# Why does the Earth spin? **URL:** \ **Category:** Factual Questions\ **Created:** [December 1, 2000, 3:00pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578 "2000-12-01T15:00:50Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![furnishesq](https://avatars.discourse-cdn.com/v4/letter/f/977dab/32.png) [@furnishesq](https://boards.straightdope.com/u/furnishesq)\ **Post date:** [December 1, 2000, 3:00pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/1 "2000-12-01T15:00:50Z")
I know that the Earth (as well as the other planets) revolves around the Sun due to gravity. However, why does the Earth spin (the reason why we have day and night and not a permanent dark side and light side). On the same token the moon does not spin, it simply revolves with the Earth (and thus a permanent dark side of the moon). Why is this? ---
**Author:** ![pldennison](https://avatars.discourse-cdn.com/v4/letter/p/9fc29f/32.png) [@pldennison](https://boards.straightdope.com/u/pldennison)\ **Post date:** [December 1, 2000, 3:39pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/2 "2000-12-01T15:39:02Z")
> [@](#): > > \*Originally posted by furnishesq \* > **I know that the Earth (as well as the other planets) revolves around the Sun due to gravity. However, why does the Earth spin (the reason why we have day and night and not a permanent dark side and light side).** Because the Earth, and the other planets, condensed from material in an accretion disk around the sun, which was itself spinning. Conservation of energy and angular momentum tells us the rest of the story. \*\* > [@](#): > > On the same token the moon does not spin, it simply revolves with the Earth (and thus a permanent dark side of the moon). > \*\* This is not true. The same side always faces the earth due to its synchronous rotation, but the whole thing gets illuminated during it’s orbital cycle. During New Moon, the sun is illuminating the face we never get to see from Earth. See [http://www.seds.org/nineplanets/nineplanets/luna.html](http://www.seds.org/nineplanets/nineplanets/luna.html) . ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [December 1, 2000, 5:19pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/3 "2000-12-01T17:19:08Z")
Pldennison is correct on all accounts. When stuff forms in space, it usually does so while _spinning_. How’s that for a concise answer? I have also heard people ask, “Why does the Earth _continue_ to spin?” or “What force _keeps_ the Earth spinning?” This can be answered with the following question: “What is _stopping_ the Earth from spinning?” Answer: Nothing. Therefore, it continues to spin. Note that this is not really true; due to cosmic dust and other stuff, the Earth _is_ slowing down. Slowly. ---
**Author:** ![Uniball](https://avatars.discourse-cdn.com/v4/letter/u/e5b9ba/32.png) [@Uniball](https://boards.straightdope.com/u/Uniball)\ **Post date:** [December 1, 2000, 5:22pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/4 "2000-12-01T17:22:15Z")
Earth doesn’t spin, silly! If it did, we would all be dizzy by now! ---
**Author:** ![Stimpy](https://avatars.discourse-cdn.com/v4/letter/s/779978/32.png) [@Stimpy](https://boards.straightdope.com/u/Stimpy)\ **Post date:** [December 1, 2000, 5:22pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/5 "2000-12-01T17:22:24Z")
what year will it stop? do i need to move house? 🙂 ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [December 1, 2000, 5:27pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/6 "2000-12-01T17:27:43Z")
welcome to the SDMB! pldennison and Crafter\_Man got it right, but maybe I can help clarify it. The solar system formed from a cloud of matter. Gravity pulled the material together. It did not come together symmetrically, so there was some overall spin that got faster as the stuff got closer together (like an ice skater who pulls in his/her arms to spin faster). Gravity pulled this material into what became the sun and planets. Since they started off with the rotation, they kept going (not enough friction in space to stop it). The moon does spin. It just does takes the same amount of time for it to orbit the Earth as it does to spin once on its axis…so we only see one side. ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [December 1, 2000, 5:31pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/7 "2000-12-01T17:31:37Z")
> [@](#): > > \*Originally posted by Stimpy \* > \*\*what year will it stop? do i need to move house? 🙂 > \*\* don’t worry about it [http://itss.raytheon.com/cafe/qadir/q396.html](http://itss.raytheon.com/cafe/qadir/q396.html) ---
**Author:** ![Wildest\_Bill](https://avatars.discourse-cdn.com/v4/letter/w/b9e5f3/32.png) [@Wildest\_Bill](https://boards.straightdope.com/u/Wildest_Bill)\ **Post date:** [December 1, 2000, 5:34pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/8 "2000-12-01T17:34:04Z")
\*Originally posted by Phobos \* \*\*welcome to the SDMB! > [@](#): > > The solar system formed from a cloud of matter. How do you know what God is? ---
**Author:** ![Zenster](https://avatars.discourse-cdn.com/v4/letter/z/cdc98d/32.png) [@Zenster](https://boards.straightdope.com/u/Zenster)\ **Post date:** [December 1, 2000, 6:01pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/9 "2000-12-01T18:01:44Z")
Because there is nothing in this entire universe that stands still. ---
**Author:** ![SSgtBaloo](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@SSgtBaloo](https://boards.straightdope.com/u/SSgtBaloo)\ **Post date:** [December 1, 2000, 6:16pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/10 "2000-12-01T18:16:02Z")
> [@](#): > > \*Originally posted by Zenster \* > \*\*Because there is nothing in this entire universe that stands still. \*\* Amen, brother! I have a 3-year-old son, and he’s proof enough for me of that! ~~Baloo ---
**Author:** ![Doctor\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doctor_jackson/32/32_2.png) [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)\ **Post date:** [December 1, 2000, 6:17pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/11 "2000-12-01T18:17:26Z")
Zenster, you obviously haven’t met my brother-in-law. ---
**Author:** ![Uniball](https://avatars.discourse-cdn.com/v4/letter/u/e5b9ba/32.png) [@Uniball](https://boards.straightdope.com/u/Uniball)\ **Post date:** [December 1, 2000, 9:39pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/12 "2000-12-01T21:39:08Z")
> [@](#): > > \*Originally posted by Zenster \* > \*\*Because there is nothing in this entire universe that stands still. \*\* Tell that to my penis, that damn traitor! ---
**Author:** ![keeper0](https://avatars.discourse-cdn.com/v4/letter/k/b19c9b/32.png) [@keeper0](https://boards.straightdope.com/u/keeper0)\ **Post date:** [December 1, 2000, 11:00pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/13 "2000-12-01T23:00:58Z")
[warning the following information has been pulled from deep and dark recesses of knowledge] There is a slight braking force due to gravity, I believe. This is sometimes called “tidal effects”. Visualize a planet orbiting the sun. The gravitational force of the sun deforms the shape of the planet into an ellipsoid rather than a sphere. (We can see this with the tides in the ocean, but it happens to a smaller degree with the land). Now, as I understand it, the center of mass of the planet has shifted slightly sunward from the center of rotation. When the planet rotates, it takes some energy since the center of mass moves further from the sun. But, then the planet re-deforms so that the ellipsoid again points toward the sun. The short answer is that some of the kinetic (spin) energy of the planet is lost through friction as the planet deforms under tidal pressure. So, for orbiting bodies in which the gravitational force pulling on it is large enough relative to its own mass, it will stabilize into a spin such that it maintains the same face to the object it is orbiting. (stops deforming and maintains a stable ellipsoid) The moon does this around the earth, (but not the earth with respect to the moon because our mass is higher). Mercury does it around the sun. ---
**Author:** ![jmullaney](https://avatars.discourse-cdn.com/v4/letter/j/85e7bf/32.png) [@jmullaney](https://boards.straightdope.com/u/jmullaney)\ **Post date:** [December 1, 2000, 11:10pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/14 "2000-12-01T23:10:04Z")
> [@](#): > > \*Originally posted by Zenster \* > \*\*Because there is nothing in this entire universe that stands still. \*\* Actually, you stay still and the rest of the universe moved relative to you. But to adress the OP: hampsters, lots and lots of hampsters. [http://www.hampsterdance.com](http://www.hampsterdance.com) ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [December 1, 2000, 11:21pm UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/15 "2000-12-01T23:21:59Z")
> [@](#): > > \*Originally posted by Wildest Bill \* > \*\*\*Originally posted by Phobos \* > \*\* > > > [@](#): > > > > The solar system formed from a cloud of matter. > > How do you know what God is? \*\* I made no such claim. Why do you assume I am making a religious statement? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 2, 2000, 1:39am UTC](https://boards.straightdope.com/t/why-does-the-earth-spin/43578/16 "2000-12-02T01:39:45Z")
Actually, Mercury doesn’t always show the same face to the Sun. It’s got what’s called a harmonic lock: Its rotation period is exactly 2/3 of its orbital period. The causes for this are similar to tidal lock, but slightly more complicated: It’s got a bit of a permanent deformation, and a rather eccentric orbit (as planets go, at least). With that period, the long face alternately points either directly towards or directly away from the Sun whenever it’s at perihelion (closest point to the Sun in its orbit).