Moreover, in a system of two bodies mutually acting upon each other, since the… — Isaac Newton

Moreover, in a system of two bodies mutually acting upon each other, since the differences between their centres and the common centre of gravity of both are reciprocally as the bodies, the relative motions of those bodies, whether of approaching to or of receding from that centre, will be equal junong themselves. Therefore since the changes which happen to motions are equal and directed to contrary parts.