# Moment of inertia of the sun

1. Dec 16, 2011

### AndersLau

I want to find the rotational energy of the sun.
Erot=1/2*I*ω2

m=1.9891*10^30 kg
r=6.955*10^5 km

When Im using the formula for a sphere's moment of inertia: I=2/5*m*r2
I'm getting 3.848671797*1047 km*m2

Can i find the moment of inertia in another way? the moment of inertia needs to be in *1046 , to get the rotational energy right.

Thank you.

2. Dec 16, 2011

### D H

Staff Emeritus
A uniform density sphere is a fairly bad model even for the earth, which is more or less solid. For a gas it is an incredibly lousy model. The sun's core contains about half of the total mass of the sun but only 1.5% of its total volume.

Unless you want to get into astrophysics, you are going to need an empirical value. From http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html, I/MR2 for the sun is 0.059. Compare that for a uniform density sphere, where I/MR2=2/5 or 0.4.

3. Dec 16, 2011

### AndersLau

Thank you very much for the answer, that sorts it out. Much appreciated.
I'm waiting with the astrophysics :)