Only earth orbits in a circular motion?

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Discussion Overview

The discussion centers around the nature of Earth's orbit compared to other planets in the solar system, specifically addressing whether Earth orbits the Sun in a circular motion while other planets do so elliptically. Participants explore concepts related to orbital dynamics, stability, and the definitions of orbits.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that all planets, including Earth, orbit the Sun in elliptical paths, with Earth’s orbit being slightly elliptical rather than perfectly circular.
  • Warren discusses the flawed concept of a "perfectly circular orbit," arguing that any perturbation would lead to an elliptical orbit.
  • Another participant mentions that the Earth's orbit is not perfectly circular and varies in distance from the Sun by about 5,000,000 km throughout the year.
  • There is a discussion about the stability of circular orbits, with some participants noting that small perturbations can lead to deviations into elliptical orbits.
  • One participant highlights that Earth's orbit is least inclined because it serves as the reference plane, thus having an inclination of 0.000 degrees.
  • Another participant questions the accuracy of the inclination statement, noting that the inclination has changed over time and is not strictly zero.
  • There are humorous exchanges about the terminology and the nature of the discussion, with some participants poking fun at each other’s comments.
  • Warren mentions the need to consider the Earth's ephemeris to accurately describe its orbit.

Areas of Agreement / Disagreement

Participants generally disagree on the characterization of Earth's orbit, with some asserting it is circular while others emphasize its elliptical nature. The discussion remains unresolved regarding the implications of these differing views on orbital dynamics.

Contextual Notes

The discussion includes various assumptions about orbital stability, the definitions of circular and elliptical orbits, and the implications of perturbations. There is also a lack of consensus on the precise nature of Earth's orbital characteristics.

vtas25
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I was reading and came across an article which stated that all the planets orbitted the sun elliptically while the Earth was the only planet to orbit the sun in a circular motion...


Is this true?
Also what technology did we use to figure this out??

Thanks


Vtas25
 
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vtas25 said:
I was reading and came across an article which stated that all the planets orbitted the sun elliptically while the Earth was the only planet to orbit the sun in a circular motion...


Is this true?
Also what technology did we use to figure this out??

Thanks


Vtas25

All the planets, including the Earth, orbit in ellipses. Neptune and Venus actually orbit closer to a circle than the Earth does.

As to technology, telescopes.
 
No orbit, including the Earth's, is perfectly circular. The Earth's orbit is, in fact, an ellipse. The distance between the Earth and the Sun varies by about 5,000,000 km throughout the year.

The very concept of a "perfectly circular orbit" is flawed, since the circle is not a stable orbit. Stability implies self-correction: if you perturb a stable system, it naturally returns to the same stable state. A circular orbit is not stable, because any perturbation, even a passing comet, would irreversibly perturb the orbit into an ellipse.

- Warren
 
In orbital dynamics, a circular orbit is generally considered stable if small perturbations cause its radial position to oscillate about the initial radius of the circular orbit. In the Keplerian case, this is the same as saying that the object enters an elliptical orbit. Unstable circular orbits, on the other hand, will exponentially deviate from their initial radius after being perturbed. This can occur, for example, near the event horizon of a black hole.
 
Well, SpaceTiger, I was trying to make it easy to understand: if you start with a circle, any perturbation, no matter how small, leaves you with an ellipse (or other conic section, I suppose).

- Warren
 
chroot said:
Well, SpaceTiger, I was trying to make it easy to understand: if you start with a circle, any perturbation, no matter how small, leaves you with an ellipse (or other conic section, I suppose).

I understand what you were trying to say and it's not wrong -- it's just an issue of terminology. The general circular configuration isn't so much unstable as it is "special". It occupies an infinitesimally small region of parameter space.
 
Last edited:
You can tell the orbit isn't circular by counting the days from Spring Equinox to Fall Equinox, versus the days from Fall Equinox to Spring Equinox. Earth is closest to the Sun right around the New Year.
 
Earts orbit

selfAdjoint said:
You can tell the orbit isn't circular by counting the days from Spring Equinox to Fall Equinox, versus the days from Fall Equinox to Spring Equinox. Earth is closest to the Sun right around the New Year.


I read that Earth's orbit was significantly different from that of all the other planets..How true is this statement...




Vtas25
 
vtas25 said:
I read that Earth's orbit was significantly different from that of all the other planets..How true is this statement...




Vtas25


Not true at all. Earth's orbit is slightly elliptical, close enough to circular that you can't tell it's an ellipse unless you pay close attention. And that pretty much describes the orbits of the other (traditional) planets too. Some are a little more elliptical, others a little less, but there isn't any deep pattern, it bears all the earmarks of accidental initial conditions just randomly distribution a small varying eccentricity on the various planets. Oh, Jupiter and Saturn have a certain "tide locking", and the Earth and Moon show another, but that again is slight and historically contingent, not "deep".
 
  • #10
Earth has the least inclined orbit in the solar system. :wink:
 
  • #11
tony873004 said:
Earth has the least inclined orbit in the solar system. :wink:
Har har.

It's also suspicious that its distance from the Sun is exactly 1AU - no more, no less! :wink:
 
  • #12
DaveC426913 said:
Har har.

It's also suspicious that its distance from the Sun is exactly 1AU - no more, no less! :wink:

Well, this is actually not so... the AU is defined as the mean distance between the Earth and Sun. The actual distance at any given time varies from a little less to a little more than 1 AU.

- Warren
 
  • #13
chroot said:
...the AU is defined as the mean distance between the Earth and Sun. The actual distance at any given time varies from a little less to a little more than 1 AU.

- Warren
Well, this is actually not so... From http://ssd.jpl.nasa.gov/?faq#B05
Why isn't AU the average Earth-Sun distance?
The astronomical unit (AU) may be defined as the distance from the Sun that a massless particle would require for a circular, Keplerian orbit with an exact period of 2*pi/0.01720209895.
 
  • #14
  • #15
I know, I was was poking fun at tony's "least inclined orbit" quip. (To be accurate, I should have said Earth's mean orbit is exactly 1AU.)

Earth's orbit is least inclined because Earth is the reference plane, thus by definition its incline is 0.000 degrees.
 
  • #16
DaveC426913 said:
Earth's orbit is least inclined because Earth is the reference plane, thus by definition its incline is 0.000 degrees.

I know, tony's quip was correct simply because an orbit only has one plane, so its inclination with respect to itself is always zero. Your re-quip, while funny, was just a bit misleading. I didn't mean to kill the humor. :biggrin: It's just my PF fact-checking trigger finger at work.

- Warren
 
  • #17
You're all terribly nerdy.

Carry on.
 
  • #18
DaveC426913 said:
Earth's orbit is least inclined because Earth is the reference plane, thus by definition its incline is 0.000 degrees.
That's not quite right either.

On January 1, 2000 at 12:00, the inclination of the orbit of the Earth / Moon barycenter was 0.000. Taking it 1 more digit would have yielded some non-zero numbers, but I'm not sure why since the J2000 epoch defined the plane.

It's since drifted and is now 0.001. If you had only expressed it as 0.00 I'd have nothing to nitpick.:biggrin:

The Earth itself always has a little z-axis action going on as it gets tugged up and down by the Moon. That's why the barycenter is used.

Uh oh. I thing Space Tiger was right. :bugeye:
 
  • #19
SpaceTiger said:
You're all terribly nerdy.

Carry on.


"Terribly nerdy" is an oxymoron. Try magnificently nerdy or brilliantly nerdy instead.
 
  • #20
Okay, so the Earth doesn't generally orbit at its supposed orbital distance, nor does it generally orbit in its supposed orbital plane.

Argh. I guess we'll just have to list the Earth's ephemeris and say "that's the orbit." (Anyone know where I can find the Earth's ephermis?)

- Warren
 

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