Rotational inertia physics question?

Question

Find the ratio of the rotational inertia of the Earth for rotation about its own axis to its rotational inertia for rotation about the Sun.

Answer 1

1. Assume that the Earth is a solid sphere or radius r and mass m.
2 Assume that the Earth is so far away from the Sun that it's radius r is very small in comparison to the radius of its orbit R and Earth becomes sort of a point mass.

For a solid sphere
I= (2/5)mr^2

For point mass
I=mR^2

Ratio=(2/5)mr^2 / mR^2
Ratio=(2/5)(r / R)^2