Why are planets, comets etc orbiting the sun in elips? True, they can maintain their ellipse orbits by changing their rotational speeds steadily, but how (that of comets is the oddest especially)?

curioucity:

<i>Why are planets, comets etc orbiting the sun in elips?</i>

It is possible to prove mathematically that the path of any object acted on by a central, inverse-square force, must be one of the three conic sections (parabola, hyperbola or ellipse). Planets and comets are acted on by the inverse-square force of gravity, which always points towards the sun, so they follow ellipses. (They don't have enough energy to follow hyperbolic or parabolic orbits.)

<i>True, they can maintain their ellipse orbits by changing their rotational speeds steadily, but how (that of comets is the oddest especially)?</i>

I don't know what you mean by this. Can you explain, please?

True, they can maintain their ellipse orbits by changing their rotational speeds steadily, but how (that of comets is the oddest especially)?Thnk about it. When something is going up, it slows down. When it's coming down, it speeds up.

When a comet is heading toward the Sun, it speeds up. When it's heading away, it slows down.

It's a continuous exchange between gravitational potential energy and kinetic energy.

From Dictionary of Astronomy: The orbits pursued by bodies moving under the influence of a gravitational field are conic in nature (conic sections). For example, if we imagine launching space vehicles from a given height, h, above the Earth (and in a direction parallel to the Earth's surface), there are five possibilities.

(1) If the velocity is less than circular velocity, the craft will strike the Earth's surface.
(2) If the velocity is precisely equal to the circular velocity, a circular orbit will result.
(3) If the velocity is greater than circular velocity but less than escape velocity, the craft will follow an elliptical orbit.
(4) If the velocity is precisely equal to escape velocity, a parabolic trajectory will be pursued.
(5) A velocity in excess of escape velocity results in a hyperbolic trajectory.It should be noted that the source, Iain Nicolson's Dictionary of Astronomy, is over 25 years old.

Perhaps these links will explain it more clearly, though the references for the second are forty and fifty years old. Maybe I shouldn't worry about the age of Nicolson's volume.

- Eccentricity (http://scienceworld.wolfram.com/physics/Eccentricity.html)
- Elliptical Orbit (http://scienceworld.wolfram.com/physics/EllipticalOrbit.html)

Enjoy. I'm not a fluent mathematician and should not attempt any more detailed contribution at this time.

- Nicolson, Iain. Dictionary of Astronomy: Terms and Concepts of Space and the Universe. New York: Barnes & Noble, 1980.

:m:,
Tiassa :cool:

Not quite. Kepler discovered that the orbits of planets (and comets?) are elliptical, but did not explain why. Newton did that some time later.

to Pete

I think the one about the exchange of potential gravity energy and kinetic energy makes quite sense to me, thanks...
Still, it's quite funny to think that they don't travel in circle instead, in which they are supposed to have constant energy (and movement).....
Oh, wait....... do the forces of other orbitting matters also cause this ellipse orbit phenomena?

Originally posted by curioucity
to Pete

I think the one about the exchange of potential gravity energy and kinetic energy makes quite sense to me, thanks...
Still, it's quite funny to think that they don't travel in circle instead, in which they are supposed to have constant energy (and movement).....
Oh, wait....... do the forces of other orbitting matters also cause this ellipse orbit phenomena?

Objectss in orbit always have a constant energy regardless of the shape of the orbit. Elliptical orbits are the norm. In fact, a circular orbit is just a elliptical orbit with an eccentricity of exactly zero. And since an ellipse can have any eccentricity of 0 to 1, the chances of an orbit having an eccentricity of exactly one is extremely small.

Hi, a very insightful question.

Inertial orbits are circular. For an orbit to be anything other than circulat, energy must have been added or taken from the system.

Since all the solar bodies are moving OUT in their orbit, this outward force causes an elliptical orbit with precession.

from Electrodynamic Spin Gravity Theory.

:)

Why are the orbits elliptical ? Well simply, because they are ;), and because, as James pointed out, the only solutions to inverse-square laws like gravitational interaction are the conic intersections. Some people give arguments for circular orbits, so I guess it is time I give one why there should be elliptic orbits.

Gravity in outer space is just the same as gravity on earth. If you let something go, it will drop towards the heavier mass (on earth this would be the earth that attracts everything, in our solar system the sun). The earth (and other planets) literally falls towards the sun! It passes the sun, but the path gets curved along the sun. After the gravitational "sling", you have a slight overshoot, so you move a bit further away before the sun's gravity wins again and throws you in for another gravitational sling. This "moving a bit further away" stretches the orbit to an elliptical orbit, putting the sun in the focal point of the ellipse.

This might sound a bit plastic, but this is really what happens. Just like all the satellites, space stations are continously in free fall around the earth, so are we around the sun.

Bye!

Crisp

SORRY

>>Gravity in outer space is just the same as gravity on earth. If you let something go, it will drop towards the heavier mass

This just isn't what space probes experience.. You should check NASA's sites re orbits and trajectories.

Basically space does not behave as Newton thought.

:)

Originally posted by Zarkov
SORRY

>>Gravity in outer space is just the same as gravity on earth. If you let something go, it will drop towards the heavier mass

This just isn't what space probes experience.. You should check NASA's sites re orbits and trajectories.

Basically space does not behave as Newton thought.

:)

Okay, here's a Nasa site directly related to orbits and trajectories,

http://www.hq.nasa.gov/office/pao/History/conghand/traject.htm

Take particular note of the following quote from that site:

"The basic types of paths in space are determined by the gravitational-attraction properties of concentrated masses of material and the laws of motion discovered by Newton."

Originally posted by Zarkov
This just isn't what space probes experience.. You should check NASA's sites re orbits and trajectories.

Basically space does not behave as Newton thought.

It does up to a very high degree of accuracy. But I do not want to drag in general relativity here ;)

>> It does up to a very high degree of accuracy

I agree the Universe has a very high degree of accuracy, if you know what you are doing, BUT, objects sent into space do not necessarily behave as the layman might think.

For example, let us fire a rocket from Earth to the Sun, easy ??? NO it is not.

I was talking of orbital characteristics, as in how to attain them, how to move from one sysyem to another, I have no time for GR and it's fantasies.



:)

"For example, let us fire a rocket from Earth to the Sun, easy ??? NO it is not."

Sure it is. You point the big pointy thing on top of your rocket towards the sun, give it a slight push and gravity will do the rest!

"I was talking of orbital characteristics, as in how to attain them, how to move from one sysyem to another, I have no time for GR and it's fantasies."

Clearly, you have not heard of the perihelion shift of mercury and how GR explains that. I would call that quite an important orbital characteristic.

Originally posted by Zarkov
>> It does up to a very high degree of accuracy

I agree the Universe has a very high degree of accuracy, if you know what you are doing, BUT, objects sent into space do not necessarily behave as the layman might think.

For example, let us fire a rocket from Earth to the Sun, easy ??? NO it is not.

:)

Take your rocket, point it in the opposite direction of the Earth's orbital velocity and fire it until it has achieved a Delta V of 30km/sec.

It will now fall into the Sun. Simple, and all within the Laws of Newton.

Yes that is correct... it took scientists a while to work that out. I am an advocate of an altered Newtonian gravity..

see Electrodynamic Spin Gravity, this section.

The point I was making was that you just can not hit the Sun by firing a rocket directly at the Sun.

:)

Originally posted by Zarkov
Yes that is correct... it took scientists a while to work that out. I am an advocate of an altered Newtonian gravity..

see Electrodynamic Spin Gravity, this section.

The point I was making was that you just can not hit the Sun by firing a rocket directly at the Sun.

:)

But since the answer I gave is based on un-altered Newtonian Gravity and Physics, there is no reason for an altered version to explain it (except for those modifications involving GR).

Oh, and you could hit the sun by firing a rocket directly at it, it would just take a lot more energy to do so.

>> Oh, and you could hit the sun by firing a rocket directly at it, it would just take a lot more energy to do so.

How much energy have you got ??

mmmh, just because one theory can be used in one instance does not make it universal.

Just as an aside, maybe best answered in the ESG thread.... explain why the Moon stays in orbit around the Earth, when the Sun "attracts" it twice as strongly as the Earth (in Newtonian gravity).

Just a little exercise.

:)

:rolleyes:

Originally posted by Zarkov

mmmh, just because one theory can be used in one instance does not make it universal.


Newtonian Physics doesn't just work in this one instance, It is universally applicable (within the aforementioned limits imposed by GR)


Just as an aside, maybe best answered in the ESG thread.... explain why the Moon stays in orbit around the Earth, when the Sun "attracts" it twice as strongly as the Earth (in Newtonian gravity).

Just a little exercise.

:)

The answer to this is the fact that both the Earth and Moon are in freefall around the Sun. As a result, The acceleration due to the gravity of the Sun is the the same for both. (There are slight variations due to the fact the the Moon is sometimes slighty closer or further form the Sun, this only has the effect of changing the shape of the Moon's orbit around the Earth slightly. )

Since the acceleration on both due to the Sun is the same, it can safely be ignored if you are viewing the system from the reference of the Earth, and you get the Moon following an eliptical path around the Earth.

If on the other hand, you consider the Moon's heliocentric path, you would note that it follows a very shallow sinewave shape that weaves in and out on either side of the Earth's orbit. ( it does not perform little "loop the loops".

Here is where the factor of the Sun's relative pull will come into play, if you check the curve of this path, you will note that it is always concave to the Sun. (The Moon is always falling towards the Sun)

So from this view, one could say that the Moon and Earth co-orbit the sun and mutually perturb each other.

The scenerio would look something like this:

The Moon starts directly behind the Earth and with a slight anti-sunward component to its velocity As it swings outward of the Earth, the Earth's gravity pulls it forward, accelerating it as it does so. It passes the Earth on the side away from the Sun and starts to pull ahead of the Earth, the Earth is now pulling the Moon back towards the Sun and starts to slow it down. By the time it gets directly ahead of the Earth it is now moving sunward, and is staring to lose speed compared to the Earth. When the Earth catches up with it, it will be about 384,000 km sunwards of the Earth, and lost its sunward velocity vector. As it falls behind the Earth, the Earth's pull on it will start pulling it away from the Sun and speed it up again, until it ends up behind the Earth again, like it started before, just further along in the Earth's orbit.


Again, from the Earth's point of view, the Moon simply follows an elliptical orbit around it.

In order for the Sun to rip a moon out of orbit away from a planet, the moon must be further than the following distance from the Planet.

R = Dsp(Mp/Ms)^(2/5)

Where Dsp is the distance from Sun to Planet,
Mp is the mass of the planet
Ms is the mass of the Sun.

For the Earth, this distance is 927,000 km or about 2.4 times the present distance of the moon.

Again, all of this is easily explainable through an understanding of Newtonian Physics.

whoa!:confused: :eek:

Does this mean that the moon is in elliptical orbit, and the earth is located in the elliptical focus which is farther from the sun (you know, ellipse has two focuses)?

Originally posted by curioucity
whoa!:confused: :eek:

Does this mean that the moon is in elliptical orbit, and the earth is located in the elliptical focus which is farther from the sun (you know, ellipse has two focuses)?

It means that the Earth and Moon share the same orbit around the Sun, the Moon just weaves in and out from the sun with as it does so, moving from one side of the Earth to the other. This is the Moon's Heliocentric path. (as measured from the Center of the Sun)

The two foci of the Earth's orbit around the Sun are only about 5 million km apart, which places the second focus well inside the orbit of Mercury.

The Earth is located at the focus of the Moon's geocentric path. The Moon's geocentric path has it traveling an ellipse around the Earth.(technically, the focus is located just under the crust of the Earth, and the Earth's center traces a small ellipse around it in step with the Moon. )

BTW, the second focus of the Earth-Moon system is located about 32,000 km above the Earth's surface.

Originally posted by James R
curioucity:

<i>Why are planets, comets etc orbiting the sun in elips?</i>

It is possible to prove mathematically that the path of any object acted on by a central, inverse-square force, must be one of the three conic sections (parabola, hyperbola or ellipse). Planets and comets are acted on by the inverse-square force of gravity, which always points towards the sun, so they follow ellipses. (They don't have enough energy to follow hyperbolic or parabolic orbits.)

<i>True, they can maintain their ellipse orbits by changing their rotational speeds steadily, but how (that of comets is the oddest especially)?</i>

I don't know what you mean by this. Can you explain, please?

Not understanding a word could I just say one? Gravity.

>> It means that the Earth and Moon share the same orbit around the Sun, the Moon just weaves in and out from the sun with as it does so, moving from one side of the Earth to the other.


Electrodynamic Spin Gravity would not agree with this "obvious" conclusion..
There is far more to the Moon's orbit than that.

Try applying this false logic to the moons of say Neptune.

Nope the Earth-Moon system is complete in itself, and does not require the Sun at all.

:)

Mucho grassy ass for that, Zark!!

Did you know that the moon is leaving the earth at the rate of 1 1/2 inches per year? I found that out on NASA site.

Hi Devil, yea most if not all of the moons are moving away from their parent, and so I suspect are all the planets, away from the Sun parent.

:)

Originally posted by Zarkov
>> It means that the Earth and Moon share the same orbit around the Sun, the Moon just weaves in and out from the sun with as it does so, moving from one side of the Earth to the other.


Electrodynamic Spin Gravity would not agree with this "obvious" conclusion..
There is far more to the Moon's orbit than that.

Try applying this false logic to the moons of say Neptune.

Nope the Earth-Moon system is complete in itself, and does not require the Sun at all.

:)

Don't put words in my mouth. Where did I say that the Earth-Moon system required the Sun?

It was you who brought up the effect of the Sun's Gravity on the Moon, in your question. I was pointing out that you cannot consider the Sun's gravitational effect on the Moon in isolation. You have to consider the effect on the whole Earth-Moon system. In which case, you have two options:

You can consider the things with respect to the Sun, in which you deal with the heliocentric paths of the Earth and Moon, which is what I was discussing in the passage you quoted.

OR

You can consider things with respect to the Earth-Moon system itself, in which you only need consider the differential between the Sun's gravitational effect on the Earth and Moon. (not the difference between the gravitational effect of the Sun and Earth on the Moon) Since the relative distance between the Earth and Moon is very small compared to the distance between the Earth-Moon system and the Sun, this differential is too small to do anything other than reshape the Moon's orbit around the Earth. (Essentially the effect here is to elongate the Moon's orbit along the Earth-Sun line. Thus when the major axis of the Moon's orbit is near parallel with this line, the eccentricity on the Moon's orbit around the Sun is increased, and when it runs perpendicular to this line, the eccentricity is decreased. The strength of this effect also varies over the course of the year as the Earth-Moon sytem varies its distance form the Sun)

So, while existance of the Sun isn't required for the Earth-Moon orbital system's existance, The existance of the Sun requires that we consider its effects on the Earth-Moon system if we are to get a complete picture.

As far as Neptune's moons go, they also can be analyzed in terms of their respective heliocentric paths. The fact that they orbit Neptune nearly perpendicular to Neptune's orbital plane, merely means that their heliocentric pathes are a little more complex, that's all.

Again, they can also be analyzed in terms of the Neptunian sytem itself, with consideration made to the Sun's effect. ( a much smaller effect due to the distance of the Neptune from the Sun.)

Either method is equally valid, one is just easier to put in practice than the other.

Zark, serious question from a person unschooled in these matters - if the earth is possibly pulling away from the sun, albeit at a fractional rate, why are we not cooling - instead of talks of global warmth. Or - is man compensating by screwing up the planet ourselves?

Originally posted by Red Devil
Zark, serious question from a person unschooled in these matters - if the earth is possibly pulling away from the sun, albeit at a fractional rate, why are we not cooling - instead of talks of global warmth. Or - is man compensating by screwing up the planet ourselves?

First you need to understand why the moon is moving away from the Earth.

It is due to the Tides. As the Moon creates tidal bulges on the Earth, these bulges are pulled forward by the Earth's rotation. As the Moon tries to pull the bulges back into alignment, it produces a torque which slows the Earth's rotation and tranfers angular momentum to the Moon. As a result, the Moon is pushed into a higher orbit.

Now, for the Earth to do the same with respect to the Sun, the same mechanism would be involved. Now the Earth is 81 times more massive than the Moon, so for that part, its tidal effect on the Sun would be larger. But the Earth is also 400 times further away from the Sun than the Moon is from the Earth, and tidal effects fall off by the cube of the distance. Thus the Relative tidal effect of the Earth on the Sun is much smaller than that of the Moon on the Earth, so Earth's recession rate would be much smaller than the Moon's.

But let's say, just for the sake of argument, that the Earth receded at the same rate as the Moon, 4cm a year. At that rate, in 1 million years, the Earth would be 40 km further away from the Sun than now.

That increase would be to its average distance. Since the Earth already travels around the Sun in an ellipse, its distance from the Sun varies by 5 million kilometers over the course of a year. A 40 km increase in average distance would not be noticeable. (Especially considering the fact that the even the 5 million kilometer variation has a negligible effect on Earth's temperature. The Earth is actually closest to the Sun during the Northern Hemisphere's winter.)

In fact, the Earth varies it's distance from the Sun by a few thousand km every few weeks, as it and the moon do their monthly waltz around their mutual center of gravity.

40 km over 1 million years would be a drop in the ocean.

Thanks for that - appreciated. I might use that on a page on my domain about the moon leaving orbit, albeit slowly ;) Very well put sir!

Originally posted by Red Devil
Thanks for that - appreciated. I might use that on a page on my domain about the moon leaving orbit, albeit slowly ;) Very well put sir!

You're welcome.

One thing, the Moon will never actually leave Earth orbit. Once the Earth's Rotation has slowed to the point that it matches the period of the Moon's orbit, the Moon will stop receding. This will happen long before the Moon gets far enough away to leave orbit.

There is no guarantee that this situation will ever come about either. As the sun ages it will eventually swell into a Red giant, possibly all the way out to Earth's orbit, vaporizing the Earth and Moon in the process. This could happen before the Moon reaches its maximum distance from the Earth.

What is interesting is to examine what would continue to happen if the Earth and Moon did survive for an extended period.

After the Earth's rotation matches the Moon's orbit, the Sun continues to have a tidal effect on the Earth, working to slow the Earth's rotation further. This will cause the tidal bulge to lag behind the Moon. The result will be to pull the Moon back in to a lower orbit.

If this process were allowed to continue until its conclusion, the Moon would eventually move close enough to the Earth to break up into a ring system.

Of course, it all becomes moot if the Earth and Moon don't survive the Sun's death pangs in the first place.

The NASA page that I read eons ago stated that the moon was originally 15 times closer to the earth when it formed. Imagine moonrise on that basis!! Talk about fill the sky!!

Here's my url for what I read:

http://www.mikekemble.com/space/discovery2.html

I would be interested in your comments, but you have already stated some excellent points.

Red Devil, the Moon of the Earth is a queer one, at approx 1% of Earth mass, it is massive compared to other moons/planets (usual 0.1%).

I believe from evidence that the satellites of the Sun, and the son's of satellites, moons, are brought about by a fission method (ejection). This would imply that maybe the Moon was ejected by the Earth, or it could have been ejected from the Sun approximately at the same time the Earth was ejected.

The Moon being of less geomagnetic mass than the Earth, so it was spun in orbit by the Earth's field.

Yep, the Moon is going away from the Earth and therefore would have been a lot closer many whiles ago.

:)

First I think I want to apologize if this thing has been stated somewhere in the thread, because I don't read that deatiled...

You said that the revolving moon slows down the earth. How can this be? Is it because the moon's revolution's direction is against earth's rotation's? Or what?

action & reaction , for every action by the moons gravitational pull there is reaction from the earths gravitational pull. And vice versa.

Janus: I have updated that page with your replies ;)