# Only earth orbits in a circular motion?

1. Aug 15, 2006

### vtas25

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

2. Aug 15, 2006

### Janus

Staff Emeritus
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.

3. Aug 15, 2006

### chroot

Staff Emeritus
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

4. Aug 16, 2006

### SpaceTiger

Staff Emeritus
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.

5. Aug 16, 2006

### chroot

Staff Emeritus
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

6. Aug 16, 2006

### SpaceTiger

Staff Emeritus
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: Aug 16, 2006
7. Aug 16, 2006

Staff Emeritus
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.

8. Aug 21, 2006

### vtas25

Earts orbit

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

Vtas25

9. Aug 21, 2006

Staff Emeritus

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. Aug 22, 2006

### tony873004

Earth has the least inclined orbit in the solar system.

11. Aug 22, 2006

### DaveC426913

Har har.

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

12. Aug 22, 2006

### chroot

Staff Emeritus
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. Aug 22, 2006

### tony873004

Well, this is actually not so... From http://ssd.jpl.nasa.gov/?faq#B05

14. Aug 22, 2006

### chroot

Staff Emeritus
15. Aug 23, 2006

### DaveC426913

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 defintion its incline is 0.000 degrees.

16. Aug 23, 2006

### chroot

Staff Emeritus
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. It's just my PF fact-checking trigger finger at work.

- Warren

17. Aug 23, 2006

### SpaceTiger

Staff Emeritus
You're all terribly nerdy.

Carry on.

18. Aug 23, 2006

### tony873004

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.

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.

19. Aug 23, 2006

Staff Emeritus

"Terribly nerdy" is an oxymoron. Try magnificently nerdy or brilliantly nerdy instead.

20. Aug 23, 2006

### chroot

Staff Emeritus
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