whether the Sun rose in the east or west on Venus, but its sidereal day is longer than its sidereal year.

Though this suggests another way to ask the question: if Venus were merely to rotate faster (say, a sidereal day of 10 (Earth) days) but in the same direction as it currently does and nothing else changed, would the Sun rise in the east or the west on this Venus?

It just occurs to me that I have omitted a possibility -- I have assumed that feet downward and face northward, that west would be to the left and that east would be to the right -- that also doesn't have to be true, though I doubt that convention would be broken.

Jens
2012-Oct-12, 12:23 AM
It just occurs to me that I have omitted a possibility -- I have assumed that feet downward and face northward, that west would be to the left and that east would be to the right -- that also doesn't have to be true, though I doubt that convention would be broken.

Sorry, but that's simply a question of how you define "east" and "west". You can define east in two ways. One is "the direction that is to your right when you are facing north," and the other is, "the direction from which the sun rises." If you define it as (1), then by definition what you right is always true. If you define it as (2), then it would be opposite on a planet rotating the other direction.

And note that even for "north" and "south," you have a similar problem. One way of defining north is "the direction that is to your left when you're facing east," and another is "the direction that the N needle on a compass points." You might also ask magnetically which way is north on Venus (I'm not sure).

fagricipni
2012-Oct-12, 01:20 AM
You might also ask magnetically which way is north on Venus (I'm not sure).

That one I can answer: Venus has no magnetic field to speak of.

Jens
2012-Oct-12, 01:22 AM
That one I can answer: Venus has no magnetic field to speak of.

Right, thanks. And in that case, then, I think that the assignment of the words "north" and "south" is somewhat arbitrary.

Ivan Viehoff
2012-Oct-15, 02:35 PM
If you look at the diagram of Uranus at the sub-heading "magnetic field" here http://en.wikipedia.org/wiki/Uranus you will see it refers to
(1) The "North pole"
(2) The "rotational North pole" - which is the opposite to (1)
(3) The magnetic "North pole" - which is in the Southern hemisphere by (1)

Uranus has an axial tilt of 97 degrees, so, like Venus, it is in effect retrograde - looking down on the North pole at (1) it is rotating clockwise.

What makes (1) the North Pole is that it lies the same side of ecliptic as Earth's North Pole. In fact the axial tilt is measured relative to the plane of Uranus's orbit, which is 0.7 degs different from the ecliptic, but fortunately there is no planet with an axial tilt sufficiently close to 90degs and an orbital plane sufficiently different from the ecliptic that this is confusing. Though I believe that there are some planets with an orbital plane sufficiently different from the ecliptic that if they had Uranus's axial tilt it would be confusing, though this probably isn't true if you include all the smaller bodies, and you might have to make some arbitrary or local decisions.

What makes (2) the rotational north pole is that if you look down on it the planet rotates anticlockwise.

So my belief is that the colloquial North Pole is the pole the same side of the ecliptic as earth's North pole, but you can also refer to the rotational north pole. The north magnetic Pole on Uranus is in the southern hemisphere, as was also repeatedly true on Earth.

There are likely also some small bodies in the solar system with "tumbling" motions where it would be difficult to say objectively which end was north other than by arbitrary decision.