# oh lord help me: Is orbit perpetual motion? **URL:** \ **Category:** Factual Questions\ **Created:** [December 29, 2002, 12:14am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421 "2002-12-29T00:14:27Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [December 29, 2002, 11:53pm UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/21 "2002-12-29T23:53:14Z")
> [@](#): > > \*Originally posted by Osiris \* > That is why many moons (and Mercury) have fallen into an orbit-rotation ratio where one side is always facing their more massive partner. Not Mercury (at least not yet). From [http://pds.jpl.nasa.gov/planets/welcome/mercury.htm:](http://pds.jpl.nasa.gov/planets/welcome/mercury.htm:) > [@](#): > > Rotation period (length of day in Earth days)…58.65 > Revolution period (length of year in Earth days)…87.97 ---
**Author:** ![Dr\_Paprika](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr_paprika/32/3042_2.png) [@Dr\_Paprika](https://boards.straightdope.com/u/Dr_Paprika)\ **Post date:** [December 30, 2002, 12:20am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/22 "2002-12-30T00:20:41Z")
“In this house we obey the laws of thermodynamics!” ---
**Author:** ![Fern\_Forest](https://avatars.discourse-cdn.com/v4/letter/f/71c47a/32.png) [@Fern\_Forest](https://boards.straightdope.com/u/Fern_Forest)\ **Post date:** [December 30, 2002, 12:31am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/23 "2002-12-30T00:31:03Z")
> [@](#): > > \*Originally posted by rowrrbazzle \* > \*\*Not Mercury (at least not yet). From [http://pds.jpl.nasa.gov/planets/welcome/mercury.htm:](http://pds.jpl.nasa.gov/planets/welcome/mercury.htm:) \*\* Oops, you’re right, I misspoke. I meant to say something about simple rations of orbit to rotation but couldn’t come up with a good way to say it so slipped into something a little different and suddenly Mercury no longer makes sense. Mercury has a 2:3 orbit to rotation ratio. Interestingly enough 2:3 exists in a stability pocket meaning it will take a lot of energy to get it into a 1:1 ratio. So it wont be going there unless something drastic happens. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [December 30, 2002, 3:38am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/24 "2002-12-30T03:38:23Z")
> [@](#): > > It is my understanding that although the orbiting body appears to be in perpetual motion, you could not use that motion to do any useful work[.] HEY! You sayin’ I’m not doing anything useful? ---
**Author:** ![CasperQ](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@CasperQ](https://boards.straightdope.com/u/CasperQ)\ **Post date:** [December 30, 2002, 3:53am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/25 "2002-12-30T03:53:33Z")
Here’s the truth, the whole truth and… well, anyway - it’s at least in layman’s terms!! 😃 > [@](#): > > The reason that I mention this is that some science books actually call the earth’s orbit perpetual motion. Well, it is close, but not perfect. found here [http://www.jimloy.com/physics/perpet.htm](http://www.jimloy.com/physics/perpet.htm) I love layman’s terms…!!! ---
**Author:** ![CasperQ](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@CasperQ](https://boards.straightdope.com/u/CasperQ)\ **Post date:** [December 30, 2002, 3:58am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/26 "2002-12-30T03:58:10Z")
By the way - layman’s terms being much clearer than my prof’s (and this is relevant to the point that there is nothing with ‘no’ gravitational pull / friction): > [@](#): > > Physics 101: > There is a simple formula for caluclating the gravitational pull between any two objects…it is Fg = Gm1m2/r^2…where Fg = the Force of gravity, G is the gravitational constant (9.8 m/s^2 here on earth) m1 is the mass of the first object, m2 the mass of the second and r is the distance between the 2 objects…therefor-since the distance between the 2 objects (the sun and the photon) is quite small when the photn reaches the surface, r is small r^2 (r squared) makes the Fg even smaller…point being-that is when and where the Fg is the STRONGEST…also note (very interestingly) that since no matter how big r is-Fg between ANY TWO OBJECTS IN THE UNIVERSE is never zero…somethiing to think about during your next hike through the woods. And if my prof is reading this (I suspect he is online here somewhere!) it still makes me smile!! ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [December 30, 2002, 4:17am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/27 "2002-12-30T04:17:49Z")
Here’s an easy way to visualize tidal forces. For an extended body in orbit only its center of mass has the correct angular velocity for the given orbit. Any part of the object that is further out from the central force is moving too fast, and is therefore trying to pull away from the CoM, and vice versa for parts that are closer in. So you get two oppositely directed force vectors trying to pull the body apart. Also, these oppositely directed vector fields aren’t equal across the extended object so you get tangential components pointing to the center of the body and it’s these components, for instance, that cause the tides on Earth. ---
**Author:** ![bryanmcc](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@bryanmcc](https://boards.straightdope.com/u/bryanmcc)\ **Post date:** [December 30, 2002, 9:36pm UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/28 "2002-12-30T21:36:17Z")
> [@](#): > > \*Originally posted by CasperQ \* > **where…G is the gravitational constant (9.8 m/s^2 here on earth)** Actually, G is the universal gravitational constant, equal to about 6.673x10^-11 m^3/s^2kg. What you are quoting is the acceleration due to gravity on the earth’s surface. Acceleration due to gravity is a local phenomenon, while G is a universal constant. ---
**Author:** ![ForgottenLore](https://avatars.discourse-cdn.com/v4/letter/f/5e9695/32.png) [@ForgottenLore](https://boards.straightdope.com/u/ForgottenLore)\ **Post date:** [December 30, 2002, 10:34pm UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/29 "2002-12-30T22:34:30Z")
Regarding the OP, here’s my two cents: Planetary orbits are not perpetual motion. Here’s a way of visualizing it that may make the reason a little more obvious. If we follow general relativity we should think of gravity as a curvature of spacetime, and _not_ as an attractive force. From Newton’s laws of motion we know that an object in motion tends to stay in motion, unless acted on by an outside force. This is not perpetual motion, just a statement about inertia. With this in mind, we can think of a planet orbiting a star as following the _straightest possible path through curved spacetime._ That’s not perpetual motion. It’s just Newton’s law in relativistic terms. This is not very obvious on the face of it because we only see it in the three spatial dimensions. To us it looks like a closed loop. If we plot the orbital motion in the two relevant space dimensions and one time dimension, however, the graph looks like a helix, not a closed loop. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [December 30, 2002, 10:45pm UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/30 "2002-12-30T22:45:05Z")
Forget tidal friction (ok…don’t forget it cuz it counts). I thought the earth (or any orbiting body) lost energy due to gravitational waves. Much as a cork bobbing in the water loses energy (the ripples moving away from it) so too does the earth (or any planet). IIRC Stephen Hawking calculated that the earth is losing enough energy in this fashion to power an average toaster (how you would convert the gravitational energy to a useful form is anybody’s guess). Eventually, if you wait long enough, the earth will spiral into the sun. Not much use in waiting though as this time period will take FAR longer than the life of our sun. One way or the other we will all be long gone before we see this happen. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 31, 2002, 1:22am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/31 "2002-12-31T01:22:00Z")
> [@](#): > > \*Originally posted by Dr\_Paprika \* > \*\*“In this house we obey the laws of thermodynamics!” \*\* See now if Brad Pitt had formulated the laws of thermodynamics… “The first law of thermodynamics is that energy can be changed from one form to another, but it cannot be created or destroyed…” (dramatic pause) “…The second law of thermodynamics is that ENERGY CAN BE CHANGED FROM ONE FORM TO ANOTHER BUT IT CANNOT BE CREATED OR DESTROYED” ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [December 31, 2002, 2:41am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/32 "2002-12-31T02:41:55Z")
> [@](#): > > \*Originally posted by Osiris \* > \*\*…Mercury [has] fallen into an orbit-rotation ratio where one side is always facing their more massive partner. \*\* This is not correct about Mercury. If it were, then the length of one rotation of the planet would equal the length of one revolution of the planet about the sun. A solar year on Mercury is almost 88 earth days. A synodic day on Mercury (midday to midday) is almost 176 earth days. In comparison, our moon DOES hold the same face to its more massive partner, and so by definition, the period of one rotation equals the period of one revolution. Hope this helps clarify… - Jinx ---
**Author:** ![Ficer67](https://avatars.discourse-cdn.com/v4/letter/f/c57346/32.png) [@Ficer67](https://boards.straightdope.com/u/Ficer67)\ **Post date:** [December 31, 2002, 2:51am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/33 "2002-12-31T02:51:31Z")
I thought that friction between the Earth and its oceans was turned into heat and radiated. This cannot be the reason that the moon’s orbit is getting larger can it? ---
**Author:** ![Ficer67](https://avatars.discourse-cdn.com/v4/letter/f/c57346/32.png) [@Ficer67](https://boards.straightdope.com/u/Ficer67)\ **Post date:** [December 31, 2002, 2:53am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/34 "2002-12-31T02:53:34Z")
Mercury has its orbit because it was struck by a huge meteor many many moons ago ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [December 31, 2002, 2:54am UTC](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421/35 "2002-12-31T02:54:12Z")
Ah, but Rarrwlbazzle, may I explain that your post is in error. ## quote: ## Rotation period (length of day in Earth days)…58.65 Revolution period (length of year in Earth days)…87.97 The error is that the rotation period you cite is the _sidereal_ period. This means it is the time for a star to return to the same spot (as recorded by an observer) from one consecutive planetary rotation to the next. For earth, this is 23h 56m, not 24h 00m. You see, the sidereal period accounts for the movement of a planet in its orbit AND the rotation about its axis. Although the sun is a star, sidereal time does NOT include the sun. The measure of the sun to return to the same spot from one consecutive planetary rotation of Mercury to the next is its synodic period of almost 176 earth days. The synodic period of Mercury is so great because it is moving so fast along its orbit. I hope this helps clarify…? Sidereal time and sidereal period can be a tough concept to understand. :eek: - Jinx [Previous page](https://boards.straightdope.com/t/oh-lord-help-me-is-orbit-perpetual-motion/145421.md?page=1)