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.
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?