Earth-Sun Gravity Question

[yonderboy]

Hi -- sorry if these are stupid easy questions, but... what keeps the earth from falling into the sun? Also, does anyone know if the earth spins around its axis in the same direction as it orbits the sun, or the opposite?

[ydoaPs]

its orbit. it's tangental velocity keeps it from succombing to the centripedal force of gravity. basically, it is always falling into the sun, but it is moving fast enough past it, that it always misses.

[yonderboy]

ok, cool. why is it tilted on its axis?

[Sisyphus]

Probably as a result of the collisions of the various planetoids which originally formed the earth. It just worked out to be the sum of the angular momentums of those bodies.

[Janus]

Also, does anyone know if the earth spins around its axis in the same direction as it orbits the sun, or the opposite?

 

The same direction. In fact, The Moon also orbits the Earth in the same direction. In addition, all the planets of the Solar system orbit in the same direction and the majority of the moons of those planets.

 

It is so prevalent that it is called "direct" motion, while orbiting or rotating in the opposite direction is called "retrograde" motion. There are exceptions, but they are just that, exceptions, such as some moons that are captured asteroids (For complicated reasons it is easier for a planet to capture another body into a retrograde orbit), or due to some major event in the body's past.

[JustStuit]

Well if you look at it from general realativity, the earth is in a straight orbit but the sun bends space time from all its mass which causes earth's straight path to not be straight. Is this correct?

[[Tycho?]]

Well if you look at it from general realativity, the earth is in a straight orbit but the sun bends space time from all its mass which causes earth's straight path to not be straight. Is this correct?

 

Pretty much yeah, but its not very useful to think of an orbit as a straight line in this context, its more just confusing.

[JustStuit]

Why is the orbit elliptical then?

[sunspot]

One conceptual problem of the earth rotating around the sun is perpetual motion. If the gravitational force is perpendicular to the motion and the earth's velocity allows it to overcome the force, where does the energy come from so the earth can overcome the work created by the sun's gravity and maintain velocity. Most space junk eventually falls to the earth unless we periodically added energy to maintain the orbit.

 

Relativity sort of explains this with a curving of space time. Does that mean that since we are in that curved space/time that our standards of length and time need to be adjusted to compensate? If we go to Mars will time and distance be slightly different?

[insane_alien]

Most space junk eventually falls to the earth unless we periodically added energy to maintain the orbit.

 

This is because the earths atmosphere provides friction. this slows the orbital velocity and it eventually crashes in to the big rock below it.

[s pepperchin]

From what I can figure the earth's orbit is an ellipse because the sun is moving through space. It is like if you did that experiment with the mass on a string, but replace the string with a rubber band. Then start running while the mass spins. I think that you would get an ellipse. I've never sat down to do the math behind it, but it just always made sense to me. As far as issues of spin and the Earth falling into the Sun, I am pretty sure that conservation of momentum (linear and angular) and energy play a big part in that.

[ecoli]

One conceptual problem of the earth rotating around the sun is perpetual motion. If the gravitational force is perpendicular to the motion and the earth's velocity allows it to overcome the force, where does the energy come from so the earth can overcome the work created by the sun's gravity and maintain velocity. Most space junk eventually falls to the earth unless we periodically added energy to maintain the orbit.

 

This is mostly due to friction... space is not empty, especially for things orbiting around a planet with an atmosphere. (this is according to my astonomy prof. btw)

[insane_alien]

From what I can figure the earth's orbit is an ellipse because the sun is moving through space. It is like if you did that experiment with the mass on a string, but replace the string with a rubber band. Then start running while the mass spins. I think that you would get an ellipse. I've never sat down to do the math behind it, but it just always made sense to me. As far as issues of spin and the Earth falling into the Sun, I am pretty sure that conservation of momentum (linear and angular) and energy play a big part in that.

 

Nope. the orbit is elliptical because the momentum is not the right magnitude or direction to support a circular orbit. the direction is only correct at points on the orbit and so is the magnitude but different sets of points.

[sunspot]

The elliptical orbit implies that the earth sees various amounts of gravity on its journey around the sun. An ellipse has two foci, which would amount to two apparent centers of gravity. Such a journey also implies that the velocity of the earth around the sun is not constant. It slows down to a minimum, and then speeds back up to a maximum, only to slow down again, etc. If gravity is pulling inward, where does the earth get the energy to not be pulled in?

 

In space journeys, we can use gravity to slingshot space vehicles. But these are hyperbolic orbits instead of elliptical. I have no problem with that geometry. But an elliptical orbit implies two centers of gravity that keeps something in place.

 

The relativity/gravity explanation raises an interesting conceptual problem. Time and distance measurements on the earth should vary slightly over the year since the space/time warping seen by the earth varies along the orbit due to two centers of gravity.

[[Tycho?]]

The elliptical orbit implies that the earth sees various amounts of gravity on its journey around the sun. An ellipse has two foci' date=' which would amount to two apparent centers of gravity. Such a journey also implies that the velocity of the earth around the sun is not constant. It slows down to a minimum, and then speeds back up to a maximum, only to slow down again, etc. If gravity is pulling inward, where does the earth get the energy to not be pulled in?

 

In space journeys, we can use gravity to slingshot space vehicles. But these are hyperbolic orbits instead of elliptical. I have no problem with that geometry. But an elliptical orbit implies two centers of gravity that keeps something in place.

 

The relativity/gravity explanation raises an interesting conceptual problem. Time and distance measurements on the earth should vary slightly over the year since the space/time warping seen by the earth varies along the orbit due to two centers of gravity.[/quote']

 

You need to look up Keplers laws to get rid of these misconceptions you have. Basic circular motion could also help, to explain to you why the earth does not fall into the sun.

[Janus]

The elliptical orbit implies that the earth sees various amounts of gravity on its journey around the sun. An ellipse has two foci' date=' which would amount to two apparent centers of gravity. Such a journey also implies that the velocity of the earth around the sun is not constant. It slows down to a minimum, and then speeds back up to a maximum, only to slow down again, etc. If gravity is pulling inward, where does the earth get the energy to not be pulled in?

 

In space journeys, we can use gravity to slingshot space vehicles. But these are hyperbolic orbits instead of elliptical. I have no problem with that geometry. But an elliptical orbit implies two centers of gravity that keeps something in place.

 

[/quote']

No, an elliptical orbit only has one CoG which occurs at one of the foci. The other focus doesn't enter into the orbit except as an abstraction.

 

The thing that needs to be remembered is that while gravity is pulling "inward" the planet has a sideways velocity to that pull. If that inward pull wasn't there, the planet would continue on in a straight line at that velocity. What the inward pull of the Sun does is curve that path. If the pull and velocity match just right, you get a circular orbit. (one way to envisage this is that the centripetal force of gravity is just enough to cancel out the centrifugal effect of the planet trying to fly away in a straight line. )

 

As such, the planet's total energy is constant, as the potential energy due to its distance fom the Sun and the kinetic energy due its velocity are both constant.

 

So, what about an elliptical orbit? Let's start with a circular orbit and then decrease the planet's velocity a bit. Now the centripetal force of gravity causes the planet's path to curve more tightly. As it does so, the planet moves closer towards the Sun. This causes it to lose potential energy, and just like a ball dropped from your hand, it compensates by gaining kinetic energy and it speeds up. As it it moves in and gains speed, eventually it gains enough speed that the tendancy to fly away from the Sun begins to dominate again, and the path that was curving more and more towards the Sun begins to "flatten out". At a point 180 degrees aorund the Sun from where it started, it will be moving perpendicular to the force of the Sun. It has reached perihelion. It is now moving its fastest and the force of gravity is not sufficient to hold it this close. It starts to pull away from the Sun, shedding speed as it gains potential energy. It follows a path that mirrors its inward path, and eventually ends up right back where it started, to do it all over again.

 

You will note that as its loss and gain of potential energy is perfectly balanced by a gain and loss of Kinetic energy, the total energy of the Planet, again never changes.

 

So to answer your question as to where thae planet gets the energy to keep from falling into the Sun, it gets it from the kinetic energy due to the planet's orbital velocity.

[sunspot]

Ecoli made a point about friction within our upper atmosphere causing space junk to lower its velocity causing it to fall to the earth. That seems reasonable. But it rasies another question; isn't the earth in the sun's atmosphere. We call it the solar wind. Where does the earth get the energy to overcome this friction and not end up with a decaying orbit?

[CanadaAotS]

It probably does have a decaying orbit, but the "solar wind friction" would be so small that it would take a few billion years to be noticeable.

 

About the elliptical orbits, think of this. When you throw a ball in the air it takes a parabolic trip but what if the earth wasn't in the way? it would orbit the center of the Earth in a very elliptical orbit.

[Janus]

Ecoli made a point about friction within our upper atmosphere causing space junk to lower its velocity causing it to fall to the earth. That seems reasonable. But it rasies another question; isn't the earth in the sun's atmosphere. We call it the solar wind. Where does the earth get the energy to overcome this friction and not end up with a decaying orbit?

 

Two points:

 

The solar wind is exceedingly thin. if you just consider the Earth's orbital path, the Earth collides with less than 6 grams of mass per sec. Gvien that the mass of the Earth itself is 6,000,000,000,000,000,000,000,000,000 grams, it would be a long time before this led to any significant slowing.

 

The other point is that the Solar wind is "blowing" out away from the Sun. This tends to push the Earth out away from the Sun. Since the speed of the solar wind varies between 200 and 800 km/sec and the Earth's orbital velocity is 30 km/sec, this "push" exceeds any drag.

 

But then again, this push is very, very, very small.

[[Tycho?]]

Ecoli made a point about friction within our upper atmosphere causing space junk to lower its velocity causing it to fall to the earth. That seems reasonable. But it rasies another question; isn't the earth in the sun's atmosphere. We call it the solar wind. Where does the earth get the energy to overcome this friction and not end up with a decaying orbit?

 

The effect is super super super tiny. And besides, the Earth's orbit is decaying, just by a very small amount.

[insane_alien]

'']The effect is super super super tiny. And besides, the Earth's orbit is decaying, just by a very small amount.

 

this is true. i think a study found that it would take mercury 120 billion years venus 540 billion and earth over a trillion years to fall into the sun. since its going to blow up in about 5 i'd say that its not anything to worry about.

[sunspot]

One of the creation scenarios of the earth has to do with asteroids and space debris focusing to become the earth. Since the sun is the major player in the solar system, it was probably responsible for pulling this stuff it. The question I have, with the average trajectory of this space debris inward toward the sun, why didn't the earth get banged off course into the sun?