1. The problem statement, all variables and given/known data Explain why both the Earth and Moon must rotate around their common centres of mass, rather than the moon about the centre of the mass of the earth. 2. Relevant equations sum of clockwise moments=sum of anti-clockwise moments 3. The attempt at a solution I want to use Principle of Moments to explain this but the earth and moon must be in rotational equilibrium for this to happen right? I do not understand if one of the conditions for rotational equilibrium (total torque=0) is true. If so, how?