In astrophysics, the Darwin–Radau equation gives an approximate relation between the moment of inertia factor of a planetary body and its rotational speed and shape. The moment of inertia factor is directly related to the largest principal moment of inertia, C. It is assumed that the rotating body is in hydrostatic equilibrium and is an ellipsoid of revolution. The Darwin–Radau equation states[1]

${\displaystyle {\frac {C}{MR_{e}^{2}}}={\frac {2}{3\lambda }}={\frac {2}{3}}\left(1-{\frac {2}{5}}{\sqrt {1+\eta }}\right)}$

where M and Re represent the mass and mean equatorial radius of the body. Here λ is the d'Alembert parameter and the Radau parameter η is defined as

${\displaystyle \eta ={\frac {5q}{2\epsilon }}-2}$

where q is the geodynamical constant

${\displaystyle q={\frac {\omega ^{2}R_{e}^{3}}{GM}}}$

and ε is the geometrical flattening

${\displaystyle \epsilon ={\frac {R_{e}-R_{p}}{R_{e}}}}$

where Rp is the mean polar radius and Re is the mean equatorial radius.

For Earth, ${\displaystyle q\approx 3.461391\times 10^{-3}}$ and ${\displaystyle \epsilon \approx 1/298.257}$, which yields ${\displaystyle {\frac {C}{MR_{e}^{2}}}\approx 0.3313}$, a good approximation to the measured value of 0.3307.[2]

## References

1. ^ Bourda, G; Capitaine N (2004). "Precession, nutation, and space geodetic determination of the Earth's variable gravity field". Astronomy and Astrophysics. 428: 691–702. arXiv:. Bibcode:2004A&A...428..691B. doi:10.1051/0004-6361:20041533.
2. ^ Williams, James G. (1994). "Contributions to the Earth's obliquity rate, precession, and nutation". The Astronomical Journal. 108: 711. Bibcode:1994AJ....108..711W. doi:10.1086/117108. ISSN 0004-6256.