Eliptical orbits.

[peter]

Quick question.

Keplers first law states that a planet moves in an eliptical orbit s around the sun with the sun at one of its foci.

Can anyone tell [or even better, show] me how 'eliptical' the eliptical orbit of earth is. If I were staring down on it from above, would it be nearly circular, but just a bit flatter or noticably sort of 'egg degree' flatter.

And [second question if you're an accountant] are the eliptical orbits of all of our companion planets in the solar system the same 'degree' of elipse as the others [and ours], or do we all have our own particular amount of elipse and if so [third] what causes this?

[Worked out in my head in the bath this morning (as an excercise in self-dementia checking) that the speed of the earth as it moves around the sun is [very approximately] 70,000 miles per hour. Was I in striking distance?]

[Zarathustra]

Just off the top of my head (no Google check), I'm pretty sure the orbit of the earth is very nearly circular, so that you wouldn't be able to tell from 'looking from above.' If this weren't the case, we'd notice huge shifts in the temperature. For instance, in the northern hemisphere, it would actually cancel out the effect of the earth's tilt on the seasons, since we're closer to the sun during our winter.

I'm not sure of the orbital speed, but you're probably in the ballpark.

I'd say each planet has its own elliptical orbit. As to what produces this, it's probably the effects of all the bodies in the solar system interacting, tugging, etc., as the solar system settled down to its current order. I know that Jupiter migrated from closer to the sun, then back out again. But even if there were only one planet in the solar system, it would be very surprising if it settled down to a perfectly circular orbit. That's just one special case of an ellipse, and very rare, I'd imagine. Nature isn't perfect.

[Vraith]

Yea, earth is quite close to circular. I think??? it's only off by 3%-ish from perfect, so I doubt you could detect it without instruments.

And each has it's own variations.
Some are highly elliptical...if you include comets, some are radically elliptical.
And z is also right that all the planets tug on each other, and various shiftings have taken...and are taking...place.

There are a lot of causes of the variations...but almost all of them are, one way or another, related to the perpetual tuggings, and collisions or near collisions between massive bodies.

your speed calculation matches my vague memory [my memory says 'approximately 65k.']

As an aside...I believe [again vague recall] the Kepler's calculations work descriptively...you can get the orbit/draw the "map" and be correct...but
physically it isn't what is happening. What is happening is that the Sun and the Earth are both orbiting a point between the two. The Sun is far more massive, so that point is inside its body...but not exactly at it's center.
[heh...super-technically, even that isn't really what's happening...since everything is moving through space. It's probably pretty close on human scales...but long term, it probably makes a spirograph kind of thing.]

[peter]

And I suppose in terms of Einstein you'd have to think in terms of warped regions of space-time all circulating and fluctuating around each other [do space time variations have that same effect as light waves do.....'interference' is it called? - what a 'cosmic dance' that would turn out to be! Curioser and curioser as Alice said ].

[wayfriend]

152.5m is 3.4% further than 147.5m.

[Vraith]

And I suppose in terms of Einstein you'd have to think in terms of warped regions of space-time all circulating and fluctuating around each other [do space time variations have that same effect as light waves do.....'interference' is it called? - what a 'cosmic dance' that would turn out to be! Curioser and curioser as Alice said ].

Hee...I never thought of that question. Now I'll probably have to look around and find out.
But my impression/intuition/total guess would be:
If gravity functions like other forces...then there should be gravitons, and also gravity waves, so there SHOULD be similar effects/occurences. Including interference. Which is weird...two gravity waves meet synchronized, boom, gravity spike!. opposite---bang! Zero-grav!
That would be one hell of a bumpy ride...if they were big ones. I suspect, since gravity is so weak, that the only places you'd see waves that size is a place where two [or more] neutron stars or black holes were really really close together and moving really damn fast [without the velocity, they'd be one object]...and to feel them, you'd have to be pretty close by. Close enough to get ripped up, I bet.
Of course, some say gravity isn't like other forces...it definitely causes some model conflicts right now.
Peeps are looking for gravity waves, though...or at least signs that there have been gravity waves at least once, at the beginning of the universe.
Some say it isn't a force at all.
Still...even if not wave-like interference, curved/folded and moving fabric of space time has to do something. Perhaps the massive objects/gravity wells ARE the "interference pattern" of that fabric.

[Avatar]

Peter: www.shatters.net/celestia

Turn on the orbit lines and you can look down from "above" and see for yourself.

--A

[peter]

Good posts Guys and nice link Av! I'm off to play in space for a while .

[Avatar]

Haha, have fun. There's all sorts of add-ons you can download. Better textures, asteroids, Kuiper belt objects, all sorts of stuff.

If you know the date of an eclipse, you can go to that date and watch the shadow of it cross the earth, or you can follow Halley's comet along its orbit or whatever.

--A