Talk:Orbital eccentricity

Gravitation eccentricity formula as written looks like e>=1 always, please explain

Could someone address the formula e = SQRT(1+2*energy*h^2/u^2) which seems to imply e>=1 always, since h^2 and u^2 are always positive. To match the planetary data, it seems like the equation should be e = 2-SQRT(1+2eh^2/u^2). Perhaps we could plug in numbers for one of the examples?2607:F140:6000:1C:551C:8D2:AC56:37E5 (talk) 16:41, 29 October 2018 (UTC)Ralph Berger

For gravitational calculations, the "energy" term is not always positive. Total energy is equal to the kinetic energy plus the potential energy. The kinetic energy ( (m*v^2)/2 ) is always positive, but the potential energy (-G*Mp*m/r) is always negative that approaches zero as (r) approaches infinity. So total energy can only be positive if the kinetic energy is greater than the potential energy, which is the same as saying that the satellite has enough velocity (kinetic energy) to escape the gravity well of the primary. I.E., that it is at or above the escape velocity. For this reason, when the energy term is non-negative you get open trajectories (parabolas, hyperbolas), but when it is negative you get closed trajectories (circles, ellipses) or "orbits".

RBarryYoung (talk) 12:37, 15 August 2020 (UTC)

Other eccentricity

what about Eccentricity (mathematics) ? there's other equation, which is wrong ?

They are both right, they are just calculating the eccentricity using different variables/parameters so it is a different function. But they will both give the same answer if you use parameters from the same orbit.

RBarryYoung (talk) 12:48, 15 August 2020 (UTC)

Missing info

The effect of excentricity on insolation is missing. —Preceding unsigned comment added by 130.184.251.50 (talk) 22:28, 26 October 2009 (UTC)

Checking Facts After Reverting to Older Version

I just reverted the article to an earlier version because of inserted vandalism. The value of the eccentricity of Earth's orbit was also changed in the same vandalism. I assume the previous value was correct but I'm uncertain so it should be checked by someone who knows better than I. --Pigman (talk • contribs) 21:17, 26 November 2006 (UTC)

Ice Age?

Is that part about eccentricity and longer winters "favorable for triggering the next ice age" actually true? It set my bogometer off, so I thought I'd ask if anybody else has heard that theory. The only reference in the paragraph is that AOL page, which seemed a little un-scientific and POV. --W0lfie 20:26, 8 December 2006 (UTC)

pet peeve: please don't use "theory" to mean "conjecture" in a science talk page. a theory doesn't just mean some random idea in science. —Preceding unsigned comment added by 99.233.20.151 (talk) 05:05, 19 March 2008 (UTC)

The piece of text mentioned above seems to be referring in some way to the well-known (in climatological circles anyway) Milankovitch hypothesis - plenty of references to this can be found on the Wikipedia Milankovitch cycles and related pages. Hays, Imbrie and Shackleton Science 1976 is a good place to start http://www.es.ucsc.edu/~pkoch/EART_206/09-0303/Hays%20et%2076%20Science%20194-1121.pdf, but there are plenty of subsequent papers, both pro and anti, but the current consensus would be that eccentricity variations did play a significant role in the timing of late Pleistocene glaciations up to the present time. CF a very recent paper examining how this could play out through the dynamics of ice sheets: http://www.nature.com/nature/journal/v500/n7461/full/nature12374.html Some, of course, would beg to differ, but not me. You can certainly call it a hypothesis. It's perhaps worth noting that the next ice age may well have been, so to speak, put on ice for a while: http://www.cam.ac.uk/research/news/ice-age-interrupted. Orbitalforam (talk) 15:21, 19 June 2014 (UTC)

Only known or only one?

I reckon that this formulation is somewhat wrong: "Planet Neptune's largest moon Triton is believed to be the only astronomical body that has a perfectly circular orbit with an eccentricity of 0." Is it not quite harsh to claim that Triton is the only body in the entire universe to have a perfectly circular orbit? Either it should be stated that it is in our Solar system or it should state that it's the only know body... or both. --Bilgrau 11:46, 23 June 2007 (UTC)

Is it even possible for something in nature to be perfectly anything? I suspect that the O.E. of Triton is close to but not exactly equal to 0. 65.167.146.130 (talk) 15:46, 31 October 2008 (UTC)

Definition Missing

Am I wrong, or is there no actual definition of eccentricity given in this article? I read the whole thing, but I still have no real idea how it is defined. --01:13, 16 December 2009 (UTC) —Preceding unsigned comment added by 90.162.97.176 (talk)

• "deviates from a perfect circle " is the answer. Telecine Guy 21:05, 1 September 2016 (UTC)

Graph

Graph of the variation of eccentricity of rocky planets (Mercury, Venus, Earth, Mars) does not agree with upper value for Earth in the text beside the graph, assuming the heavy curve is for the Earth. (I do not know if there is a problem with the other three planets, obviously Earth is most important from human perspective.) If the upper value in the text occurs outside the time span of the graph, an explanation would be in order. G Warren Coleman wd4nit@yahoo.com — Preceding unsigned comment added by 76.105.116.92 (talk) 05:45, 16 November 2011 (UTC)

The graph is only plotted for the next 50,000 years, the text in the article refers to the last 750,000 years. -- Kheider (talk) 15:58, 16 November 2011 (UTC)

Rosette orbits

Regarding the definition in the lead, the orbit of stars and clusters through the galaxy follow a non-Keplerian rosette path. This orbit can still be assigned an orbital ellipticity, even though it isn't used in the same mathematical sense.[1][2][3] Regards, RJH (talk) 17:02, 11 August 2012 (UTC)

Graph again

The graph is hard to fathom. The vertical scale at the origin disagrees with the vertical scale on the right by X10. Is this an error?. If not, an explanation is needed on what the two scales mean. Also, what is the meaning of the four arrows on the diagram? SpinningSpark 10:12, 1 November 2013 (UTC)

The arrows point to which of the two scales to use. The eccentricity of Mercury and Mars are much higher than Venus and Earth. -- Kheider (talk) 11:49, 1 November 2013 (UTC)

Well I think that needs explaining in the caption. Even better would be a plot to a log scale which would give a true picture of which planets were more eccentric. Do you have data or algorithms that would allow this to be replotted in another application? It needs redoing anyway, it's a bit pixely and it is already tagged for recreation as an SVG. SpinningSpark 20:34, 1 November 2013 (UTC)

Removal of potentially misleading climate statement

Under climate it states "Apsidal precession slowly changes the place in the Earth's orbit where the solstices and equinoxes occur (this is not the precession of the axis). Over the next 10,000 years, northern hemisphere winters will become gradually longer and summers will become shorter. Any cooling effect in one hemisphere is balanced by warming in the other—and any overall change will, however, be counteracted by the fact that the eccentricity of Earth's orbit will be almost halved[citation needed], reducing the mean orbital radius and raising temperatures in both hemispheres closer to the mid-interglacial peak.". This seems presumptuous and remains uncited. While the total solar insolation may balance out, Many factors could influence the effects on the climate such as exposure to land vs water etc. Unless a counter argument can be posed i think it should be removed or reworded. Nickmista (talk) 09:10, 14 February 2015 (UTC)

• Add ref, (I did not place the text here, but it looks close to facts. Thank you, Telecine Guy 21:07, 1 September 2016 (UTC)

Eccentricity of comets

Currently, the text of the article says "The eccentricity of comets is most often close to 1. Periodic comets have highly eccentric elliptical orbits with eccentricities just below 1 ...". This is totally nonsense! If you go to the JPL Small-Body Database Search Engine, select "object kind: Comets", select "Limit by object characteristics: (select orbital parameter)" and choose "period (years) < 200 " for periodic comets, you will get 701 periodic comets. Then if you refine the search by adding the limit "eccentricity < 0.9", you will still get 659 periodic comets. And so on. What I want to say is, that only a small fraction of periodic comets have "eccentricities just below 1"! This would be true only for non-periodic comets - out of 343 non-periodic (period > 200 years) comets only 4 have e < 0.9. The eccentricity of periodic comets is most often below 0.7, namely, 566 out of 701 comets, that is more than three fourths. 467 periodic comets of 701 have eccentricities: 0.3 < e < 0.7 .

Will anyone object if I correct that part in the article's text? Sergei Schmalz (talk) 15:16, 29 March 2015 (UTC)

Drawing your own conclusions from a primary source database is original research and we should not be doing that. There are all kinds of possible pitfalls with that. We should be using a reliable secondary source to provide the analysis. However, you could use this book source which says that short-period comets (p<100 years by their definition) have eccentricities "mostly between 0.2 and 0.7". SpinningSpark 17:54, 29 March 2015 (UTC)

Of course, it's a sort of original research, but I've done that just to prove my words (otherwise, I just could have edited the article's text right away). Of course, this research can't be used as a source for quotation, or as a reference. And of course, I wouldn't use the same words of my research within the article's text. I'd just write down the conclusion. I'm an astronomer, doing research and observation of minor planets/comets, so I know there is no pitfall in this case. My conclusion is faultless. At the same time, we can't use the web-link which you're providing here as a reference, just by the reason that I can't view it, I get a message: "You have reached a page that is unavailable for viewing or reached your viewing limit for this book.", someone else could have the same problem. To sum up, I would just correct the article's text with correct values now, but we'd need references to be added later on. Sergei Schmalz (talk) 18:37, 29 March 2015 (UTC)

Strangely enough your web-link works for me now. Alright, let's use it and I'd suggest the following new text for the article (with the only remark, that we'd still need a reference for non-periodic comets, which is missing in that book):

Text to be replaced: The eccentricity of comets is most often close to 1. Periodic comets have highly eccentric elliptical orbits with eccentricities just below 1; Halley's Comet's elliptical orbit, for example, has a value of 0.967. Non-periodic comets follow near-parabolic orbits and thus have eccentricities even closer to 1.

New text: Comets have very different values of eccentricity. Periodic comets have eccentricities mostly between 0.2 and 0.7. Non-periodic comets follow near-parabolic orbits and thus have eccentricities closer to 1.

Sergei Schmalz (talk) 20:12, 29 March 2015 (UTC)

References

That seems good to me. The Halley's comet example could be retained to show there are some exceptions (it is even mentioned on the same page in the source). By the way, sources not viewable online are still acceptable as references. One more thing, books should be citded with the cite book template to give the correct formatting (italic title, access date is not required etc etc). SpinningSpark 22:41, 29 March 2015 (UTC)

Done. Sergei Schmalz (talk) 23:03, 29 March 2015 (UTC)

Eccentricity 1 can be an ellipse?

If an orbit is one dimensional (like an object in vertical freefall), its semiminor axis zero, so the eccentricity=1, while I'd still call it an ellipse. So it seems like we have to say eccentricity=1 AND semimajor axis=infinity as both needed to define a parabola. Tom Ruen (talk) 04:48, 21 January 2017 (UTC)

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Claim not in cited work

The claim that...

"Comet McNaught has a hyperbolic orbit while within the influence of the planets..."

... is not supported by the citation for that sentence. The cited article makes no mention of "hyperbolic" — Preceding unsigned comment added by 23.119.204.117 (talk) 21:10, 28 April 2018 (UTC)

JPL shows that in Nov 2006 the orbit of 2006 P1 was hyperbolic. That does not mean the orbit will be hyperbolic when outside the influence of the planetary region of the Solar System. -- Kheider (talk) 17:57, 16 August 2020 (UTC)

Eccentricity of Neptune disagrees with Neptune article

This article lists the eccentricity of Neptune as 0.0086, while the Neptune page has 0.009456. I don't know which is correct. — Preceding unsigned comment added by Probablyarobot (talk • contribs) 08:56, 3 November 2018 (UTC)

Different epochs (dates) will generate different values. Neptune's heliocentric eccentricity varies from 0.0069 (2029-Aug) to 0.0146 (2023-Jan) as a result of Jupiter's 12 year orbit. Neptune's barycentric (Sun+Jupiter) eccentricity is a more stable 0.0087. -- Kheider (talk) 14:33, 16 August 2020 (UTC)

The animated gifs are unfactual?

The animated gifs, https://upload.wikimedia.org/wikipedia/commons/a/ae/Binary_system_orbit_q%3D3_e%3D0.gif, do not represent realistic motion because the periods of both orbits are shown as the same, whereas in reality the one with a longer major axis has longer period, according to Keplers laws. IMO, these gifs are likely to cause more confusion than any benefit because orbital eccentricity is a property of the whole orbit and animation is really unnecessary, but a wrong animation is worse than unnecessary. I am deleting them. Kotika98 (talk) 15:49, 12 August 2021 (UTC)

m² discrepancy in the Definitions section?

I'm having difficulty reconciling the two equations for e in the Definitions section. They differ where one has EL²/mα² and the other has εh²/μ². I'm reading ε = E/m, h = L/m, and μ = Gm. When I plug these three values into the second definition I get EL²/m³α² instead of EL²/mα². Why am I getting this extra m² in the denominator? Should μ = G instead of μ = Gm or h = L instead of h = L/m or what? Vaughan Pratt (talk) 07:22, 2 February 2022 (UTC)

On further consideration, for e = 0 (a circular orbit), both equations would need EL²/mα² and εh²/μ² to be -½. But none of E, m, or ε = E/m can be negative. So now I'm even more mystified. Vaughan Pratt (talk) 07:46, 2 February 2022 (UTC)

There is no discrepancy

To be more precise, μ = GM, the mass of the body being orbited (the center of mass), and hence ε h² / μ² = ( E / m ) * ( L / m )² / G² M² = E L² / G² M² m³ = E L² / ( G² M² m² m ) = E L² / α² m. where I have used M as the mass of the body being orbited (the center of mass), and m as the reduced mass of the orbiter, m_red , as it appears in the article. In addition, please keep in mind that in all of physics, bound states (such as closed orbits) have total negative energy. Therefore, the total energy, E, as a quantity is a negative number with the following bounds  - G M m / r < E < 0. It never gets more negative than zero kinetic energy ( v = 0 ), and is never less negative than approaching zero, which at E = 0 the orbit stops being bound and becomes an escape parabola, consistent with e = 1. MMmpds (talk) 15:09, 2 February 2022 (UTC)

Thanks, I see my mistakes: I used M for the reduced mass, and neglected to include the potential energy in E. Very out of practice. Vaughan Pratt (talk) 20:43, 2 February 2022 (UTC)