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Denis12
2006-Jan-08, 12:34 AM
I have seen on a chart of the orbits of the planets around the sun that earths orbit around the sun is not so exentric but nearly round,is that true? And which planet has the most exentric and the most round orbit?

Saluki
2006-Jan-08, 01:24 AM
You can find everything you need here:

http://vathena.arc.nasa.gov/curric/space/planets/planorbi.html

As to your specific questions, Venus is the nearest a circle at an eccentricity of 0.007. Pluto has the most exccentric orbit of the currently accepted planets at 0.248. Note that the definition of planet is under some pretty strenuous debate right now, and in the near future, you might find that either (a) Pluto is no longer considered a planet, or (b) there will be one or more additional planets added to the list.

For reference, an eccentricity of 0 would be a perfect circle, an eccentricity greater than 0 but less than 1 is an elipse, an eccentricity of exactly 1 is a parabola (not gravitationally bound by the sun), and an eccentricity of greater than 1 is a hyperbolic orbit (again, not gravitationally bound).

Edit: Missed a question. Earth's orbit is indeed nearly circular at an eccentricity of 0.017, but it is third on the list as Neptune's orbit has an eccentricity of only 0.09.

cjl
2006-Jan-08, 02:07 AM
What he said ^^^^^

Also, the most eccentric bodies in the solar system are comets, which can have eccentricities approaching one. The most eccentric planet other than pluto (since pluto is relatively controversial as far as whether it is a planet) is mercury, with an orbital eccentricity of .206.

The_Radiation_Specialist
2006-Jan-08, 04:38 PM
What about Sedna? Last time I was playing around with Celestia and saw Sedna's orbit was very eccentric. I increased the time and saw that Sena orbit the sun every thousands of years. Is that true? Or a bug in the software?

aurora
2006-Jan-08, 05:46 PM
Denis12, if you read the responses, can you thank the respondents for their answers?

You ask a lot of questions, and express frustration when someone doesn't hop right to work to answer your questions, but you rarely come back and acknowledge the response.

A little politeness would go a long way.

grant hutchison
2006-Jan-08, 05:55 PM
What about Sedna? Last time I was playing around with Celestia and saw Sedna's orbit was very eccentric. I increased the time and saw that Sena orbit the sun every thousands of years. Is that true? Or a bug in the software?It's true. The data Celestia uses come from the relevant Minor Planet Electronic Circular: MPEC 2004-E45 (http://cfa-www.harvard.edu/mpec/K04/K04E45.html). A period of 12260 years, an eccentricity of 0.86.

Grant Hutchison

hhEb09'1
2006-Jan-08, 06:10 PM
I have seen on a chart of the orbits of the planets around the sun that earths orbit around the sun is not so exentric but nearly round,is that true?A path can be very eccentric, and perfectly round.

For instance, using Saluki's figure of 0.017 for the eccentricity of Earth, and a distance to the Sun of 150,000,000 km, the Earth's distance to the Sun differs by 2 times 0.017 times 150,000,000 km, or 5 million km, about 400 times the diameter of the Earth. The Sun is offset from the center of the orbit (eccentric) by half that, in other words. However, if one were to fit a perfect circle to Earth orbit, the radius of the Earth's orbit would differ from a perfect circle by little more than the diameter of the Earth.

grant hutchison
2006-Jan-08, 07:36 PM
A path can be very eccentric, and perfectly round.I'm having difficulty resolving that one. Do you mean something other than "circular" when you say "perfectly round"? Circular orbits have eccentricities of zero, and increasing eccentricities result in increasing deviations from circularity. A value of 0.017 seems like a low eccentricity resulting in a near-circular orbit.

Grant Hutchison

Gsquare
2006-Jan-08, 08:02 PM
A path can be very eccentric, and perfectly round.



Yea, and I have no problem with pain ...as long as it doesn't hurt. ;)

G^2

hhEb09'1
2006-Jan-09, 04:50 PM
I'm having difficulty resolving that one. Do you mean something other than "circular" when you say "perfectly round"?No, I do mean circular.
Circular orbits have eccentricities of zero, and increasing eccentricities result in increasing deviations from circularity. A value of 0.017 seems like a low eccentricity resulting in a near-circular orbit.I would also say that 0.017 is a relatively low eccentricity--but that means that the Sun is offset from the actual center of the Earth's orbit by 2.5 million km--over 200 Earth diameters. That's the meaning of ec-centric, an offset from the center.

However, the circularity of the Earth's orbit about the actual center of the orbit is such that the Earth doesn't deviate much more than one Earth diameter from a perfectly circular path.

PS: I corrected the 200 number to make it in line with my previous post, and calculated that if the semimajor axis were 150,000,000 km, the semiminor axis would be 21,000 km less. So, the Earth path would be no more than 10,500 km from that of a circle 149,989,500 km in diameter. So, that's less than one Earth diameter from the perfectly circular path.

grant hutchison
2006-Jan-09, 05:30 PM
Well, these are just geometric properties of the ellipse: the distance from centre to focus is ae, the semiminor axis is a.sqrt(1-e2). The first distance is always less than the second until the eccentricity gets over sqrt(0.5) = ~0.7.
I see the point you're making, but I guess we're just not interpreting "very eccentric" and "perfectly round" the same way: to me, something that is perfectly round has an eccentricity of zero, by definition; and a very eccentric ellipse would have an eccentricity of more than, oh, 0.5 or so.

Grant Hutchison

worzel
2006-Jan-09, 05:42 PM
and a very eccentric ellipse would have an eccentricity of more than, oh, 0.5 or soSureley that's an e3 - (an exceedingly eccentric ellipse)

hhEb09'1
2006-Jan-09, 05:54 PM
I see the point you're making, but I guess we're just not interpreting "very eccentric" and "perfectly round" the same way: to me, something that is perfectly round has an eccentricity of zero, by definition; and a very eccentric ellipse would have an eccentricity of more than, oh, 0.5 or so.A perfectly round path (as I said, not necessarily a free orbit) can be eccentric, so I think we are using the terms the same way.

The point I was making was that the eccentricity of the Earth, .017, is a measure of how far the Sun is displaced from the center of the orbit, whereas the deviation from "roundness" is a hundred times less than that.

grant hutchison
2006-Jan-09, 06:13 PM
A perfectly round path (as I said, not necessarily a free orbit) can be eccentric, so I think we are using the terms the same way.Ah, I'd missed the distinction between "path" and "orbit" that you are making. A curve of zero eccentricity (a circle) may be placed eccentrically (in relation to some externally defined central point). No argument.

Grant Hutchison

hhEb09'1
2006-Jan-09, 06:25 PM
No argument.If you ever have an argument with anything I say, just let me know, and I'll change my mind. :)

I thought it was interesting that the difference could be a couple orders of magnitude--that that was the constraint that Kepler was under when he decided the paths were elliptical, rather than for instance eccentric circles. This makes the distinction not the difference between 0 and .017, but 0 and say .00017.

grant hutchison
2006-Jan-09, 06:33 PM
If you ever have an argument with anything I say, just let me know, and I'll change my mind. :)I frankly wouldn't advise that as a general policy ... Sometimes I argue in favour of things that turn out not to be so.

Grant Hutchison

hhEb09'1
2006-Jan-09, 06:39 PM
I'm just going to go with the odds :)

astromark
2006-Jan-09, 06:50 PM
And consider this; The planets have arrived at there orbital paths out of a uncertain solar disk of debris., They are doing there best, but the sun keeps moving. Around and around we go. . . . .
Venus, Neptune, Earth. 0.017 out of round, is not much at 150 m/km. is it.?

hhEb09'1
2006-Jan-09, 07:09 PM
0.017 out of round, is not much at 150 m/km. is it.?my point was that it is actually more like .00017 out of round, though it has an eccentricity of .017