Axis Wobbling

[Githyanki]

I've often heard Earth's large moon prevents axis-wobbling, or when the axis-tilt changes drastically. However, when I observe the seven other planets, only one appears to perhaps be experiencing axis wobbling.This leads me to believe axis-wobbling doesn't really occur; am I right?

[Hornblower]

I've often heard Earth's large moon prevents axis-wobbling, or when the axis-tilt changes drastically. However, when I observe the seven other planets, only one appears to perhaps be experiencing axis wobbling.This leads me to believe axis-wobbling doesn't really occur; am I right?

As I think I understand it, our large Moon causes a relatively rapid precession which prevents troublesome resonances with the other planets which otherwise would cause large, chaotic variations in the axial tilt. The tilt is constrained to a range of about 22 to 24 degrees.

I don't think we have actually observed any precession of any of the other planets, at least not with enough precision to ascertain any secular variations. We simply have not been able to observe their spin axis orientations for a long enough time with the necessary precision. What we can do is to run computer simulations based on what we know about the effect of gravitational perturbations on spinning oblate spheroids. If I am not mistaken, such simulations of Mars predict very large excursions of its axial tilt. It has no large moons to stabilize it the way ours does.

[Moonhead]

I've often heard Earth's large moon prevents axis-wobbling, or when the axis-tilt changes drastically. However, when I observe the seven other planets, only one appears to perhaps be experiencing axis wobbling.This leads me to believe axis-wobbling doesn't really occur; am I right?

Wow, I just wrote a post asking almost the same thing, but my browser crashed on submitting! A good thing, as you phrased it better than I did


In addition, I read this function of the moon is thought to be one of the things that makes the earth so suitable for life. But how bad would this 'axis-wobbling' actually be?

[Githyanki]

Thanks, though, I had difficulty in phrasing it.

[Robert Tulip]

I've often heard Earth's large moon prevents axis-wobbling, or when the axis-tilt changes drastically. However, when I observe the seven other planets, only one appears to perhaps be experiencing axis wobbling.This leads me to believe axis-wobbling doesn't really occur; am I right?

There are two components of axis wobble for the earth. Obliquity of the axis is the 41,000 year oscillation of the angle of tilt between ~22.1° and 24.5°. Precession of the equinox is the 25,800 year wobble of the axis around the ecliptic pole like the wobble of a gyroscope. These orbital cycles are caused by lunisolar torque.

[Hernalt]

Mars "Wobbling" Axis

"During the past ten million years, Earth's axial tilt has only varied between about 22 and 24.5 degrees, because our relatively large Moon helps maintain a stable tilt. But Mars, which has two tiny moons, has experienced more extreme changes in its axial tilt - between 13 and 40 degrees over timescales of about 10 to 20 million years."

[Jeff Root]

Why is Mars's rotational axis thought to have varied so much?
Is there any observational evidence for such variation?

-- Jeff, in Minneapolis

[tusenfem]

Why is Mars's rotational axis thought to have varied so much?
Is there any observational evidence for such variation?

-- Jeff, in Minneapolis

Here is a paper by Dehant et al. (2009): Lander radioscience for obtaining the rotation and orientation of Mars

Here is a paper by Spenke et al. (2005): Polar wander on Mars: Evidence in the geoid

And a paper by Bouquillon & Souchay (1999): Precise modeling of the precession-nutation of Mars

You can click on the "citations to the article" to get more papers.

[Cougar]

I've often heard Earth's large moon prevents axis-wobbling, or when the axis-tilt changes drastically.

The latter. At least, I read that somewhere, and not often. I think it was Paul Davies. The contention was, there would be nothing to stop the axis of rotation from flipping over entirely. Local environments would be generally unstable....

[grapes]

There are two components of axis wobble for the earth. Obliquity of the axis is the 41,000 year oscillation of the angle of tilt between ~22.1° and 24.5°. Precession of the equinox is the 25,800 year wobble of the axis around the ecliptic pole like the wobble of a gyroscope. These orbital cycles are caused by lunisolar torque.

Plus the much shorter-term seasonal wobble, and a little bit longer free wobble--the Eulerian wobble or Chandler wobble.

[Tim Thompson]

I've often heard Earth's large moon prevents axis-wobbling, or when the axis-tilt changes drastically. However, when I observe the seven other planets, only one appears to perhaps be experiencing axis wobbling.This leads me to believe axis-wobbling doesn't really occur; am I right?

The problem is that "axis wobbling" could mean almost anything, so the answers mostly miss the point. First, a definition: The obliquity of a planet is the angle between the spin axis of the planet and the line perpendicular to the plane of the planet's orbit. The French physicist Jaques Laskar has shown that, contrary to prior belief, the obliquity of a planet can, in general, vary over fairly large angles in a relatively short time (Laskar & Robutel, 1993). This is one of many examples of "chaos" in the solar system, which was once thought to be more an example of clock-work determinism. Earth is stabilized against such chaotic excursions of its obliquity by tidal forces between Earth & the moon (Laskar, Joutel & Robutel, 1993). Venus may be stabilized at least to some extent by its very dense & massive atmosphere (Correia & Laskar, 2001). Mars, lacking any stabilizing influence, has a strongly chaotic obliquity (Laskar, et al., 2004).

So it is the stabilizing of this chaotic effect that you have heard about. The obliquity of Earth, about 23.4 degrees, is more a result of tidal torque from the moon than anything else. The oscillating cycles mentioned by Robert Tulip are the Milankovitch cycles (which are extensively verified in the geological record). The Chandler Wobble mentioned by grapes is a totally different beast, an example of true polar wander, it is a motion of the planet with respect to the spin axis (a "slipping" of the planet through the spin axis), which is caused by redistribution of matter inside the planet (or in the atmosphere and/or oceans), changing the planetary moment of inertia, and thus causing the planet to seek a new orientation relative to its spin axis. These are all deterministic & predictable effects, not examples of chaos.

[iantresman]

Back in 1955, astronomer Thomas Gold wrote that if the Earth were a perfect sphere, it:

'would possess no stability of its axis of rotation at all; the smallest beetle walking over it would be able to change the axis of rotation relative to markings on the surface by an arbitrarily large angle'. (Gold T, 1955 Nature "Instability of the Earth's Axis of Rotation" 175 526-9)