View Full Version : Is Earth's orbit decaying?


wesmorris
06-03-03, 11:39 AM
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?

chroot
06-03-03, 12:49 PM
What dpes it mean for a force to be efficient?

- Warren

wesmorris
06-03-03, 01:02 PM
Originally posted by chroot
What dpes it mean for a force to be efficient?

- Warren

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).

Persol
06-03-03, 01:25 PM
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:)

jcsd
06-03-03, 01:26 PM
Ok The answer to this one is in my head somewhere:bugeye:

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).

AntonK
06-03-03, 01:41 PM
Originally posted by wesmorris
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).

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

chroot
06-03-03, 02:29 PM
wesmorris:

Congratulations, you answered your own question.

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

wesmorris
06-03-03, 02:37 PM
Thanks guys!

chroot
06-03-03, 02:41 PM
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

wesmorris
06-03-03, 02:48 PM
Originally posted by chroot
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

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.

MacM
06-03-03, 11:03 PM
chroot,



I kind of miss the "crackpot killer" below your name though.

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


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

AntonK
06-04-03, 11:59 AM
Originally posted by MacM
chroot,




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


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

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

You get the analogy.

-AntonK

James R
06-10-03, 10:50 PM
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.

Janus58
06-11-03, 08:36 AM
Originally posted by Andre
OK Thanks, James.

Now I wonder, Venus already overshot tidal lock and is rotating backwards slightly, yet Earth is nowhere near such a thing. The general characteriscs, like weight, radius, orbit distance are roughly in the same order of magnitude (70-80% or so). Yet Venus has stopped rotating and Earth will not do so for the next few billion years? What makes the differcence?

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.

Blindman
06-11-03, 08:38 AM
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.

MacM
06-11-03, 01:42 PM
Andre,

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

ElectricFetus
06-11-03, 01:58 PM
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.

Loco
06-11-03, 03:17 PM
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...

(Q)
06-11-03, 03:59 PM
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.

James R
06-11-03, 09:41 PM
Venus may well have been hit by something reasonably big during the formation of the solar system. This could have imparted its current spin.

ryans
06-11-03, 09:55 PM
Maybe Venus was home to an ancient civilisation that endowed itself in nuclear winter, with the nukes all fired in the same diraection thus imparting angular momentum to it. It would certainly explain why there are so many clouds there and why there was a sudden burst of life on earth that many millions of years ago (the ones that survived came here). I thought that since crack pottery is the taste of the month, I might have ago at my own crackpot theory and see how many of you I convince. You know you will never be able to prove me wrong.

ElectricFetus
06-11-03, 11:11 PM
Actually Venus and Mercury have very slow rotations, it seems that only mars and earth have high-speed rotations… Earths case can easily be blamed on the world colliding impact that formed are moon and modern earth. Mars also sustain heavy impacts but nothing that harsh.

The inner planets are very slowly falling into the sun, the outer planets are very slowly moving away. Why? I don’t know, this is just something I read out of a astronomy book some time ago.

ElectricFetus
06-11-03, 11:12 PM
by the way ryan we can prove you wrong: the amount of nukes needed turn its rotation backwards would have blown venus in half!

NileQueen
06-12-03, 12:03 AM
So if the core of a planet is solid, it is going to slow down? No...I think Mars does not have an active core, does it? And it takes 24.6 Earth days to spin a day, and 687 Earth days to make a revolution around the sun.
Wait, it seems Mars has a liquid core: March 2003 news
http://www.space.com/scienceastronomy/mars_core_030306.html
"Additional information for the study came from previous measurements of the precession of Mars' rotation. Like Earth, the Red Planet's axis of rotation is not steady, but instead proscribes an imaginary cone-shaped feature in space as it shifts over time.

It takes 170,000 years for the Martian axis to make one revolution, Konopliv said, effectively drawing a circle on some imaginary plane above the pole. "The precession rate indicates how much the mass of Mars is concentrated toward the center. A faster precession rate indicates a larger dense core compared."

In addition to iron, the new study indicates there is also some lighter element in the core, possibly sulfur, Yoder told SPACE.com. The study also pinned the core's radius down to between 945 and 1,143 miles (1,520 and 1,840 kilometers)."

Mars is a rikwid creechuh. But it doesn't have a dynamo like Earth does.

A body in motion tends to stay in motion, a body at rest tends to remain at rest. Maybe Venus never really got started moving....

Or being so much closer to the sun, maybe the sun's magnetic field has slowed their spin. Could it be a magnetic rather than a friction cause?

Janus58
06-12-03, 06:59 PM
Originally posted by Andre
There are more possibilities


There is a law of maintaining constant turning momentum. As Earth looses turning momentum the moon compensates for the total Earth- moon system with moving away from the Earth increasing momentum at the same rate(roughly 3 cm per year). Now, (speculating), the sun - earth system also should be working towards a tidal lock as happened with Venus and is on its way with Mercury. So by loosing turning momentum the Earth may well be moving away from the sun, maintaining angular momentum, similar as the moon. So, Earth orbit may be extending from the sun instead of being pulled in. Any confirmation of that?

The actual outward movement of the Earth would be caused by the Earth's tidal effect on the Sun. (Just like the outward drift of the Moon is due to the Moon's tidal effect on the Earth.)

This effect would be very small and such a drift would be very slow.

But his does bring up an interesting point: The tidal locking of the Earth with the Sun. This would be pretty straight forward (Assuming there was enough time for it to happen), with the Earth slowly losing rotational speed until it matched its period of revolution. There is one fly in the ointment however, the Moon.

Right now, both the Moon and sun work to slow the Earth's rotation, and eventually the Earth's Rotation will match that of the moon's period. But what happens then?

The Sun will continue to try to slow the Earth. But as it does, this means that the Earth will rotate slower than the moon's period. This means the Tidal action between the Earth and Moon will begin to tug the Moons backwards in its orbit, as it now tugs it forward. The moon will begin to impart some of it's angular momentum back to the Earth, trying to speed up the Earth's rotation while falling into a lower orbit.

Now at first, the moon will have moved far enough away from the Earth that the Sun will win, and the Earth will continue to slow. But as the Moon falls into a lower and lower (and faster and faster) orbit, Its tidal influence will increase and eventually surpass that of the Sun. The Earth would begin to speed up again trying to catch up with the Moon.

The moon would continue to drift further and futher inwards. The interesting question is how far? Will it drop below the Rouche limit and break up? (In which case, the Earth will develop a ring system. ) Or will a new equilibrium state be reached with the Earth and moon tidally locked again?