Earths orbit

[Pebbilita]

What makes the earths orbit around the Sun eliptical? Why not a constant distance all the way around? Thank you!

[vascop]

There is no reason according to the Physics why an orbit should not be circular. A circle is just a particular kind of ellipse, and the probability of the conditions being such that the orbit is a circle are much smaller than for an ellipse. It's like saying if I pick up from the floor of a wood two straight sticks, they are ALWAYS going to give me the sides of a rectangle. Now a square is a particular case of a rectangle (when both sticks are the same length). This situation is much less likely to occur than the rectangle situation.
So circular orbits are much less likely than elliptical ones.

[jake-the-peg]

An elipse as a length and a width. A circle is just a special type of elipse where these are equal.

How eliptical the orbit is is measured by it's "eccentricity" which is a related to this.

Earth has an eccentricity of .017, it's only a few percent off of being totally circular

Venus' orbit is much more circular Mercury's is much less so. There appears to be no real rhyme or reason to why some planets have a more circular orbit than others.

[fo3nix]

As Jake says, it's only extreme maths pedantry that makes it elliptical. It is for most intents and purposes, circular.

Same thing for the Earth being a sphere. It's slightly squashed, and of course isn't perfectly smooth. But for most purposes, it's a perfect sphere.

[R1Geezer]

If you drew the orbit on a peice of paper it would look circular, the difference in minor/major access is very small. Probably caused by the gravitaion of other bodies in the solar system pulling it out of shape over the billions of years.

[Pebbilita]

Thanks Geezer! I was wondering what the effect of gravity was. I thougth that if the gravitational force of the sun was constant around itself then the planets should be "pulled" to a constant distance. But I guess that if each planets gravity was affecting another it would have an effect on the shape of the orbit. Thanks! (affect/effect - sorry still not sure which is which! )

[Clanad]

Kepler tells us that all orbits are ellipses. However, if one were to consider only a two body system (Sun and Earth) the elliptical orbit would be quite a bit different than is displayed in reality. A three body system consisting of the Sun, Earth and our Moon actually predicts an orbit of the Earth nearly exactly that which it is.
Many references will state that only the Sun's gravitational force affects the Earth's orbit. However Newton's inverse square law tells us that any and all bodies with mass exert gravitational forces on each other, including our Earth. We know that the Moon contributes to the wobble of our Earth's rotation, for example. Calculations indicate that all of the planetary bodies gravitational forces (especially the gas giant Jupiter) combined produce only an additional 1/10,000th gravitational force as that of the Sun (since the added forces tend to cancel each other out). But, intuitivelyat least, this must contribute to the ellipse.
Additionally, it's unclear if the center of mass of our Sun is actually equivalent to the center of gravity. It's proposed that the center of gravity (due to a number of reasons) is more near the surface on one side of the Sun. This too, (if found to be accurate) could contribute to our slight off center circular orbit...
The final, definitive answer, I suspect will be derived from Kepler, Newton and especially Einstein's demonstration of gravity being a curvature of the space/time fabric.

But that's another discussion, no?

[mibn2cweus]

To understand present day planet compositions, positions, orientation and orbits it might be helpful to study the various mechanisms governing solar system formation and evolution:

http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System

http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/Origin-and-Evolution-of-the-Solar-System.topicArticleId-23583,articleId-23488.html

[mibn2cweus]

. . . AB broke the second link at the comma. You may need to copy/paste the complete url into your browser's address bar:
1. Click & drag mouse through complete url to select, then Copy
2. Click once on address bar to highlight, then Paste, then Enter
3. Complain to AB Editor ;o)