What keeps the moon from crashing into the Earth?

[Armitage]

Originally posted by: Descartes

Originally posted by: Armitage

Originally posted by: Descartes

Originally posted by: Mo0o
The ball has a tangential velocity to the surface of the earth with a constant force inwards. In essen it's constantly falling into the earth but since its traveling trangentially as well, it maintains an geosynchronous orbit.

No body can maintain orbit indefinitely, I believe. There are fairly stable points referred to as Lagrange Points, but those too are only stable over a period relative to the body's masses, velocity, etc. Eventually a body will succomb to one of two things: Decay in the form of gravitational or, in the case of earth, atmospheric; An increase in orbital velocity from some influence (another planet perhaps, an Aristotelian angle pushing it around, etc.) that exceeds its escape velocity and thus pushes it beyond its orbit.

Not really - if you are out of the drag regime (say perigee about 10000 Km altitude) but still in an orbit where the earth is the primary gravitational influence by a large factor, the object will remain in orbit essentially forever. The shape and orientation will change due to various perturbative factors (3rd body, earth geopotential, solar radiation pressure, etc.). But it won't decay or escape.

What I was arguing was the idea of "essentially"; essentially by what timeframe? I'm merely pointing out that over a sufficient timeframe nothing remains forever; hence my use of the word indefinitely...

Until the sun turns into a red giant and expands to envelope the earth - at that point, all bets are off

 

[JulesMaximus]

Originally posted by: plastick

Originally posted by: TheStu
look at it this way...

Stand on top of mount everest and level everything else lower than the peak... so it becomes a nice triangular mountain... now take a ball and throw it as hard as you can... the flight path of the ball is straight for a while and then falls right? Now haul up a cannon and shoot the same ball out of it... the flight path is straight again and farther this time before it falls to the earth... now, haul up a rocket engine for the ball and launch so that the straight part of the flight path takes it out of the atmosphere, now let the ball fall... it will fall straight down, but the earth's gravity will pull it back in, correcting its path


The moon is much the same, it actually <i>is</i> falling toward teh earth, but it is also moving forward in relation to the earth, so the earth's gravity is there to keep the orbit of the moon consistent... the moon doesnt crash into the earth because it is falling past it (you sort of have to think of this in 2D)

falling past it.... thats kind of what i was thinking.. its constantly missing it because of... see i need more understanding though.

thanks for your help

It is moving fast enough not to fall into the Earth but not fast enough to escape the pull of Earth's gravity.

 

[Descartes]

Originally posted by: Armitage

Originally posted by: Descartes

Originally posted by: Armitage

Originally posted by: Descartes

Originally posted by: Mo0o
The ball has a tangential velocity to the surface of the earth with a constant force inwards. In essen it's constantly falling into the earth but since its traveling trangentially as well, it maintains an geosynchronous orbit.

No body can maintain orbit indefinitely, I believe. There are fairly stable points referred to as Lagrange Points, but those too are only stable over a period relative to the body's masses, velocity, etc. Eventually a body will succomb to one of two things: Decay in the form of gravitational or, in the case of earth, atmospheric; An increase in orbital velocity from some influence (another planet perhaps, an Aristotelian angle pushing it around, etc.) that exceeds its escape velocity and thus pushes it beyond its orbit.

Not really - if you are out of the drag regime (say perigee about 10000 Km altitude) but still in an orbit where the earth is the primary gravitational influence by a large factor, the object will remain in orbit essentially forever. The shape and orientation will change due to various perturbative factors (3rd body, earth geopotential, solar radiation pressure, etc.). But it won't decay or escape.

What I was arguing was the idea of "essentially"; essentially by what timeframe? I'm merely pointing out that over a sufficient timeframe nothing remains forever; hence my use of the word indefinitely...

Until the sun turns into a red giant and expands to envelope the earth - at that point, all bets are off

Well, we have a few billion years to figure out how to get out of here Perhaps not even that long...

 

[Armitage]

Originally posted by: Descartes

Originally posted by: Armitage

Originally posted by: Mo0o
The ball has a tangential velocity to the surface of the earth with a constant force inwards. In essen it's constantly falling into the earth but since its traveling trangentially as well, it maintains an geosynchronous orbit.

A geosynchronous orbit is a particular class of orbit that is (nearly) circular and has a period equal to the rotation period of the earth (24 hours). If the inclination is 0 as well it will also be geostationary in addition to being geosyncronous. The end effect is that the satellite will appear to be stationary in the sky as seen from earth.

Are you sure the moon is geosynchronous? If so, how do we have tides? Perhaps you're instead referring to gravitional locking?

I'm sure the moon is *not* geosyncronous. I don't think Mo0o was referring to the moon above, but rather orbits in general. I was just pointing out that geosync is a special class of orbits.

 

[George P Burdell]

Originally posted by: Descartes

Originally posted by: Armitage

Originally posted by: Mo0o
The ball has a tangential velocity to the surface of the earth with a constant force inwards. In essen it's constantly falling into the earth but since its traveling trangentially as well, it maintains an geosynchronous orbit.

A geosynchronous orbit is a particular class of orbit that is (nearly) circular and has a period equal to the rotation period of the earth (24 hours). If the inclination is 0 as well it will also be geostationary in addition to being geosyncronous. The end effect is that the satellite will appear to be stationary in the sky as seen from earth.

Are you sure the moon is geosynchronous? If so, how do we have tides? Perhaps you're instead referring to gravitional locking?

The moon isn't geosynchronous IIRC.

 

[Descartes]

Originally posted by: spidey07
IIRC, orbit is really nothing more than perpetually falling.

that whole gravity thing.

Newton was a clever fellow.

 

[Descartes]

Originally posted by: Armitage

Originally posted by: Descartes

Originally posted by: Armitage

Originally posted by: Mo0o
The ball has a tangential velocity to the surface of the earth with a constant force inwards. In essen it's constantly falling into the earth but since its traveling trangentially as well, it maintains an geosynchronous orbit.

A geosynchronous orbit is a particular class of orbit that is (nearly) circular and has a period equal to the rotation period of the earth (24 hours). If the inclination is 0 as well it will also be geostationary in addition to being geosyncronous. The end effect is that the satellite will appear to be stationary in the sky as seen from earth.

Are you sure the moon is geosynchronous? If so, how do we have tides? Perhaps you're instead referring to gravitional locking?

I'm sure the moon is *not* geosyncronous. I don't think Mo0o was referring to the moon above, but rather orbits in general. I was just pointing out that geosync is a special class of orbits.

Doh!! You're right. Please accept my apologies...

 

[sao123]

Originally posted by: plastick

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

okay thanks man.. im searching for orbital velocity

Easy demonstration why...
1) the moon will never crash into the earth
2) the moon is moving away from the earth


Tie a rock onto the end of a string. Twirl the string. Feel the rock pulling away from your hand?
if the earths gravity wasnt so strong, the moon would fly away. If the moon suddenly stopped, it would fall to the earth.

 

[plastick]

Originally posted by: sao123

Originally posted by: plastick

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

okay thanks man.. im searching for orbital velocity

Easy demonstration why...
1) the moon will never crash into the earth
2) the moon is moving away from the earth


Tie a rock onto the end of a string. Twirl the string. Feel the rock pulling away from your hand?
if the earths gravity wasnt so strong, the moon would fly away. If the moon suddenly stopped, it would fall to the earth.

SO you are saying that the "rope" represents the pull of gravity?

 

[sao123]

Originally posted by: plastick

Originally posted by: sao123

Originally posted by: plastick

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

okay thanks man.. im searching for orbital velocity

Easy demonstration why...
1) the moon will never crash into the earth
2) the moon is moving away from the earth


Tie a rock onto the end of a string. Twirl the string. Feel the rock pulling away from your hand?
if the earths gravity wasnt so strong, the moon would fly away. If the moon suddenly stopped, it would fall to the earth.

SO you are saying that the "rope" represents the pull of gravity?

Essentially.

 

[sao123]

Originally posted by: sao123

Originally posted by: plastick

Originally posted by: sao123

Originally posted by: plastick

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

okay thanks man.. im searching for orbital velocity

Easy demonstration why...
1) the moon will never crash into the earth
2) the moon is moving away from the earth


Tie a rock onto the end of a string. Twirl the string. Feel the rock pulling away from your hand?
if the earths gravity wasnt so strong, the moon would fly away. If the moon suddenly stopped, it would fall to the earth.

SO you are saying that the "rope" represents the pull of gravity?

Essentially.

The reason why the moon is actually moving at 3.8 cm per year away from the earth is far more complex...

Quoted from a reputable source...

The Moon's orbit (its circular path around the Earth) is indeed getting larger, at a rate of about 3.8 centimeters per year.

The reason for the increase is that the Moon raises tides on the Earth. Because the side of the Earth that faces the Moon is closer, it feels a stronger pull of gravity than the center of the Earth. Similarly, the part of the Earth facing away from the Moon feels less gravity than the center of the Earth. This effect stretches the Earth a bit, making it a little bit oblong. We call the parts that stick out "tidal bulges." The actual solid body of the Earth is distorted a few centimeters, but the most noticable effect is the tides raised on the ocean.

Now, all mass exerts a gravitational force, and the tidal bulges on the Earth exert a gravitational pull on the Moon. Because the Earth rotates faster (once every 24 hours) than the Moon orbits (once every 27.3 days) the bulge tries to "speed up" the Moon, and pull it ahead in its orbit. The Moon is also pulling back on the tidal bulge of the Earth, slowing the Earth's rotation. Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit, making the Moon's orbit bigger (but, a bit pardoxically, the Moon actually moves slower!).
The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now.

 

[SparkyJJO]

Take a rock or something, attach it to a string, and spin it. The rock goes in a circle around your hand. Reason is it has forward momentum, but the string is keeping it from flying away from your hand, so it has to curve. The forward momentum keeps it at the string's full length.

Take that to the earth and moon - the moon has forward momentum, but the earth's gravity acts as the "string" holding it. And it won't slow down because unlike the rock on the string there are no other forces acting upon it (other gravity sources or air friction for example).

That's why

 

[LordMorpheus]

The moon might be moving slightly away from the earth, but here is something interesting - The reason that only one face of the moon ever faces the earth is because the earth creates tidal forces on the moon. The tidal forces that the moon creates on the earth is slowing the rotation of the earth around its axis, so eventually only one face of the earth will face the moon. Good stuff.

 

[DrPizza]

Originally posted by: plastick

Originally posted by: Mo0o

Originally posted by: plastick

Originally posted by: Mo0o
The ball has a tangential velocity to the surface of the earth with a constant force inwards. In essen it's constantly falling into the earth but since its traveling trangentially as well, it maintains an geosynchronous orbit.

okay its making more sence now...


btw: nice sig to have... ROFL

Just think of it this way. A ball is traveling tangentially to the earth. Every time it moves an inch, gravity pulls it into the earth a little. But then it travels tangentially again so it gets furthur out from earth once again. Then gravity pulls it back in to where it was before, in terms of its distance from the earth. Granted the motion is not discrete(is that teh word?) like that, that's a picture you should have in your head. So basically if gravity was to stop, the moon would shoot off into space in a tangential vector to its position to the earth at teh time gravity stopped

yeah i get it now... both forces are in effect continuously at the same time and there is a "sloped" result. That result is dependant on those and other variables.. certian variables are what i was trying to understand ...etc

thanks

Except, the forward motion isn't a "force" - the only force involved is that provided by gravity.

 

[plastick]

I want to restate my question....

How does the Earth's spinning effect the moon? Does it at all?

 

[Descartes]

Originally posted by: plastick
I want to restate my question....

How does the Earth's spinning effect the moon? Does it at all?

I don't remember the specific math, but the rotation of the bodies adds to the total tidal force, and its that tidal force that ultimately results in the moon being gravitationally locked from earth's perspective.

 

[dxkj]

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

If this is the case, how long ago did the moon begin to orbit the earth? Was the moon an asteroid that got caught up in the perfect way as it passed earth slowly? I dont get it

It seems to round , ie more round than chunky, to have come from the earth.

For that matter why are so many planets round? Are they actually pretty round? is it because of gravity in a vaccuum? wtf?


im confused now

 

[Mo0o]

Originally posted by: dxkj

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

If this is the case, how long ago did the moon begin to orbit the earth? Was the moon an asteroid that got caught up in the perfect way as it passed earth slowly? I dont get it

It seems to round , ie more round than chunky, to have come from the earth.

For that matter why are so many planets round? Are they actually pretty round? is it because of gravity in a vaccuum? wtf?


im confused now

I believe it was a giant asteroid that slammed into Earth and when a massive chunk ofEarth+Asteroid broke off, the moon is what's formed. This was during the VERY early stages of Earth's life.

I believe mathemtically for a giant moving body to be moving slow enough and at the right trajectory and distance from earth for it to be captured in orbit is too rare and unrealistic to be an explanation

 

[dxkj]

yeah i just figured it all out


large objects are round because their gravity crushes any deviations that would stick out far enough to be noticeable.

im not sure how a large object could break off a piece of earth, allow earth to reshape into a sphere, and still keep the moon in orbit, but im sure it did somehow i guess

 

[Questionmark]

bananas.....don't ask me how....or why.....bananas


----------------------------
You can only be young once. But you can always be immature. ~Dave Barry

 

[Mo0o]

Originally posted by: dxkj
yeah i just figured it all out


large objects are round because their gravity crushes any deviations that would stick out far enough to be noticeable.

im not sure how a large object could break off a piece of earth, allow earth to reshape into a sphere, and still keep the moon in orbit, but im sure it did somehow i guess

I think the Earth was half molten at the time, even now, past the mantle its just molten rock, nothing solid

 

[dxkj]

Originally posted by: Mo0o

Originally posted by: dxkj
yeah i just figured it all out


large objects are round because their gravity crushes any deviations that would stick out far enough to be noticeable.

im not sure how a large object could break off a piece of earth, allow earth to reshape into a sphere, and still keep the moon in orbit, but im sure it did somehow i guess

I think the Earth was half molten at the time, even now, past the mantle its just molten rock, nothing solid

but its so cold in space, wouldnt the surface be solid?

I guess the crust is a miniscule part of the earth, so it wouldnt matter since it could just reform after it got smacked up

 

[Chryso]

Magic

 

[Descartes]

Originally posted by: dxkj

Originally posted by: Descartes
It has to do with its orbital velocity. It's sufficient enough to keep the moon from crashing into earth through gravitational decay, and as someone else said it's actually enough such that the moon is receding in its orbit slightly every year.

If this is the case, how long ago did the moon begin to orbit the earth? Was the moon an asteroid that got caught up in the perfect way as it passed earth slowly? I dont get it

It seems to round , ie more round than chunky, to have come from the earth.

For that matter why are so many planets round? Are they actually pretty round? is it because of gravity in a vaccuum? wtf?


im confused now

There are many different theories of how the solar system formed, and that includes the planets and their moons. The most common theory I believe is the nebula theory, and that simply states that our solar system began as a nebula. Various density inhomogeneities in the gas and dust started to accrete, and over time enough accumulated to form our planets. The momentum of the accumulating dust and particles caused the early planets to rotate, and it's that rotation along with gravity that made them round.

 

[Descartes]

Originally posted by: dxkj

Originally posted by: Mo0o

Originally posted by: dxkj
yeah i just figured it all out


large objects are round because their gravity crushes any deviations that would stick out far enough to be noticeable.

im not sure how a large object could break off a piece of earth, allow earth to reshape into a sphere, and still keep the moon in orbit, but im sure it did somehow i guess

I think the Earth was half molten at the time, even now, past the mantle its just molten rock, nothing solid

but its so cold in space, wouldnt the surface be solid?

I guess the crust is a miniscule part of the earth, so it wouldnt matter since it could just reform after it got smacked up

Molten is the wrong word. Earth started from an accretion disk of dust and gas, so it's not like there was this massive amount of molten "rock" floating through space that happened to later form the earth.