# Is Earth's orbit decaying?

Discussion in 'Physics & Math' started by wesmorris, Jun 3, 2003.

1. ### wesmorrisNerd Overlord - we(s):1 of NValued Senior Member

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So is it? If it isn't, then isn't the gravitational interaction between the earth and the sun 100% efficient? I'm sure that's got to be wrong, so can someone straighten me out?

3. ### chrootCrackpot killerRegistered Senior Member

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What dpes it mean for a force to be efficient?

- Warren

5. ### wesmorrisNerd Overlord - we(s):1 of NValued Senior Member

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I dunno... I'm a bit confused and was hoping you could straighten me out. Seem that to keep the earth in orbit, work has to be done right?

Eh, maybe I should just figure it out myself, but I was hoping for easy answers from better physicists than myself. I don't deal with this junk enough, but you know.. when I apply a force to a wood block on a table (and it moves), energy is lost to blah blah.. That's sort of where I'm getting stuff stuck in my brain. Seems like since there's no friction, the earth's orbit is blah blah.

I get it now. Sorry, just confused. Conservation of energy, intentionality. In other words, if I'm moving the block, it's the function of moving the block that loses energy.. but technically the process of energy exchange is always 100% efficient from the perspective of conservation of energy.. (since all energy expended is accounted for).

A friend of mine at work though said he thought he'd heard that the earth was closing in on the sun just a smidge all the time. That's what brought on my stupid questions. That also made me wonder how the earth could maintain orbit with zero energy loss (in it's orbit, not spin).

7. ### PersolI am the great and mighty Zo.Registered Senior Member

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Originally posted by wesmorris
A friend of mine at work though said he thought he'd heard that the earth was closing in on the sun just a smidge all the time.
You are thinking of stability. Basically it the earth is moving around the sun too fast it will leave orbit, too slow and it will crash into the sun. We must be damn close to the correct velocity to have been in orbit this long. Any change in orbit would be small.

That also made me wonder how the earth could maintain orbit with zero energy loss (in it's orbit, not spin).

Even if the earth did crash into the sun, it would have zero energy loss. That energy is just being converted.

Correction: The earth wouldn't have any energy loss, until it crashed into the sun. The there is no more earth

8. ### jcsdRegistered Senior Member

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Well first off, I definitely know that the Earth's orbit is not "100% efficient", for a start the Earth is losing angular momentum as it hits particles and objects in space, but this is only a tiny amount.

The Earth's orbit is not decaying quickly enough for it to fall into the sun, before the sun dies (infact solar radiation is probably pushing it away more than the orbit decay is bringing it in).

9. ### AntonKTechnomageRegistered Senior Member

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Well, for the work aspect of it, work is the integral of force dot displacement where dot refers to a dot product. In a perfectly circular orbit (I know we're not...but hear me out) there is no work because the force is always perpendicular to the displacement and thus, with an angle of 90, the dot product goes to 0. To find out why the work on an elliptical orbit is 0, you have to take the integral around the entire orbit. It'll still come out to 0 I believe because there is both positive work and negative work being done (depending on where in the orbit you are). Thus, the sum is 0.

-AntonK

10. ### chrootCrackpot killerRegistered Senior Member

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wesmorris:

In a true, perfect vacuum, the two bodies (the Sun and Earth) would orbit their common center of mass, following the exact same ellipses, forever -- without losing energy, velocity, or anything else.

The universe is, of course, not a perfect vacuum! The Sun emits light, some of which impinges upon the Earth's surface. The force is in opposition to the gravitational "pull" of the Sun, and tends to push it out to more distant orbits.

On the other hand, as has been mentioned, the Earth also encounters space junk all the time -- you could think of the Earth as bustling through a thin soup of space sludge. This material exerts what is effectively a drag force, and tends to slow the Earth down, which tends to push it toward closer orbits.

Physical problems are often fraught with this kind of stuff: we analyze the very simplest possible example of a phenomenon (two bodies moving in a total perfect vacuum) to extract the basic physical laws. To fully predict the Earth's real trajectory for billions and billions of years, however, is impossible. Passing comets, asteroids, perhaps even stars -- they'll all perturb the orbit and screw it up.

In general, astrophysicists consider orbits to be "chaotic," and thus are an example of a much wider field of problems. For any small piece of time centered around a known data point, you can pretty much predict exactly where the Earth will be. In the case of our own orbit, a billion years or so seems reasonable. Beyond that time, though, the errors begin to get pretty severe -- there are just too many variables, and they all accumulate and cause "feedback." We can say with pretty good confidence where the Earth will be in a hundred thousand years -- but we have little confidence that we know where it will be in four billion years.

Note as well that this is classical physics -- wherein energy states are continuous and forces can be any arbitrarily small magnitude. In quantum physics, things are more rigid -- you can often make predictions for a system that are valid for all of eternity!

- Warren

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Thanks guys!

12. ### chrootCrackpot killerRegistered Senior Member

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Also, if you need fodder for science-bashing, look into the "n-body problem" -- virtually all systems with 3 or more bodies cannot be described analytically (with equations). They are truly chaotic!

Guess how many bodies the Solar System has?

- Warren

13. ### wesmorrisNerd Overlord - we(s):1 of NValued Senior Member

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Eh, let the crackpots do the science-bashing. I was just trying to straighten out a friend and confused me in the process. I've heard about the n-body problem before (I think when I was in school, I think my first physics course mentioned it as a tidbit, the prof. (Clifford Thompson, supposedly a nationally infamous test writer) was awesome).. but it's been a while. Regardless, I appreciate your assistance Warren. I was bummed when you said you were leaving because physics forums was back up. Glad to see you're still here. I kind of miss the "crackpot killer" below your name though.

Thanks to the rest of you guys as well.

14. ### MacMRegistered Senior Member

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chroot,

- Warren
--------------------------------------------------------------------------------

I hadn't noticed. You're not becoming a convert on us are you.

15. ### AntonKTechnomageRegistered Senior Member

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An exterminator that stops killing bugs does not become a bug, he's simply no longer an exterminator

You get the analogy.

-AntonK

16. ### James RJust this guy, you know?Staff Member

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Andre,

I think you're right that if there was time the Earth would eventually become tidal locked with the Sun. However, in practice the Sun will become a red giant and engulf the Earth long before that happens.

17. ### Janus58Valued Senior Member

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Planet's don't typically "overshoot" tidal lock. Once they attain it, they stay there unless something external disturbs the system.

Considering the fact that Mercury isn't even tidally locked, it seems reasonable that Venus' slight retrograde rotation is due to some other explanation.

Also, I should point out that tidal forces fall off by the cube of distance, so the tidal forces acting on Venus are about 2.64 times those acting on the Earth.

18. ### BlindmanValued Senior Member

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1,425
What makes the differcence?

Da Moon...

Billions of years from now the oceans might disappear into the earth (due to slowing of plate tectonics). Oceans being the major drag in Earth, Moon and Sun motion. Just a thought.

19. ### MacMRegistered Senior Member

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Battle

Andre,

Go ahead and join. We have only lost a couple battles, not the war. We are restocking our arsenals.

20. ### ElectricFetusSanity going, going, goneValued Senior Member

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yes the earths orbit is decaying, but that does not matter because the earth will burn up as the sun goes nova way before we falls into the sun.

21. ### LocoRegistered Member

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I don't know if this might help anymore, but you talked about energy loss later, and to get things straightened out for those of you who didn't know. Energey do not at any time dissapear, though it changes form quite often. Energy can even become matter, and otherwise. Just... you know... :bugeye: I'm sure you don't care anyway... so... I'm just gonna go now... See you...

22. ### (Q)Encephaloid MartiniValued Senior Member

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Janus

Considering the fact that Mercury isn't even tidally locked, it seems reasonable that Venus' slight retrograde rotation is due to some other explanation.

One of the more interesting theories I had heard was that Venus was part of a 3-gear system when the solar system was a proto-system. Mercury and Earth were the two other “gears” in the system in that they both followed the proto-system’s original counter-clockwise rotation and that Venus was caught up between them and was forced to rotate clockwise very similar to 3 gears where the inside gear will always rotate in the opposite direction from the outside gears.

23. ### James RJust this guy, you know?Staff Member

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Venus may well have been hit by something reasonably big during the formation of the solar system. This could have imparted its current spin.