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View Full Version : Venus rotation vs gas giant moons



Tom Mazanec
2014-Feb-02, 02:57 AM
Venus points one side towards Earth at closest approach to our planet, and does not have synchronous rotation as a result.
So why don't gas giant moons have one side point to the next moon out as they make closest approach to them, preventing them from having their actual synchronous rotation?

Noclevername
2014-Feb-02, 09:44 AM
Venus points one side towards Earth at closest approach to our planet, and does not have synchronous rotation as a result.
So why don't gas giant moons have one side point to the next moon out as they make closest approach to them, preventing them from having their actual synchronous rotation?

Gas giant moons are deeper in their planets' gravity wells than Venus is to the Sun's gravity. They get a much stronger tide, to lock their rotation face-on relative to their primary. They have less "wiggle room" as a result.

grapes
2014-Feb-02, 11:24 AM
And, I think the matchup between Earth and Venus is close, but not a resonance.

Wiki cites a paper from 1979 ("the hypothesis of a spin-orbit resonance with Earth has been discounted "), and also says Venus's rotation has slowed by 6.5 minutes in the last 16 years.
http://en.m.wikipedia.org/wiki/Venus

That's pretty fast slowing, at that rate it'll come to a complete stop in 861,630 years. :)

Jeff Root
2014-Feb-02, 09:09 PM
I didn't see any reason given in the Wikipedia article for
the stupendously humongous change of 6.5 minutes in the
length of Venus's day in just 16 Earth years. It seems
impossible. Even a change of 6.5 seconds would be very
surprising. I've seen this figure before. Is it reliable?
If so, does it have a strong connection with the fact that
Venus's rotation is retrograde?

Is it possible that this was reported incorrectly, and the
6.5 minutes is instead actually the cumulative change in
time of day over 16 years, due to a change in the length
of the day of only a few seconds?

-- Jeff, in Minneapolis

Hornblower
2014-Feb-02, 10:45 PM
I didn't see any reason given in the Wikipedia article for
the stupendously humongous change of 6.5 minutes in the
length of Venus's day in just 16 Earth years. It seems
impossible. Even a change of 6.5 seconds would be very
surprising. I've seen this figure before. Is it reliable?
If so, does it have a strong connection with the fact that
Venus's rotation is retrograde?

Is it possible that this was reported incorrectly, and the
6.5 minutes is instead actually the cumulative change in
time of day over 16 years, due to a change in the length
of the day of only a few seconds?

-- Jeff, in Minneapolis
My bold. I think that is correct. I did some number crunching when that came up some time ago, and the numbers added up.

Jeff Root
2014-Feb-02, 11:43 PM
Ah, I recall now some discussion here about the possibility
that Venus's winds might be the cause, since its atmosphere
is so massive.

-- Jeff, in Minneapolis