Earth's orbit not perfect ellipse

[Delta2]

TL;DR Summary

Earth's orbit not perfect ellipse from the classical, relativistic, and quantum mechanical point of view

Listen to the following arguments:

• Earth's orbit isn't perfect ellipse because classically there is the gravitational field of moon and possibly of Mars and Venus which affect it
• According to general relativity isn't perfect ellipse because there is the curvature of space time which doesn't work exactly classically like Newtonian gravity and there are also gravitational waves which may affect its orbit
• According to quantum mechanics, Earth is a big set of quantum mechanical particles which don't have definite orbits, therefore Earth doesn't have a definite elliptic orbit

So which of the three arguments is more valid to you?

 

[malawi_glenn]

Listen to the following arguments:

Do you have a sound file available so that I can LISTEN to them?

 

[Bandersnatch]

They are all valid. The way you listed them, they're just sorted in the order of increasing negligibility.

 

[Delta2]

They are all valid. The way you listed them, they're just sorted in the order of increasing negligibility.

Yes I also think exactly that. First argument is the most important and practical I think while the other two are mostly theoretical.

 

[malawi_glenn]

According to quantum mechanics, my brain is a huge set of quantum mechanical particles which don't have definite orbits, therefore my brain doesn't have a definitive answer

 

[Ibix]

First argument is the most important and practical I think while the other two are mostly theoretical.

IIRC the anomalous precession of the Earth's orbit is detectable with modern observations.

 

[Delta2]

Sorry what do you mean by anomalous precession? I guess something to do with the general relativity?

 

[PeterDonis]

IIRC the anomalous precession of the Earth's orbit is detectable with modern observations.

I believe we have detected it now for all the inner planets (out to Mars).

what do you mean by anomalous precession? I guess something to do with the general relativity?

Yes, it's the extra precession predicted by GR over and above what Newtonian physics would predict due to the effects of all the other planets.

 

[Ibix]

Sorry what do you mean by anomalous precession? I guess something to do with the general relativity?

Yes. One of the earliest successes of GR was explaining a 43 seconds of arc per century drift in the location of the perihelion of Mercury's orbit compared to a Newtonian model. The figure for Earth's perihelion shift is much smaller but now detectable.

 

[malawi_glenn]

I believe we have detected it now for all the inner planets (out to Mars).

Saturn too iirc

 

[Baluncore]

So which of the three arguments is more valid to you?

As a multiple choice question, I would have to answer D.

 

[FactChecker]

The argument about the Moon (as opposed to other planets) is a little suspect. If you consider the Earth and Moon as one object, you might have no objection to the ellipse.
The argument using quantum theory is so negligible that there is not enough space in an entire book to put all the zeros after the decimal point to quantify it. That can not be defended seriously.

That being said, the first two (even including the Moon) are probably measurable deviations from an ellipse using today's technology. The third one is not.

 

[Delta2]

If you consider the Earth and Moon as one object

You mean their CoM does perfect ellipse around the Sun. OK but no in my mind I couldn't see them as a single object, Earth rotates around their CoM so Earth doesn't do perfect ellipse.

 

[PeterDonis]

You mean their CoM does perfect ellipse around the Sun.

Not quite. There are still effects from the other planets, and there is still the extra precession from GR. But the CoM's orbit is closer to a perfect ellipse than the path of the Earth or the Moon.

 

[Delta2]

@PeterDonis, @Ibix, in the moon-earth CoM frame, Earth (and moon) do a circle orbit or an ellipse?

 

[PeterDonis]

@PeterDonis, @Ibix, in the moon-earth CoM frame, Earth (and moon) do a circle orbit or an ellipse?

Ellipse. Both of them are in elliptical orbits about the CoM. The CoM is actually inside the Earth (since the Earth is 81 times as massive as the Moon but the Earth-Moon distance is only 60 times the Earth's radius), so the Earth's "orbit" about the CoM is actually more like a wobble.

 

[Orodruin]

In fact, the Moon’s orbit has an eccentricity of 0.055, meaning its distance will vary about 10% between perigee and apogee. (Earth’s orbit around the common CoM will have the same eccentricity.)

Because of this, you can have things like annular solar eclipses when the Moon is too far away to cover the full Sun due to the umbra not reaching the Earth’s surface, but the antumbra doing so.

 

[pbuk]

[*]Earth's orbit isn't perfect ellipse because classically there is the gravitational field of moon and possibly of Mars and Venus which affect it

As mentioned above we usually separate out the effects of the Moon and consider the orbit of the (barycentre of the) Earth-Moon system.

Neither Mars nor Venus have a great impact on this: I haven't checked but from memory the main classical factors causing deviation from an ellipse for the Earth-Moon system orbiting the Sun are in descending order:

• Jupiter
• Saturn
• Other planets combined
• Tidal effect of Sun on Earth-Moon system
• Solar quadrupole moment (the equatorial bulge of the sun)
• Tidal effect of Earth-Moon system on Sun

 

[jbriggs444]

Even in the Newtonian model and in the absence of other planets, the barycenter of the Earth-moon pair would not follow a perfect ellipse in its path around the Sun.

Newton's shell theorem allows us to treat the gravitational effect of a system as if it were a point mass, so as long as the system is spherically symmetric. The Earth-moon system is not spherically symmetric.

Edit: this would be the "Tidal effect of the Sun on the Earth-moon system" in the post just preceding.

 

[swampwiz]

Do you have a sound file available so that I can LISTEN to them?

Maybe this?