PDA

View Full Version : Tidal Lock: Earth - Moon == Sun - Earth?

Kinthalis
11-11-2003, 07:24 AM
I know that our moon is in tidal lock with our planet.

it therefore always has one side facing us (with a litle wobbling).

This is a pretty common thing since I believe most if not all planetary satellites in our solar system are tidal locked.

If so, I'm tempted to say that all orbiting bodies will eventually become tidal locked. Which would in turn lead me to believe that at some point in the future the earth would become tidal locked with the sun.

Is this true?

And for bonus points ;)

What is the relationship between weight/size orbital height and the time it takes for a body to become tidal locked?

How long did it take our moon to become locked?

CurtC
11-11-2003, 08:30 AM
The tidal effect is caused by (for example, let's take the Earth-Moon) the Earth's gravity being stronger on one side of the Moon than the other. And that's because near side of the Moon is significantly closer to the Earth than the far side.

With the Earth/Sun, there is much less of a tidal effect, because of the greater distance to the Sun. The side of the Earth where it's noon is 0.0086% closer to the Sun that the side where it's midnight, and this difference doesn't make for a very strong tidal effect.

So yes, there is a tendency to tidally lock the Earth, but it's so small that my gut feel tells me it will never happen in the five billion years we have left.

ftg
11-11-2003, 09:04 AM
Just to remind folks: tidal locks don't have to be 1:1. Mercury has a 3:2 tidal lock with the Sun. It will never go to a 1:1 lock as the 3:2 lock is sufficiently stable that no reasonable pertubation is going to bounce it out of that local minimum. It is likely that the Earth/Moon system will tidal lock with the Sun, but it will be a quite high ratio.

Kinthalis
11-11-2003, 09:05 AM
Hmmm, thanx for the responce CurtC.

So the tidal effect is proportionate to the distance and size of the object?

In which case, if the earth would be considerably larger and/or be closer to the sun the tidal effect would greater, correct?

Achernar
11-11-2003, 12:25 PM
The tidal force goes as:

MSUN REARTH / DS-E3

So yes, if the mass of the Sun were greater, or the size of the Earth were greater, it would have a bigger effect. But what really makes a difference is the Earth-Sun distance. That's why the Moon has more tidal influence on the Earth than the Sun does. Even though it's many orders of magnitude less massive, it's a good bit closer.

Freddy the Pig
11-11-2003, 01:34 PM
To expand on Achernar's point, we would reach "full tidal coupling" with the Moon long before reaching tidal coupling with the Sun. That is, the Earth and Moon would always present the same faces to each other. (Today the Moon always presents the same face to the Earth, but not vice versa.) I believe Pluto and Charon are mutually tidally coupled--each rotates in 6.4 days, and Charon orbits Pluto in 6.4 days.

However, the Sun will become a red giant and vaporize both the Earth and the Moon long before this happens.