h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This is a follow on question from my last.

The Earth orbits the Sun due to gravitational pull.

So why isn't the orbit circular?

Hunch, I maybe barking up wrong tree...Is it something to do with a kind of 'sling' effect?

If someone could use an analogy or modelling again as an answer, I will understand that much better I think.

But any answer will be appreciated.

I'd quite like to see a good modelling type answer too.

I *think* it's a combination of the orbiting object's velocity (which provides a sling effect) and the fact that the centre of the orbit itself isn't static but itself moves around a central point. Plus other stuff like how gravity isn't a 'force' per se but a curvature in spacetime. Entropy is possibly involved as well as a circular orbit going around a fixed point would be a very perfect and very ordered way of doing things and while there is less entropy than one might expect in the observable universe it's by no means an ordered system.

So I'll join you in hoping someone can provide an answer that can be easily modelled.

Typical diagrams of the Earth's orbit around the sun either grossly overestimate the actual ellipticity of the orbit or show the orbit at an oblique angle. On a properly scaled diagram the orbit looks circular rather than elliptic with the Sun displaced from the centre.

I'm not talking about about what diagrams look like. I'm talking about *science*. This is SEx.

The orbit is *elliptical*. I'm wondering why and hoping someone can explain the answer in a way I can easily access.

Why do you think the Earths orbit around the sun should be circular ... or why doesn't the Earth just fall into the sun? You are looking for a reason as to why the Earth's orbit is elliptical rather than circular yet you are not asking for the reason why you should naturally think the Earths orbit should be circular.

Gravity is an attractive force along the direct "line" joining the centres of the two masses. If the masses don't have any inertial motion perpendicular to that line then they will fall directly into each other and hit.

If they do have inertial motion perpendicular to this direct line then the two masses will keep falling towards each other and miss, falling towards each other and miss. In general the equations of motions will be in the form of an elipse - they fall inwards and miss.

After missing, their inertia takes them away from each other, but they "slow down" (in the "away" direction) due to their gravitation attraction, and then fall inwards again, and miss again ... "ad infinitum".

> can explain the answer in a way I can easily access.<

Love the way your intelligence took that statement onboard...given my previous posts here.

My logic is presumably understood by others here hopefully. That the earth's mass doesn't alter nor the the sun's to any degree that would affect gravity....so why isn't it circular?

There are people here that will understand the reason for my question though...

This post has been removed.

Calm down Effers, don't alienate those toe rags - they're not mind readers and are trying to help in ways that they understand.

Try this:

Tie a string round a tennis ball & go into the garden. Swing the ball around your head.
The ball's the earth, your hand's the sun, the string is gravity, the orbit is circular.

Walk down the garden, still swinging. Can you imagine that the orbit is now slightly elongated at each iteration?

OK, it's not really that way but perhaps that way illustrates that elliptical orbits are the result of the complex reactions between the bodies, their relative motion and, gravity.



Phew! Does that make sense?

I can't do the maths, but that explanation was enough to get me started.

The simple answer? Nothing's perfect.

The slightly more complex answer: the earth *doesn't* orbit the sun. The sun and the earth orbit *each other*.

If you could pin the sun to spacetime, if you could make it a fixed, invariant point, then you could make the earth go round it in a perfect circle.

But you can't. The sun, just like the earth and everything else, moves.

For another mental image, picture a hammer thrower. He/she is much, much bigger and heavier than the hammer... but in the final spins before release, the thrower isn't spinning on the spot like an axle. They're spinning AROUND a spot somewhere between them and the hammer.

So it is with the sun and earth. We are partners in a dance. http://en.wikipedia.org/wiki/File:Orbit3.gif

Sorry, Rod, that explanation is erroneous.

What you've described there is an orbit which is circular relative to the centre, and only elliptical with respect to some outside observer. In your example, the tennis ball is the same distance from you at all times (the length of the string).

That's not what's happening with the earth. The earth's orbit really is elliptical. We really ARE about three MILLION miles closer to the sun right now than we were six months ago. (Yes, we are *closer* to the sun in the winter, in the northern hemisphere).

OK, that wasn't a full answer.

Here's the thing: there are only four things a body can do if you throw it into a solar system:

1. It has a LOT of energy - in this case it zooms into the system, slingshots round the sun and heads back out, never to return.

2. It has not much energy - in this case it fall into the middle, spiralling ever closer to the sun until it falls in.

3. It has an energy somewhere in between - enough to keep it from falling in, but not enough to let it escape. It loops in close to the sun, flies back out away from it, but falls back in again... and keeps doing that forever, more or less. This is what comets do - they spend most of their time in the outer dark, and every now and then fall in close to the sun, zip round, fly back out... then come back again in a few decades, or centuries, or whatever. It's also what WE do - except the difference between our closest approach and our farther away is only about three million miles.

4. You have the absolutely PERFECT amount of energy to do something like example (3)... except your closest approach and farthest away are the same. How likely is that? For starters, you'd need a completely empty solar system, because any other planets or asteroids or whatever will exert a little pull that will soon make your perfect circle not perfect any more. You could still be in a stable orbit - but it will be elliptical, not circular.

And there we go. It's possible, in theory. But you'd need a universe consisting of exactly one star and one planet. As soon as you introduced a third body, it would get complicated.

Hoo.

I'll be on that later for proper study. Seriously thanks.

As a kid I developed a massive interest in science. I've got most of my books still. 'The Universe' was cracking. It's still on my bookcase.

Merry Christmas

"As soon as you introduced a third body, it would get complicated."

Like sex.

B

I think you may want to go through the laws of Kepler.
Read "Astronomia nova", written by Johannes Kepler about 1609, or (possibly easier) google "kepler planetary motion".

the maths are not too hard, but I don't have enough space here to write it in any detail.

#9 "... don't alienate those toe rags ..."

Oxford Dictionary
Toerag (British informal): a contemptible or worthless person.
Origin: mid 19th century: originally denoting a rag wrapped round the foot as a sock or, by extension, the wearer (such as a vagrant)

I do NOT think you need to look into Kepler's laws. It's enough to know that, in an "ideal" universe - i.e. just one start, and one planet, that the orbit could be circular, but that in any system more complicated, tiny perturbations will introduce eccentricities.

In much the same way that tiny perturbations in my childhood presumably...

and

sure, Hoovooloo .. sure ..

* prepares the stake, and stacks firewood around it *

Hoo is almost completely right in posting 12. The only thing he got wrong is his option 2, spiralling into the sun. This doesn't happen. Either it fall straight into the sun, or it goes into an elliptical orbit which is so close to the centre of the sun at the close end that the object actually hits the sun. Not a spiral.