<p>The planets are very, very close to spherical (the Earth has a variaton of about 0.1% between polar and equatorial radii).  </p>

<p>The rotational inertial for a solid sphere is</p>

<p>$$I=\frac{2}{5}mr^2$$</p>

<p>If you give us details about why you might be interested in looking at the rotational inertia of planets in greater detail, there might be more motivation for putting effort into finding an answer for you.</p>

<p>Just saying.</p>

CC BY-SA 3.0
2017-09-02T21:01:15.070Z
2017-09-02T21:01:15.070Z
David Elm
<p>The three principal moments of inertia of Earth and Moon are reported, for example, in T. J. Ahrens, <em>Global Earth Physics - A handbook of Physical Constants</em>.</p>

<p>I have found reports of the moment of inertia factor $\dfrac{C}{M R^2}$, where $C$ is the polar moment of inertia, $M$ the mass and $R$ the mean radius, for planets of the solar system. </p>

<p>Are the three principal moments of inertia for the planets of the solar system known?</p>

CC BY-SA 3.0
2017-08-30T13:15:37.443Z
I'm voting to close this question as off-topic because questions asking for references are generally undesirable.
Carl Witthoft
CC BY-SA 3.0
2017-08-30T13:47:55.413Z
@CarlWitthoft Where is this asking for references? James wants to know if the principle moments of inertia are known. Any good answer will use references, but this question is not specifically asking for someone to provide him with references.
zephyr
CC BY-SA 3.0
2017-08-30T17:24:58.477Z
@zephyr  well, sans references,  the answer is either "yes" or "no," neither of which is much help
Carl Witthoft
CC BY-SA 3.0
2017-08-30T17:26:57.007Z
@CarlWitthoft I still don't see your point. Any answer on here should provide references. If that's your metric for closing a question, all questions should be closed. James' question should only be closed (as it pertains to references) if either (a) it is clear he is asking us to do his own homework/research or (b) he is asking only for resources. As it is, neither seem to be true and this is a perfectly valid question whose answer should reference materials for support.
zephyr
CC BY-SA 3.0
2017-09-01T21:34:45.863Z
Not sure what to do with questions that can be solved by simply googling the exact same question...
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-09-02T07:11:10.430Z
I have done an extensive research before posting this question, and this question can definitely not be resolved by 'simply googling the exact same question'.
James
CC BY-SA 3.0
2017-09-03T00:11:50.907Z
@James: Weird. My second google result had all the data.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-09-03T09:30:09.503Z
Post the link then, I am curious to see it.
James
CC BY-SA 3.0
2017-09-05T15:09:06.533Z
https://en.wikipedia.org/wiki/Moment_of_inertia_factor
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-09-05T15:15:14.417Z
also the following piece of a lecture gives a nice overview how the moments are determined: astro.cornell.edu/academics/courses/astro6570/Rotation_precession_gravity_fields.pdf
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-09-05T18:37:35.920Z
Your link provides the 'moment of inertia factor', for which I knew very well that data are available, see 'I have found reports of the moment of inertia factor C/(M R^2), where C is the polar moment of inertia' in my question.  As I pointed out in my question, I am asking for a different quantity, i.e., 'the three principal moments of inertia'. 

You will easily realize that finding data for these quantities is not easy.
James
<planet><orbital-mechanics>
Moments of inertia of planets
James
# Moments of inertia of planets

(Asked by: username_0 on 2017-08-30)

The three principal moments of inertia of Earth and Moon are reported, for example, in T. J. Ahrens, *Global Earth Physics - A handbook of Physical Constants*.

I have found reports of the moment of inertia factor $\dfrac{C}{M R^2}$, where $C$ is the polar moment of inertia, $M$ the mass and $R$ the mean radius, for planets of the solar system.

Are the three principal moments of inertia for the planets of the solar system known?

**Tags:** planet, orbital-mechanics
> **Comments:**
> **username_1**: I'm voting to close this question as off-topic because questions asking for references are generally undesirable.  
> **username_2**: @CarlWitthoft Where is this asking for references? James wants to know if the principle moments of inertia are known. Any good answer will use references, but this question is not specifically asking for someone to provide him with references.  
> **username_1**: @zephyr  well, sans references,  the answer is either "yes" or "no," neither of which is much help  
> **username_2**: @CarlWitthoft I still don't see your point. Any answer on here should provide references. If that's your metric for closing a question, all questions should be closed. James' question should only be closed (as it pertains to references) if either (a) it is clear he is asking us to do his own homework/research or (b) he is asking only for resources. As it is, neither seem to be true and this is a perfectly valid question whose answer should reference materials for support.  
> **username_3**: Not sure what to do with questions that can be solved by simply googling the exact same question...  
> **username_0**: I have done an extensive research before posting this question, and this question can definitely not be resolved by 'simply googling the exact same question'.  
> **username_3**: @James: Weird. My second google result had all the data.  
> **username_0**: Post the link then, I am curious to see it.  
> **username_3**: https://en.wikipedia.org/wiki/Moment_of_inertia_factor  
> **username_3**: also the following piece of a lecture gives a nice overview how the moments are determined: astro.cornell.edu/academics/courses/astro6570/Rotation_precession_gravity_fields.pdf  
> **username_0**: Your link provides the 'moment of inertia factor', for which I knew very well that data are available, see 'I have found reports of the moment of inertia factor C/(M R^2), where C is the polar moment of inertia' in my question.  As I pointed out in my question, I am asking for a different quantity, i.e., 'the three principal moments of inertia'.   You will easily realize that finding data for these quantities is not easy.  

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## 1 answers

### Answer by username_4 on 2017-09-02. Score: 2

The planets are very, very close to spherical (the Earth has a variaton of about 0.1% between polar and equatorial radii).

The rotational inertial for a solid sphere is

$$I=\frac{2}{5}mr^2$$

If you give us details about why you might be interested in looking at the rotational inertia of planets in greater detail, there might be more motivation for putting effort into finding an answer for you.

Just saying.


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