I have been having trouble calculating the gravitational constant (G) using Kepler's third law.
given Mercury has a semi-major axis of 0.39 AU, and an orbital period of 0.24 years, and that the sun has a mass of $1.989 * 10^{30} kg$, I tried to calculate G using 
where M is the mass of the sun.
I did $39.478 *(0.24^{-2} * 0.39^3) * (1.989*10^{30})^{-1}*$ which equals to $2.04405086E-29$, but that is not the value of G which is supposed to be $6.6743*10^{-11} m^3 * kg ^{-1} * s^{-2}$. what am I doing wrong here to be so far off?
