# Orbital eccentricity

Metal Brain
Science 14 Aug '13 13:56
1. 14 Aug '13 13:56
After looking into the Milankovitch Cycles I was trying to understand why Orbital eccentricity occurs but it was hard for me to find an explanation that was easy to understand.

https://en.wikipedia.org/wiki/Orbital_eccentricity

I am a bit mathematically challenged so could someone on here please explain it to me as if I were a child?
2. 14 Aug '13 14:20
Originally posted by Metal Brain
I am a bit mathematically challenged so could someone on here please explain it to me as if I were a child?
All stable orbits in a two body system are ellipses.

To help visualize it:
If you attach a weight to rubber band and swing it around you will find you can get it to go in an ellipse. This is different from a string which enforces a circle, and different from gravity which is weaker with distance, whereas a rubber band is strong with distance, so the rubber band will very quickly change the orbit to a circle unless you swing your hand.

If you have a body (planet) that is under the influence of the gravity of another much more massive body (sun) and not directly moving towards it, the planet will accelerate towards the sun, whilst maintaining a constant additional velocity as before. So it will curve towards the sun, but miss. When it misses, is is now moving away, but will continue to curve, as it is now being pulled back at an angle to its path. So it now slows down.
If the planet is within orbital velocity, it will eventually slow down to the point where it 'falls' back and starts to accelerate towards the sun again.
This overall motion creates an ellipse.
If the bodies are perfect non-rotating spheres with nothing else involved, you should get a perfect ellipse that never changes, however when you have many bodies and tidal forces etc, then it gets more complicated hence Milankovitch cycles.
3. sonhouse
Fast and Curious
14 Aug '13 14:21
Originally posted by Metal Brain
After looking into the Milankovitch Cycles I was trying to understand why Orbital eccentricity occurs but it was hard for me to find an explanation that was easy to understand.

https://en.wikipedia.org/wiki/Orbital_eccentricity

I am a bit mathematically challenged so could someone on here please explain it to me as if I were a child?
One way of looking at it is the gravitational attraction between two bodies. You know the moon and the Earth would collide with each other at its present distance of about a quarter million miles if it were not in an orbit, so they have to be in an orbit to avoid that collision.

So lets look at a body in space, say a comet coming by the sun but it came from a trillion miles out in space. All things considered, lets say it is going very fast compared to the sun and will approach within 1 billion miles, about the distance to Jupiter. So visualize that line of approach, closest it will get is one billion miles. That is not the end of the story however. As it approaches, the path it has been traveling will bend in towards the sun.

So it comes to within a billion miles of the sun and say, just for grins, it exits the solar system but not in the straight line it was travelling before.

Now it has been deflected by some angle, say 30 degrees offset from the direction it was going before.

So you can call that the beginning of what it is to be in orbit around an object. In this case there is no orbit because it goes on its merry way but just in a different direction and its velocity is high enough it will never be captured by the sun and just go on forever unless it encounters another star.

So consider the same comet but now it comes closer to the sun and slower.

This time it has a certain velocity as it approaches the sun and at its closest approach say it is now at the orbital distance of mars. Now it swings by the sun and it changes direction but all the way round the sun. Now it might be captured by the sun and be in an orbit but the velocity it was traveling was too great to be put in to a perfect circle orbit but now goes way out, say to Jupiter, 1 billion miles out but the sun has in fact captured it and it slowly swings around and comes back towards the sun but in more or less the same path as before and that orbit starts to be a steady thing, in to the distance of mars, out to the distance of Jupiter.

That is the eccentric nature of the orbit, if you measure the longest excursion, it is about the distance from the sun to Jupiter plus the distance to mars, say 1.1 billion miles long, that part of the orbit.

But if you look at the width of the orbit it might only be 200 million miles wide so in that case the eccentricity is about 5 to 1.

Now suppose you were in a spacecraft and wanted to be in a circular orbit.

You now fire your rockets to make the long path closer to the sun and the short path apart a bit further apart, you keep doing that for a few dozen orbits and pretty soon the longest excursion is say 300 million miles and the width is now still around 200 million miles. Now you are approaching a circular orbit where the eccentricity is now a ration of 3 to 2.

See, the eccentricity is getting smaller and smaller. So a few dozen orbits later you have both the longest path away from the sun equal to the shortest path of the orbit and now it is a perfect circle, where both min and max are say 250 million miles, a circle with that diameter. Now the eccentricity is zero, there is no difference anywhere in the orbit that is a different distance to the sun, in this case it will be forever 125 million miles away from the sun and will stay in that orbit till something disturbs it, like some kind of rogue planet coming in from left field, from out of our solar system but this guy gets close enough to our little comet to screw up its nice circular orbit and when the rogue planet leaves, now the orbit is no longer a circle but an ellipse and we are back to having some kind of eccentricity, say now it is 300 million miles max and 240 million miles minimum distance from the sun for a new ration of 3.0 to 2.4. So that is its new eccentricity. And it will stay right there with that same orbit till something comes along to disturb it.

Does all that make sense?
4. 14 Aug '13 15:211 edit
Originally posted by Metal Brain
After looking into the Milankovitch Cycles I was trying to understand why Orbital eccentricity occurs but it was hard for me to find an explanation that was easy to understand.

https://en.wikipedia.org/wiki/Orbital_eccentricity

I am a bit mathematically challenged so could someone on here please explain it to me as if I were a child?
It might help to think of circles as not being distinct from ellipses,
but as being special case ellipses.

In the same way that squares are special case rectangles.

Having an elliptical orbit is easy, but a circular one requires exquisite
fine tuning that is incredibly unlikely to arise by chance.

And it's also unstable in any multi-body system as there will always be
some gravitational component pulling the orbit out of circular.

For example, if we just consider the Earth, Sun, Jupiter, system.
You can see that if we start the Earth in a perfectly circular orbit that orbit
is going to be disrupted by Jupiter.

When the Earth is between the Sun and Jupiter, Jupiter's gravity is pulling
the Earth away from the sun counteracting and weakening the Suns pull on
the Earth. Pulling the Earth outside of a perfectly circular orbit.

Conversely, when the Earth is on the far side of the sun from Jupiter the
Sun and Jupiter's gravity combine to pull harder on the Earth. Pulling the
Earth inside of a perfectly circular orbit.

More importantly...

When the earth is heading 'towards' Jupiter in it's orbit Jupiter's gravity is
helping to accelerate the Earth causing it to move outside of a perfectly
circular orbit.
And when the Earth is moving 'away' from Jupiter around it's orbit Jupiter's
gravity is decelerating the Earth causing it to fall inside of a perfectly
circular orbit.

And of course in reality the Earth has a moon, which means that both the Earth
and the Moon are swinging around their common centre of mass, while also
being pulled on unequally and variably by all the planets and asteroids in the solar
system, and all the stars and other matter beyond it.
5. 14 Aug '13 15:24
All stable orbits in a two body system are ellipses.

To help visualize it:
If you attach a weight to rubber band and swing it around you will find you can get it to go in an ellipse. This is different from a string which enforces a circle, and different from gravity which is weaker with distance, whereas a rubber band is strong with distance, so the r ...[text shortened]... ave many bodies and tidal forces etc, then it gets more complicated hence Milankovitch cycles.
Are you saying that the gravity from the other planets are changing the orbit of the Earth to become more elliptical?

Here is a link I was looking at. It says "Eccentricity is the change in the shape of the earth's orbit around the sun."

6. 14 Aug '13 15:35
It might help to think of circles as not being distinct from ellipses,
but as being special case ellipses.

In the same way that squares are special case rectangles.

Having an elliptical orbit is easy, but a circular one requires exquisite
fine tuning that is incredibly unlikely to arise by chance.

And it's also unstable in any multi-body sys ...[text shortened]... ts and asteroids in the solar
system, and all the stars and other matter beyond it.
I suspected that is what it was, but all the links I looked at never explained that.
I remember many years ago people were making a big deal about all of the planets in the solar system being on one side of the sun. I think some people were predicting tidal waves and earthquakes and other things that didn't happen. Is that the sort of thing that would make the Earth's orbit into a greater aphelion distance if the planets were aligned just right?

7. 14 Aug '13 15:43
This is the link about ice ages occurring every 100,000 years that got me thinking about the Milankovitch cycles. It is interesting.

http://www.sciencedaily.com/releases/2013/08/130807134127.htm
8. sonhouse
Fast and Curious
14 Aug '13 16:13
Originally posted by Metal Brain
This is the link about ice ages occurring every 100,000 years that got me thinking about the Milankovitch cycles. It is interesting.

http://www.sciencedaily.com/releases/2013/08/130807134127.htm
There is another effect that changes the eccentricity: Relativity. The orbit of Mercury changes by a few hundreths of a degree due to going closer and further from the sun, going deeper and less deep into the gravity well around the sun. It was a key prediction by Einstein that gave the first proof of relativity, he explained why the orbit of Mercury was precessing like it did.

So an elliptical orbit has a lot of variables, other planets, the gravitational field it is sitting in and the changes it encounters as it swings in and away from the sun.
9. 14 Aug '13 16:33
Originally posted by sonhouse
There is another effect that changes the eccentricity: Relativity. The orbit of Mercury changes by a few hundreths of a degree due to going closer and further from the sun, going deeper and less deep into the gravity well around the sun. It was a key prediction by Einstein that gave the first proof of relativity, he explained why the orbit of Mercury was pr ...[text shortened]... onal field it is sitting in and the changes it encounters as it swings in and away from the sun.
And as Googlefudge pointed out, the Moon and to a much lesser degree the stars and matter beyond it. Basically all matter.
10. sonhouse
Fast and Curious
14 Aug '13 17:54
Originally posted by Metal Brain
And as Googlefudge pointed out, the Moon and to a much lesser degree the stars and matter beyond it. Basically all matter.
It's a case of mind over matter: If you don't mind, it doesn't matterđź™‚
11. 14 Aug '13 20:45
Originally posted by Metal Brain
Are you saying that the gravity from the other planets are changing the orbit of the Earth to become more elliptical?
I didn't, but googlefudge said that circular orbits would not stay that way.

All I said was that the basic laws of gravity result in an elliptical orbit. The planet is continuously falling and bouncing out of the gravity well of the star.

Here is a link I was looking at. It says "Eccentricity is the change in the shape of the earth's orbit around the sun."