# Tidal Lock: Earth - Moon == Sun - Earth? **URL:** \ **Category:** Factual Questions\ **Created:** [November 11, 2003, 1:24pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603 "2003-11-11T13:24:46Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Kinthalis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kinthalis/32/16084_2.png) [@Kinthalis](https://boards.straightdope.com/u/Kinthalis)\ **Post date:** [November 11, 2003, 1:24pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603/1 "2003-11-11T13:24:46Z")
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? ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [November 11, 2003, 2:30pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603/2 "2003-11-11T14:30:53Z")
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. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [November 11, 2003, 3:04pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603/3 "2003-11-11T15:04:24Z")
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. ---
**Author:** ![Kinthalis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kinthalis/32/16084_2.png) [@Kinthalis](https://boards.straightdope.com/u/Kinthalis)\ **Post date:** [November 11, 2003, 3:05pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603/4 "2003-11-11T15:05:14Z")
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? ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [November 11, 2003, 6:25pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603/5 "2003-11-11T18:25:48Z")
The tidal force goes as: M[sub]SUN[/sub] R[sub]EARTH[/sub] / D[sub]S-E[/sub][sup]3[/sup] 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. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [November 11, 2003, 7:34pm UTC](https://boards.straightdope.com/t/tidal-lock-earth-moon-sun-earth/212603/6 "2003-11-11T19:34:38Z")
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.